Migrating Mvc Session State

ASP.NET MVC Session and State Migration

Overview

Migrate session state, TempData, and application-level state from ASP.NET Framework to ASP.NET Core. ASP.NET Core has no in-process session by default — session requires explicit opt-in via AddSession() plus a distributed cache provider. Getting this wrong causes silent data loss at scale or on app restart.

> Adapter overlay: If the aspnet-system-web-adapters skill is loaded, its guidance takes precedence for HttpSessionState replacement during scaffold and migrate phases — the adapter provides a session shim that defers full migration. TempData and Application State are not covered by adapters; migrate them directly using this skill.

> Related skill: migrating-global-asax covers Session_Start/Session_End event migration to session middleware registration.

Workflow

Track progress across these steps:

Migration Progress:
- [ ] Step 1: Audit state usage across project
- [ ] Step 2: Register session services and middleware
- [ ] Step 3: Migrate HttpSessionState to ISession
- [ ] Step 4: Migrate TempData provider
- [ ] Step 5: Replace Application state and HttpRuntime.Cache
- [ ] Step 6: Replace static state with DI singletons
- [ ] Step 7: Verify serialization and data round-trips

Step 1: Audit State Usage

Search the codebase for all state-related patterns. Identify which migration paths apply:

| Pattern to find | Migration path |

|---|---|

| Session[ , HttpContext.Session | Session State (Step 3) |

| TempData[ , ITempDataProvider | TempData (Step 4) |

| HttpContext.Application[ , HttpApplicationState | Application State (Step 5) |

| HttpRuntime.Cache , HttpContext.Cache | Application State (Step 5) |

| static fields holding request/user state | Static State (Step 6) |

Skip steps that have no matching patterns. If the adapter overlay applies, defer Step 3 and proceed with Steps 4–6.

Step 2: Register Session Services and Middleware

Skip this step if no Session[ usage was found in Step 1 or if the adapter overlay handles session.

Add session services and a distributed cache in Program.cs:

Before (Framework — implicit, no registration needed):

<!-- