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Sunday, 09 November 2025
UPDATE (2025-11-10): Added more practical examples from my actual codebase showing real-world patterns, trade-offs, gotchas, and the evolution from simple to sophisticated state management approaches. Includes detailed IMemoryCache patterns, ViewBag usage (good and bad), ResponseCache/OutputCache strategies, and lessons learned from production.
HTTP is famously stateless. Your app… is not. Users sign in, add items to baskets, hop between pages, return tomorrow, and expect you to remember. In ASP.NET Core (MVC, Razor Pages, Minimal APIs), there are a lot of ways to preserve and transfer state between requests. Some are per-request only. Some last for a session. Some live in the client. Some are distributed and survive server restarts. Each choice has trade-offs across security, performance, scale, and developer ergonomics.
This post catalogs the options, shows concrete, copy‑pasteable examples in all three stacks, and gives you decision guidance so you can pick the right tool for the job.
NOTE: This is part of my experiments with AI (assisted drafting) + my own editing. Same voice, same pragmatism; just faster fingers.
flowchart LR
subgraph Client
Q[Query String]
R[Route Values]
H[Headers]
F[Form/Hidden Fields]
CK[Cookies]
LS[Local/Session Storage]
end
subgraph Server
I[HttpContext.Items<br/>\nper request only]
TD[TempData<br/>\none redirect]
SS[Session]
MC[IMemoryCache]
DC[IDistributedCache]
AU[Auth Cookie / Claims]
JT[JWT / Bearer]
DB[Database / Durable Store]
BUS[Outbox / Queue]
end
Q --> Model[Model Binding]
R --> Model
F --> Model
H --> Model
CK --> App[Your Code]
Model --> App
App -->|Set| CK
App -->|Set| TD
App -->|Set| SS
App -->|Set| MC
App -->|Set| DC
App -->|Issue| AU
App -->|Issue| JT
App -->|Persist| DB
sequenceDiagram participant M as Middleware participant E as Endpoint/Controller M->>M: Compute TenantId M->>E: HttpContext.Items["TenantId"] = 42 E->>E: Read Items["TenantId"]
Middleware example (all stacks):
app.Use(async (context, next) =>
{
var tenantId = context.Request.Headers["X-TenantId"].FirstOrDefault() ?? "public";
context.Items["TenantId"] = tenantId;
await next(context);
});
app.MapGet("/whoami", (HttpContext ctx) => new { Tenant = ctx.Items["TenantId"] });
public IActionResult WhoAmI() => Json(new { Tenant = HttpContext.Items["TenantId"] });
public IActionResult OnGet() => new JsonResult(new { Tenant = HttpContext.Items["TenantId"] });
Examples
app.MapGet("/orders/{id:int}", (int id, int? page) => Results.Ok(new { id, page }));
// GET /orders/5?page=2
[HttpGet("/orders/{id:int}")]
public IActionResult Details(int id, int? page)
=> View(new { id, page });
public IActionResult OnGet(int id, int? page)
=> Page();
Generating links that preserve state:
// Razor Pages
<a asp-page="/Orders/Details" asp-route-id="@Model.Id" asp-route-page="@Model.Page">Next</a>
// MVC
@Html.ActionLink("Next", "Details", "Orders", new { id = Model.Id, page = Model.Page }, null)
app.Use(async (ctx, next) =>
{
var correlationId = ctx.Request.Headers["X-Correlation-Id"].FirstOrDefault()
?? Guid.NewGuid().ToString("n");
ctx.Response.Headers["X-Correlation-Id"] = correlationId;
await next(ctx);
});
Idempotency (pattern):
flowchart TD
C[Client POST /pay\nIdempotency-Key:k] --> S{Seen k?}
S -- No --> E[Execute charge]
E --> P[Persist result by k]
P --> R[Return 200 + result]
S -- Yes --> L[Load result by k]
L --> R
PRG pattern in MVC/Razor Pages:
sequenceDiagram participant U as User participant P as POST Action participant R as Redirect participant G as GET Action U->>P: POST form P-->>R: 302 Redirect U->>G: GET redirected G-->>U: Final page (no resubmits)
[HttpPost]
[ValidateAntiForgeryToken]
public IActionResult Save(SettingsModel model)
{
// validate & persist
return RedirectToAction(nameof(Summary), new { tab = model.SelectedTab });
}
public IActionResult OnPost(SettingsModel model)
{
return RedirectToPage("/Settings/Summary", new { tab = model.SelectedTab });
}
Minimal API example:
app.MapPost("/prefs/theme/{value}", (HttpContext ctx, string value) =>
{
ctx.Response.Cookies.Append("theme", value, new CookieOptions
{
HttpOnly = false,
Secure = true,
SameSite = SameSiteMode.Lax,
Expires = DateTimeOffset.UtcNow.AddYears(1)
});
return Results.Ok();
});
app.MapGet("/prefs/theme", (HttpContext ctx)
=> Results.Text(ctx.Request.Cookies["theme"] ?? "system"));
MVC/Razor Pages usage is identical via HttpContext.
For integrity/confidentiality, use the ASP.NET Core Data Protection system to protect payloads you put in cookies yourself.
Setup (Program.cs):
builder.Services.AddControllersWithViews().AddSessionStateTempDataProvider(); // optional
builder.Services.AddSession();
var app = builder.Build();
app.UseSession();
In MVC controller:
TempData["StatusMessage"] = "Saved!";
return RedirectToAction("Index");
In Razor Page handler:
TempData["StatusMessage"] = "Saved!";
return RedirectToPage("/Index");
In view/page:
@if (TempData["StatusMessage"] is string msg) {
<div class="alert alert-success">@msg</div>
}
flowchart LR A[POST /save] -->|TempData set| B[302 Redirect] B --> C[GET /index] C -->|TempData read consumed| D[Render message]
Configure:
builder.Services.AddDistributedMemoryCache(); // or AddStackExchangeRedisCache
builder.Services.AddSession(options =>
{
options.IdleTimeout = TimeSpan.FromMinutes(20);
options.Cookie.HttpOnly = true;
options.Cookie.IsEssential = true;
});
var app = builder.Build();
app.UseSession();
Using Session (any stack):
app.MapPost("/cart/add/{id:int}", (HttpContext ctx, int id) =>
{
var key = "cart";
var bytes = ctx.Session.Get(key);
var list = bytes is null ? new List<int>() : System.Text.Json.JsonSerializer.Deserialize<List<int>>(bytes)!;
list.Add(id);
ctx.Session.Set(key, System.Text.Json.JsonSerializer.SerializeToUtf8Bytes(list));
return Results.Ok(list);
});
Session helpers:
public static class SessionExtensions
{
public static void Set<T>(this ISession session, string key, T value)
=> session.SetString(key, System.Text.Json.JsonSerializer.Serialize(value));
public static T? Get<T>(this ISession session, string key)
=> session.TryGetValue(key, out var data)
? System.Text.Json.JsonSerializer.Deserialize<T>(data)
: default;
}
I once worked on a massive UK government IT project where session state misuse (among many other architectural sins) became a performance-killing bottleneck. The team had stuffed everything into session: user preferences, multi-step form data, search results, temporary calculations, even cached lookups that should have been in a proper cache or database.
The problem: Session state was stored in-process (ASP.NET session state in web.config, this was pre-Core days). Every request had to deserialize massive session objects. As load increased, session state ballooned to tens of megabytes per user. With thousands of concurrent users, the servers ran out of memory.
The desperate solution: We flew to HP's facility in Stuttgart to run load tests on their Superdome—at the time, Europe's most powerful Windows machine. It was a beast: dozens of Itanium processors, hundreds of gigabytes of RAM. The idea was to prove that with enough hardware, the system could meet requirements.
The result: Even on the Superdome, we couldn't hit the required concurrent user targets. The session state architecture was fundamentally broken. Vertical scaling couldn't save bad design. The session serialization/deserialization overhead, combined with memory pressure from massive session objects, meant the system simply couldn't scale—not at any reasonable cost.
The security nightmare: Worse than the performance issues, we discovered a coding error that caused session state to leak between users. User A's session data would occasionally appear in User B's session. This wasn't just embarrassing—it was catastrophic. The users were NHS staff accessing patient records and clinical systems. We had accidentally created a data protection breach mechanism that could expose sensitive medical information across different healthcare professionals' sessions.
What should have happened:
The lesson: Session state doesn't scale vertically and barely scales horizontally (even with sticky sessions or distributed stores, you're still serializing/deserializing on every request). The Superdome experiment proved that throwing hardware at architectural problems is expensive and often futile.
But more importantly: session state bugs become security vulnerabilities. Thread-safety issues, race conditions, incorrect session ID handling—these don't just cause performance problems, they can leak sensitive data between users. In a healthcare context (or banking, or any regulated industry), this is a compliance nightmare and potential criminal liability.
Modern advice:
IMemoryCache:
builder.Services.AddMemoryCache();
app.MapGet("/rates", (IMemoryCache cache) =>
{
var key = "fx:usd:eur";
if (!cache.TryGetValue(key, out decimal rate))
{
rate = 0.92m; // pretend fetch
cache.Set(key, rate, TimeSpan.FromMinutes(5));
}
return Results.Ok(rate);
});
IDistributedCache (e.g., Redis):
builder.Services.AddStackExchangeRedisCache(o => o.Configuration = "localhost:6379");
app.MapGet("/feature/{name}", async (IDistributedCache cache, string name) =>
{
var val = await cache.GetStringAsync($"feat:{name}");
return Results.Text(val ?? "off");
});
Cache-as-state anti-pattern warning: if it must be durable or authoritative, store it in a database and optionally cache it.
When you need to invalidate many related entries:
// version key: "v:products"; keys like $"{version}:product:{id}"
var version = await cache.GetStringAsync("v:products") ?? "1";
var key = $"{version}:product:{id}";
// using StackExchange.Redis directly for sets + efficient deletes
var mux = await ConnectionMultiplexer.ConnectAsync("localhost:6379");
var db = mux.GetDatabase();
var tag = "tag:category:42";
var key = $"prod:{prodId}";
await db.StringSetAsync(key, serialized, expiry: TimeSpan.FromMinutes(30));
await db.SetAddAsync(tag, key); // remember membership
// later, invalidate the whole tag
var members = await db.SetMembersAsync(tag);
if (members.Length > 0)
{
var keys = Array.ConvertAll(members, m => (RedisKey)m);
await db.KeyDeleteAsync(keys);
}
await db.KeyDeleteAsync(tag);
T GetOrAdd<T>(IMemoryCache cache, string key, Func<ICacheEntry, T> factory)
=> cache.GetOrCreate(key, e =>
{
e.AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(10);
e.SlidingExpiration = TimeSpan.FromMinutes(2);
e.Priority = CacheItemPriority.Normal;
e.Size = 1;
return factory(e);
});
static async Task<T?> GetOrSetJsonAsync<T>(IDistributedCache cache, string key, Func<Task<T>> factory, TimeSpan ttl)
{
var json = await cache.GetStringAsync(key);
if (json is not null)
return System.Text.Json.JsonSerializer.Deserialize<T>(json);
var value = await factory();
var opts = new DistributedCacheEntryOptions { AbsoluteExpirationRelativeToNow = ttl };
await cache.SetStringAsync(key,
System.Text.Json.JsonSerializer.Serialize(value),
opts);
return value;
}
Here's how I actually use IMemoryCache in my blog platform, evolved through trial and error. I'll show you three real patterns from simple to sophisticated.
This was my first caching implementation. Blog categories don't change often, so cache them for 30 minutes:
// From BaseController.cs
private const string CacheKey = "Categories";
private async Task<List<string>> GetCategories()
{
baseControllerService.MemoryCache.TryGetValue(CacheKey, out var value);
if (value is List<string> categories) return categories;
logger.LogInformation("Fetching categories from BlogService");
categories = (await BlogViewService.GetCategories(true)).OrderBy(x => x).ToList();
baseControllerService.MemoryCache.Set(CacheKey, categories, TimeSpan.FromMinutes(30));
return categories;
}
Why this works:
Gotcha I hit: Initially I used a string key "Categories". Works fine until you have multiple controllers and one accidentally reuses the same key. Now I use constants or strongly-typed keys (see the earlier section on avoiding collisions).
This caches translation status per user. It's more complex because it needs bounds and should stay alive as long as the user is active:
// From TranslateCacheService.cs - tracks translation tasks per user
public void AddTask(string userId, TranslateTask task)
{
CachedTasks CachedTasks() => new()
{
Tasks = new List<TranslateTask> { task },
AbsoluteExpiration = DateTime.Now.AddHours(6)
};
if (memoryCache.TryGetValue(userId, out CachedTasks? tasks))
{
tasks ??= CachedTasks();
var currentTasks = tasks.Tasks;
// Keep only the 5 most recent tasks
currentTasks = currentTasks.OrderByDescending(x => x.StartTime).ToList();
if (currentTasks.Count >= 5)
{
var lastTask = currentTasks.Last();
currentTasks.Remove(lastTask);
}
currentTasks.Add(task);
currentTasks = currentTasks.OrderByDescending(x => x.StartTime).ToList();
tasks.Tasks = currentTasks;
memoryCache.Set(userId, tasks, new MemoryCacheEntryOptions
{
AbsoluteExpiration = tasks.AbsoluteExpiration,
SlidingExpiration = TimeSpan.FromHours(1) // Extends on access
});
}
else
{
var absoluteExpiration = DateTime.Now.AddHours(6);
var cachedTasks = CachedTasks();
memoryCache.Set(userId, cachedTasks, new MemoryCacheEntryOptions
{
AbsoluteExpiration = absoluteExpiration,
SlidingExpiration = TimeSpan.FromHours(1)
});
}
}
Why this is different:
Mistake I made: Initially I didn't limit the number of tasks. A power user triggered 50+ translations and I had a memory leak. Now I keep max 5 per user.
Trade-off: This won't scale to millions of users. If this becomes a problem, I'd move to IDistributedCache (Redis) or store in the database with an index on userId + startTime.
This caches analytics metrics and tracks cache effectiveness using Serilog tracing:
// From UmamiDataSortService.cs - caches Umami analytics metrics
public async Task<List<MetricsResponseModels>?> GetMetrics(DateTime startAt, DateTime endAt, string prefix = "")
{
using var activity = Log.Logger.StartActivity("GetMetricsWithPrefix");
try
{
var cacheKey = $"Metrics_{startAt:yyyyMMdd}_{endAt:yyyyMMdd}_{prefix}";
if (cache.TryGetValue(cacheKey, out List<MetricsResponseModels>? metrics))
{
activity?.AddProperty("CacheHit", true);
return metrics;
}
activity?.AddProperty("CacheHit", false);
var metricsRequest = new MetricsRequest
{
StartAtDate = startAt,
EndAtDate = endAt,
Type = MetricType.url,
Limit = 500
};
var metricRequest = await dataService.GetMetrics(metricsRequest);
if (metricRequest.Status != HttpStatusCode.OK) return null;
var filteredMetrics = metricRequest.Data
.Where(x => x.x.StartsWith(prefix))
.ToList();
cache.Set(cacheKey, filteredMetrics, TimeSpan.FromHours(1));
activity?.AddProperty("MetricsCount", filteredMetrics?.Count() ?? 0);
activity?.Complete();
return filteredMetrics;
}
catch (Exception e)
{
activity?.Complete(LogEventLevel.Error, e);
return null;
}
}
What makes this production-ready:
yyyyMMdd format in key so different times on the same day share the cacheEvolution: Initially I cached for 10 minutes. But Umami metrics are heavy to fetch and don't change much, so 1 hour is fine. I discovered this by looking at the SerilogTracing data and seeing excessive cache misses.
Monitoring in action: In Seq (my log aggregator), I can query:
ActivityName = "GetMetricsWithPrefix" and CacheHit = false
This tells me my cache miss rate. If it's high, I adjust TTL or key strategy.
| Pattern | Use Case | Expiration | Size Control | Observability |
|---|---|---|---|---|
| Categories | Global, rarely-changing | 30 min absolute | Not needed (small) | Basic logging |
| Translation tasks | Per-user, bounded | 6h absolute + 1h sliding | Manual (5 items max) | None (should add!) |
| Metrics | Expensive external calls | 1h absolute | Natural (time-windowed) | Full tracing |
Key lessons:
When I don't use IMemoryCache:
Setup cookie auth:
builder.Services.AddAuthentication("Cookies")
.AddCookie("Cookies", o =>
{
o.LoginPath = "/login";
o.Cookie.SecurePolicy = CookieSecurePolicy.Always;
o.SlidingExpiration = true;
});
builder.Services.AddAuthorization();
var app = builder.Build();
app.UseAuthentication();
app.UseAuthorization();
Sign-in with claims (MVC/minimal):
app.MapPost("/login", async (HttpContext ctx) =>
{
var claims = new[]
{
new Claim(ClaimTypes.NameIdentifier, "123"),
new Claim(ClaimTypes.Name, "Alice"),
new Claim(ClaimTypes.Role, "Admin")
};
var identity = new ClaimsIdentity(claims, "Cookies");
await ctx.SignInAsync("Cookies", new ClaimsPrincipal(identity));
return Results.Redirect("/");
});
Read claims (any stack):
[Authorize]
app.MapGet("/me", (ClaimsPrincipal user)
=> Results.Ok(new { user.Identity!.Name, Roles = user.Claims.Where(c => c.Type == ClaimTypes.Role).Select(c => c.Value) }));
builder.Services.AddAuthentication("Bearer")
.AddJwtBearer("Bearer", o =>
{
o.Authority = "https://demo.identityserver.io"; // example
o.Audience = "api";
o.RequireHttpsMetadata = true;
});
Use:
[Authorize(AuthenticationSchemes = "Bearer")]
app.MapGet("/secure", () => "ok");
Mermaid overview:
sequenceDiagram participant C as Client participant STS as Token Service participant API as API C->>STS: Authenticate (username/password) STS-->>C: JWT (signed) C->>API: GET /secure (Authorization: Bearer <jwt>) API->>API: Validate signature, expiry, audience API-->>C: 200
EF Core sketch:
builder.Services.AddDbContext<AppDb>(o => o.UseSqlServer(cs));
app.MapPost("/cart/items", async (AppDb db, AddItem cmd) =>
{
var cart = await db.Carts.FindAsync(cmd.CartId) ?? new Cart(cmd.CartId);
cart.Add(cmd.ProductId, cmd.Qty);
await db.SaveChangesAsync();
return Results.Created($"/cart/{cart.Id}", cart);
});
builder.Services.AddResponseCaching();
var app = builder.Build();
app.UseResponseCaching();
app.MapGet("/products", (HttpContext ctx) =>
{
ctx.Response.GetTypedHeaders().CacheControl = new CacheControlHeaderValue { Public = true, MaxAge = TimeSpan.FromSeconds(30) };
return Results.Ok(new[] { new { Id = 1, Name = "Widget" } });
}).CacheOutput();
ETag example:
app.MapGet("/resource", (HttpContext ctx) =>
{
var version = "W/\"abc123\""; // compute based on data hash
ctx.Response.Headers.ETag = version;
if (ctx.Request.Headers.IfNoneMatch == version)
return Results.StatusCode(StatusCodes.Status304NotModified);
return Results.Text("payload");
});
I use both ResponseCache and OutputCache together on my blog for different purposes. Here's why you might want both and how they differ.
ASP.NET Core has two similar-looking caching systems:
Cache-Control, Vary) telling browsers and CDNs how to cacheThey complement each other. ResponseCache handles client/CDN caching; OutputCache handles server-side caching.
// From BlogController.cs
[Route("{slug}")]
[HttpGet]
[ResponseCache(Duration = 300, VaryByHeader = "hx-request",
VaryByQueryKeys = new[] { nameof(slug), nameof(language) },
Location = ResponseCacheLocation.Any)]
[OutputCache(Duration = 3600, VaryByHeaderNames = new[] { "hx-request" },
VaryByQueryKeys = new[] { nameof(slug), nameof(language) })]
public async Task<IActionResult> Show(string slug, string language = "en")
{
var post = await blogViewService.GetPost(slug, language);
if (post == null) return NotFound();
// ... populate user info, comments, etc ...
if (Request.IsHtmx()) return PartialView("_PostPartial", post);
return View("Post", post);
}
What happens when someone requests /blog/my-post:
OutputCache checks first: Do I have a cached response for my-post + en language?
ResponseCache sets headers: After OutputCache generates the response, ResponseCache adds:
Cache-Control: public, max-age=300
Vary: hx-request
Browser caching: Browser caches the response for 300 seconds (5 minutes). Subsequent requests from same user don't even hit the server.
CDN caching (if using Cloudflare/Fastly): CDN caches for 5 minutes. Users worldwide hit the CDN, not my server.
[ResponseCache(Duration = 300)] // 5 minutes client/CDN cache
[OutputCache(Duration = 3600)] // 1 hour server cache
Reasoning:
Evolution: Initially I had both at 5 minutes. But that meant my server was re-rendering every 5 minutes even though content rarely changes. Now:
VaryByHeader = "hx-request" // ResponseCache
VaryByHeaderNames = new[] { "hx-request" } // OutputCache
Why this matters: HTMX requests include hx-request: true header. I return different responses:
Without VaryBy, the cache would return the wrong format. With VaryBy, I cache two versions of each page.
Example:
User requests /blog/my-post → Cache key: "blog/my-post:en:hx=false" → Full HTML cached
HTMX requests /blog/my-post → Cache key: "blog/my-post:en:hx=true" → Partial HTML cached
[ResponseCache(VaryByQueryKeys = new[] { nameof(slug), nameof(language) })]
[OutputCache(VaryByQueryKeys = new[] { nameof(slug), nameof(language) })]
Problem without this: /blog/my-post?language=fr would serve the English cached version.
With VaryByQueryKeys: Separate cache entries:
/blog/my-post?language=en → Cache key includes "en"/blog/my-post?language=fr → Cache key includes "fr"Real usage from my blog list:
[Route("blog")]
[ResponseCache(Duration = 300, VaryByHeader = "hx-request",
VaryByQueryKeys = new[] { "page", "pageSize", "startDate", "endDate", "language", "orderBy", "orderDir" })]
[OutputCache(Duration = 3600, VaryByHeaderNames = new[] { "hx-request" },
VaryByQueryKeys = new[] { "page", "pageSize", "startDate", "endDate", "language", "orderBy", "orderDir" })]
public async Task<IActionResult> Index(int page = 1, int pageSize = 20, /* ... */)
Cache explosion warning: Each unique combination of parameters = separate cache entry:
page=1&pageSize=20&language=en → One entrypage=2&pageSize=20&language=en → Another entrypage=1&pageSize=10&language=en → Another entryMitigation:
ResponseCache works out of the box, but for OutputCache you need setup:
// Program.cs
builder.Services.AddOutputCache(options =>
{
options.MaximumBodySize = 64 * 1024 * 1024; // 64 MB max response size
options.SizeLimit = 100 * 1024 * 1024; // 100 MB total cache size
});
var app = builder.Build();
app.UseOutputCache(); // Must be in middleware pipeline
OutputCache is skipped for:
Set-Cookie headerCache-Control: no-storeExample where I don't use it:
// Comment submission - authenticated, POST, and per-user
[HttpPost]
[Authorize]
public async Task<IActionResult> AddComment(CommentModel model)
{
// No caching attributes - this is user-specific and changes state
}
I use Prometheus metrics (exposed via my app) to track:
// Pseudo-code for metrics
cache_hits_total{cache="output"} 45230
cache_misses_total{cache="output"} 892
My cache hit rate: ~98% for blog posts (most traffic hits the same popular posts repeatedly).
Impact:
Sometimes I'm debugging and need fresh responses every time:
// During development, comment out caching
// [ResponseCache(Duration = 300, ...)]
// [OutputCache(Duration = 3600, ...)]
public async Task<IActionResult> Show(string slug, string language = "en")
Or use environment-specific settings:
#if DEBUG
// No caching in development
#else
[ResponseCache(Duration = 300, ...)]
[OutputCache(Duration = 3600, ...)]
#endif
Better approach: Use configuration:
[ResponseCache(Duration = responseCacheDuration, ...)]
where responseCacheDuration is 0 in development, 300 in production.
Pros:
Cons:
When I'd skip caching:
My verdict: For a blog with mostly-static content and high read/write ratio, dual caching is a huge win. I wouldn't use this on admin panels or dashboards with rapidly changing data.
These three are often confused. Here's how they differ and what I actually use in production.
// ViewData: string-keyed dictionary
ViewData["Title"] = "Blog";
ViewData["Categories"] = new List<string> { "ASP.NET", "C#" };
// ViewBag: dynamic wrapper around ViewData
ViewBag.Title = "Blog";
ViewBag.Categories = new List<string> { "ASP.NET", "C#" };
// TempData: survives one redirect (backed by session or cookie)
TempData["Message"] = "Post saved!";
return RedirectToAction("Index");
| Feature | ViewData | ViewBag | TempData |
|---|---|---|---|
| Type | ViewDataDictionary |
dynamic | ITempDataDictionary |
| Lifetime | Current request | Current request | One redirect |
| Key access | String keys | Property syntax | String keys |
| Type safety | None (casts needed) | None (dynamic) | None (casts needed) |
| Compile-time checking | No | No | No |
| Survives redirect | No | No | Yes |
In my blog, I use ViewBag exclusively for passing data from controllers to shared layout (analytics, categories, etc.):
// From BaseController.cs - runs before every action
public override async Task OnActionExecutionAsync(ActionExecutingContext filterContext,
ActionExecutionDelegate next)
{
logger.LogInformation("OnActionExecutionAsync");
if (!Request.IsHtmx())
{
// Analytics settings for layout
ViewBag.UmamiPath = AnalyticsSettings.UmamiPath;
ViewBag.UmamiWebsiteId = AnalyticsSettings.WebsiteId;
ViewBag.UmamiScript = AnalyticsSettings.UmamiScript;
}
logger.LogInformation("Adding categories to viewbag");
ViewBag.Categories = await GetCategories(); // Cached list
await base.OnActionExecutionAsync(filterContext, next);
}
Then in my layout (_Layout.cshtml):
@if (ViewBag.Categories is List<string> categories)
{
<nav>
@foreach (var cat in categories)
{
<a asp-controller="Blog" asp-action="Category" asp-route-category="@cat">@cat</a>
}
</nav>
}
@if (!string.IsNullOrEmpty(ViewBag.UmamiPath))
{
<script async src="@ViewBag.UmamiScript"
data-website-id="@ViewBag.UmamiWebsiteId"></script>
}
Why this pattern works:
Mistake I made early on: I was setting ViewBag.Categories in every single action method. DRY violation and easy to forget. Moving it to OnActionExecutionAsync in the base controller solved this.
// From BlogController.cs
[Route("category/{category}")]
public async Task<IActionResult> Category(string category, int page = 1, int pageSize = 10)
{
ViewBag.Category = category; // Used in view for heading
ViewBag.Title = category + " - Blog"; // Used in layout <title>
var posts = await blogViewService.GetPostsByCategory(category, page, pageSize);
// ... populate posts model ...
if (Request.IsHtmx()) return PartialView("_BlogSummaryList", posts);
return View("Index", posts);
}
In view:
@{
ViewData["Title"] = ViewBag.Title; // Standard MVC convention for <title>
}
<h1>Category: @ViewBag.Category</h1>
This is okay because:
Title and Category properties to every view modelAnti-pattern:
// DON'T DO THIS
ViewBag.User = new UserViewModel { Name = "Scott", IsAdmin = true };
ViewBag.Posts = new List<Post> { ... };
ViewBag.Metadata = new { Tags = new[] { "a", "b" }, Date = DateTime.Now };
Problems:
ViewBag.Usr fails at runtime)Better: Strongly-typed view models:
// DO THIS instead
public class BlogIndexViewModel : BaseViewModel
{
public string Category { get; set; }
public List<PostSummary> Posts { get; set; }
public PaginationInfo Pagination { get; set; }
}
public IActionResult Category(string category, int page = 1)
{
var model = new BlogIndexViewModel
{
Category = category,
Posts = await GetPosts(category, page),
// Inherited from BaseViewModel:
Authenticated = user.LoggedIn,
Name = user.Name,
AvatarUrl = user.AvatarUrl
};
return View("Index", model);
}
Standard TempData use case:
[HttpPost]
public IActionResult SavePost(PostModel model)
{
// Save post...
TempData["SuccessMessage"] = "Post saved successfully!";
return RedirectToAction("Index");
}
public IActionResult Index()
{
// TempData["SuccessMessage"] available here (consumed on read)
return View();
}
Why I don't use TempData in my blog:
[HttpPost]
public async Task<IActionResult> Submit(ContactViewModel model)
{
if (!ModelState.IsValid)
return PartialView("_ContactForm", model); // Show errors inline
await sender.SendEmailAsync(contactModel);
// Return success view directly (no redirect)
return PartialView("_Response", new ContactViewModel
{
Email = model.Email,
Name = model.Name,
Comment = "Message sent!"
});
}
When TempData makes sense:
TempData gotcha: By default backed by cookies (since ASP.NET Core 2.0). If you put large objects in TempData, you're bloating the cookie sent with every request. For large state, use Session with a backing store or DB.
flowchart TD
A[Need to pass data to view?] --> B{What kind?}
B -->|Global layout data| C[ViewBag in BaseController]
B -->|Simple page-specific| D[ViewBag in action]
B -->|Complex model| E[Strongly-typed ViewModel]
B -->|Survive redirect| F{Using HTMX?}
F -->|Yes| G[Return partial with message]
F -->|No| H[TempData for flash message]
My rules:
Which state holder to use?
flowchart TD
A[Start Wizard] --> B{Short-lived?\nSingle browser?}
B -- Yes --> S[Session/TempData]
B -- No/Complex --> D[DB + key in route]
S --> PRG[Use PRG between steps]
D --> PRG
Example (DB + route key):
app.MapPost("/wizard/{id}", async (AppDb db, Guid id, StepInput input) =>
{
var flow = await db.Flows.FindAsync(id) ?? new Flow(id);
flow.Apply(input);
await db.SaveChangesAsync();
return Results.Redirect($"/wizard/{id}/next");
});
TempData["Flash"] = "Profile saved";
return RedirectToAction("Index");
Razor view:
@if (TempData["Flash"] is string flash) {
<div class="alert alert-info">@flash</div>
}
flowchart LR U[User] -- cart-id cookie --> S[Server] S --> DB[(Cart Table)] S <--> Cache[Distributed Cache]
Secure, HttpOnly, SameSite, IsEssential (if required by consent/functional need).classDiagram
class Items {
+Per-request only
+Great for middleware->endpoint handoff
}
class QueryRoute {
+Explicit, linkable
-User-controlled
}
class Headers {
+Tracing, idempotency
-Noisy, untrusted
}
class Cookies {
+Persist small prefs
-Size, perf, consent
}
class TempData {
+One-redirect messages
-Ephemeral
}
class Session {
+Conversational state
-Scaling complexity
}
class MemoryCache {
+Fast, in-proc
-Not shared across instances
}
class DistributedCache {
+Shared across servers
-Serialization, ops
}
class AuthCookieClaims {
+Identity, roles
-Don’t overstuff
}
class JWT {
+Stateless, cross-domain
-Revocation/rotation
}
class DB {
+Durable, authoritative
-Latency, complexity
}
Quick picks:
All three stacks sit on the same primitives (HttpContext, model binding, auth, data protection). The examples above show that the APIs differ mostly in ergonomics:
Results.*.They all share the same state mechanisms discussed here.
After showing you all these options, here's my honest assessment of what works in production for my blog platform.
| Mechanism | Frequency | Use Cases | Satisfaction |
|---|---|---|---|
| IMemoryCache | Very High | Categories, metrics, translation tasks | ⭐⭐⭐⭐⭐ Essential |
| OutputCache | High | Rendered blog posts, lists | ⭐⭐⭐⭐⭐ Huge perf win |
| ResponseCache | High | HTTP caching headers | ⭐⭐⭐⭐ Works with OutputCache |
| ViewBag | Medium | Analytics settings, page titles | ⭐⭐⭐ OK for simple stuff |
| Auth Claims | Medium | User identity, admin flag | ⭐⭐⭐⭐⭐ Right tool for auth |
| Route/Query | Medium | Pagination, filtering, slugs | ⭐⭐⭐⭐ Stateless and linkable |
| Database | Medium | Blog posts, comments, state | ⭐⭐⭐⭐⭐ Source of truth |
| Cookies | Low | User preferences (future) | ⭐⭐⭐ Haven't needed yet |
| Session | Never | - | ❌ No distributed store setup |
| TempData | Never | - | ❌ HTMX eliminates need |
| HttpContext.Items | Never | - | ❌ Haven't had use case |
| IDistributedCache | Never | - | ❌ Single server (for now) |
What I use it for:
Pros in practice:
Cons I've hit:
Scaling limit: If I get to multiple servers, I'd need IDistributedCache (Redis) for shared state. For now, single server + memory cache is perfect.
What I use it for:
Pros in practice:
Cons I've hit:
Best for: Read-heavy applications with mostly static content. Not good for personalized or real-time data.
What I use it for:
Pros in practice:
Cons I've hit:
Rule I follow: ViewBag for simple scalars only. Complex objects go in ViewModels.
What I use it for:
sub claim against config)Pros in practice:
User.Claims everywhereCons I've hit:
Best practice: Keep claims minimal and stable. Don't put frequently-changing data in claims.
Session (never used):
TempData (never used):
HttpContext.Items (never used):
IDistributedCache (never used):
Phase 1 (initial): No caching at all. Every request hit the database and rendered Markdown. Worked fine for low traffic.
Phase 2 (first optimization): Added IMemoryCache for categories. Saw immediate DB load reduction. Kept it simple: 30 min TTL, no fancy logic.
Phase 3 (scaling up): Added OutputCache for blog posts when traffic spiked. Massive performance improvement. Initial mistake: cached for 5 minutes only. Increased to 1 hour after monitoring showed content rarely changes.
Phase 4 (observability): Added SerilogTracing to metrics cache. Discovered cache misses were high due to date formatting in keys. Fixed key format to yyyyMMdd instead of full timestamps. Hit rate went from 60% to 95%.
Phase 5 (HTMX integration): Added VaryByHeader for hx-request. Initially forgot this and served full pages to HTMX requests. Debugging nightmare until I figured it out.
Current state: Happy with the stack. IMemoryCache + OutputCache + ResponseCache handle 98% of my state management needs. Database for durable state. Auth claims for identity. That's it.
Start here:
Add if needed: 6. Session (only if you must have server-side conversational state) 7. IDistributedCache (only when you scale to multiple servers) 8. Cookies (for client-side preferences, consent)
Avoid:
Key lessons:
State in web apps isn't one size fits all. Choose the lightest option that meets your needs, prefer stateless patterns when you can, and be explicit about security and lifecycle.
If you want to go deeper on how these pieces flow through the pipeline, see my series starting with Part 1: Overview and Foundation and especially the middleware and routing parts.
Happy building.
These examples deepen the earlier sections with production‑grade details you can paste into net9 minimal templates, MVC, or Razor Pages.
using Microsoft.AspNetCore.DataProtection;
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddDataProtection();
var app = builder.Build();
app.MapPost("/prefs/secure/{value}", (HttpContext ctx, string value, IDataProtectionProvider dp) =>
{
var protector = dp.CreateProtector("prefs.theme");
var protectedValue = protector.Protect(value);
ctx.Response.Cookies.Append("pref.theme.p", protectedValue, new CookieOptions
{
HttpOnly = true,
Secure = true,
SameSite = SameSiteMode.Lax,
Expires = DateTimeOffset.UtcNow.AddYears(1)
});
return Results.Ok();
});
app.MapGet("/prefs/secure", (HttpContext ctx, IDataProtectionProvider dp) =>
{
if (ctx.Request.Cookies.TryGetValue("pref.theme.p", out var v))
{
var protector = dp.CreateProtector("prefs.theme");
return Results.Text(protector.Unprotect(v));
}
return Results.NotFound();
});
Tip: In multi‑node deployments, persist Data Protection keys (e.g., to a shared file system, Redis, or Azure Key Vault) so cookies can be read across instances.
var builder = WebApplication.CreateBuilder(args);
builder.Services.AddControllersWithViews();
builder.Services.AddRazorPages();
builder.Services.AddAntiforgery(o => o.HeaderName = "X-CSRF-TOKEN");
var app = builder.Build();
app.MapGet("/antiforgery/token", (IAntiforgery af, HttpContext ctx) =>
{
var tokens = af.GetAndStoreTokens(ctx);
return Results.Json(new { token = tokens.RequestToken });
});
app.MapPost("/submit", (HttpContext ctx) => Results.Ok("posted"))
.AddEndpointFilter(async (efiContext, next) =>
{
var af = efiContext.HttpContext.RequestServices.GetRequiredService<IAntiforgery>();
await af.ValidateRequestAsync(efiContext.HttpContext);
return await next(efiContext);
});
app.MapControllers();
app.MapRazorPages();
[ValidateAntiForgeryToken] and use @Html.AntiForgeryToken() in forms.asp-antiforgery="true" if needed.IAntiforgery as shown.builder.Services.AddStackExchangeRedisCache(o => o.Configuration = "localhost:6379");
builder.Services.AddSession(o =>
{
o.IdleTimeout = TimeSpan.FromMinutes(20); // sliding
o.IOTimeout = TimeSpan.FromSeconds(2);
o.Cookie.HttpOnly = true;
o.Cookie.IsEssential = true;
});
var app = builder.Build();
app.UseSession();
Store small, compressible data only. Persist real carts/orders to a DB.
builder.Services.AddMemoryCache();
app.MapGet("/fx/{pair}", (IMemoryCache cache, string pair) =>
{
var key = $"fx:{pair.ToLowerInvariant()}";
return Results.Ok(cache.GetOrCreate(key, entry =>
{
entry.AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(10);
entry.SlidingExpiration = TimeSpan.FromMinutes(2);
entry.Size = 1; // enable size-based eviction if configured
entry.RegisterPostEvictionCallback((k, v, reason, state) =>
{
Console.WriteLine($"Evicted {k} because {reason}");
});
return 0.92m; // fetch from external service in real life
}));
});
builder.Services.AddStackExchangeRedisCache(o => o.Configuration = "localhost:6379");
app.MapGet("/feature/{name}", async (IDistributedCache cache, string name) =>
{
var key = $"feat:{name}";
var cached = await cache.GetStringAsync(key);
if (cached is not null) return Results.Text(cached);
// Lock key to prevent thundering herd (very simple approach)
var lockKey = key + ":lock";
var gotLock = await cache.SetStringAsync(lockKey, "1", new DistributedCacheEntryOptions
{
AbsoluteExpirationRelativeToNow = TimeSpan.FromSeconds(5)
});
try
{
cached = await cache.GetStringAsync(key);
if (cached is null)
{
var computed = "on"; // expensive work
var rnd = Random.Shared.Next(0, 15); // jitter
await cache.SetStringAsync(key, computed, new DistributedCacheEntryOptions
{
AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(5).Add(TimeSpan.FromSeconds(rnd))
});
cached = computed;
}
}
finally
{
await cache.RemoveAsync(lockKey);
}
return Results.Text(cached);
});
For robust locking, prefer Redis primitives (SET NX EX) via StackExchange.Redis.
using System.IdentityModel.Tokens.Jwt;
using Microsoft.IdentityModel.Tokens;
using System.Security.Claims;
var key = new SymmetricSecurityKey(System.Text.Encoding.UTF8.GetBytes("super-secret-key-please-rotate"));
var creds = new SigningCredentials(key, SecurityAlgorithms.HmacSha256);
builder.Services.AddAuthentication("Bearer")
.AddJwtBearer("Bearer", o =>
{
o.TokenValidationParameters = new TokenValidationParameters
{
ValidateIssuer = false,
ValidateAudience = false,
IssuerSigningKey = key,
ValidateIssuerSigningKey = true,
ValidateLifetime = true
};
});
var app = builder.Build();
app.UseAuthentication();
app.UseAuthorization();
app.MapPost("/token", () =>
{
var claims = new[] { new Claim(ClaimTypes.Name, "alice") };
var jwt = new JwtSecurityToken(claims: claims, expires: DateTime.UtcNow.AddMinutes(30), signingCredentials: creds);
var token = new JwtSecurityTokenHandler().WriteToken(jwt);
return Results.Json(new { access_token = token });
});
app.MapGet("/who", [Microsoft.AspNetCore.Authorization.Authorize] () => "ok");
record Todo(int Id, string Title, string Version);
var store = new Dictionary<int, Todo> { [1] = new(1, "Ship", "v1") };
app.MapGet("/todo/{id:int}", (int id, HttpContext ctx) =>
{
if (!store.TryGetValue(id, out var t)) return Results.NotFound();
ctx.Response.Headers.ETag = t.Version;
return Results.Json(t);
});
app.MapPut("/todo/{id:int}", (int id, HttpContext ctx, Todo input) =>
{
if (!store.TryGetValue(id, out var current)) return Results.NotFound();
var ifMatch = ctx.Request.Headers["If-Match"].ToString();
if (string.IsNullOrEmpty(ifMatch) || ifMatch != current.Version)
return Results.StatusCode(StatusCodes.Status412PreconditionFailed);
var next = current with { Title = input.Title, Version = $"v{DateTime.UtcNow.Ticks}" };
store[id] = next;
ctx.Response.Headers.ETag = next.Version;
return Results.Ok(next);
});
public static class TempDataJsonExtensions
{
public static void Put<T>(this ITempDataDictionary tempData, string key, T value)
=> tempData[key] = System.Text.Json.JsonSerializer.Serialize(value);
public static T? Get<T>(this ITempDataDictionary tempData, string key)
=> tempData.TryGetValue(key, out var o) && o is string s
? System.Text.Json.JsonSerializer.Deserialize<T>(s)
: default;
}
// Usage in MVC action
TempData.Put("WizardState", new { Step = 2, Name = "Alice" });
var state = TempData.Get<dynamic>("WizardState");
public class Product
{
public int Id { get; set; }
public string Name { get; set; } = string.Empty;
[Timestamp] public byte[] RowVersion { get; set; } = default!;
}
// On update
try
{
await db.SaveChangesAsync();
}
catch (DbUpdateConcurrencyException)
{
return Results.StatusCode(StatusCodes.Status412PreconditionFailed);
}
app.MapPost("/promote", async (HttpContext ctx) =>
{
var u = ctx.User;
var claims = u.Claims.ToList();
claims.Add(new Claim(ClaimTypes.Role, "Editor"));
var id = new ClaimsIdentity(claims, "Cookies");
await ctx.SignInAsync("Cookies", new ClaimsPrincipal(id));
return Results.Ok();
});
That should cover the gaps: stronger security defaults, multi‑node readiness, and real‑world patterns for caches, tokens, and conditional requests.
HttpContext.Items is a per-request bag (IDictionary<object, object?>) that lives only for the lifetime of a single request. It’s perfect for passing computed values from middleware/filters to your endpoints, controllers, and Razor Pages handlers without touching global state or long‑lived stores.
Because Items uses object keys, prefer private static object keys or a dedicated key type to avoid name collisions.
public static class ItemKeys
{
public static readonly object TenantId = new();
public static readonly object UserLocale = new();
public static readonly object PerRequestCache = new();
}
Or create a typed wrapper with extensions:
public static class HttpContextItemsExtensions
{
public static void Set<T>(this HttpContext ctx, object key, T value)
=> ctx.Items[key] = value!;
public static T? Get<T>(this HttpContext ctx, object key)
=> ctx.Items.TryGetValue(key, out var v) ? (T?)v : default;
public static T GetOrCreate<T>(this HttpContext ctx, object key, Func<T> factory)
{
if (ctx.Items.TryGetValue(key, out var existing) && existing is T typed)
return typed;
var created = factory();
ctx.Items[key] = created!;
return created;
}
}
// Program.cs
app.Use(async (ctx, next) =>
{
var tenant = ctx.Request.Headers["X-TenantId"].FirstOrDefault() ?? "public";
ctx.Set(ItemKeys.TenantId, tenant); // using extension above
// Per-request cache holder (optional)
ctx.Set(ItemKeys.PerRequestCache, new Dictionary<string, object?>());
await next(ctx);
});
// Minimal API
app.MapGet("/whoami", (HttpContext ctx) => new
{
Tenant = ctx.Get<string>(ItemKeys.TenantId),
});
// MVC Controller
public IActionResult WhoAmI()
=> Json(new { Tenant = HttpContext.Get<string>(ItemKeys.TenantId) });
// Razor Page handler
public IActionResult OnGet()
=> new JsonResult(new { Tenant = HttpContext.Get<string>(ItemKeys.TenantId) });
Use Items as a tiny cache so repeated reads within the same request don’t re-hit databases/services.
public static class PerRequestCacheExtensions
{
public static async Task<T> GetOrAddAsync<T>(this HttpContext ctx, string key, Func<Task<T>> factory)
{
var bag = ctx.Get<Dictionary<string, object?>>(ItemKeys.PerRequestCache)
?? ctx.GetOrCreate(ItemKeys.PerRequestCache, () => new Dictionary<string, object?>());
if (bag.TryGetValue(key, out var val) && val is T hit)
return hit;
var created = await factory();
bag[key] = created!;
return created;
}
}
// Usage in endpoint
app.MapGet("/profile", async (HttpContext ctx, IUserRepo repo) =>
{
var userId = ctx.User.Identity?.Name ?? "anon";
var profile = await ctx.GetOrAddAsync($"profile:{userId}", () => repo.LoadAsync(userId));
return Results.Json(profile);
});
Notes:
public class TenantFilter : IAsyncResourceFilter
{
public async Task OnResourceExecutionAsync(ResourceExecutingContext context, ResourceExecutionDelegate next)
{
var tenant = context.HttpContext.Request.Headers["X-TenantId"].FirstOrDefault() ?? "public";
context.HttpContext.Set(ItemKeys.TenantId, tenant);
await next();
}
}
// Register filter globally
services.AddControllersWithViews(o => o.Filters.Add<TenantFilter>());
Compute once, then read in logging scopes or middleware.
app.Use(async (ctx, next) =>
{
var correlationId = ctx.Request.Headers["X-Correlation-Id"].FirstOrDefault() ?? Guid.NewGuid().ToString("n");
ctx.Items["CorrelationId"] = correlationId; // string key acceptable for app-local use
using (logger.BeginScope(new { CorrelationId = correlationId }))
{
await next(ctx);
}
});
Quick rule: If it’s computed during this request and read within this request by your own code, Items is ideal.
© 2025 Scott Galloway — Unlicense — All content and source code on this site is free to use, copy, modify, and sell.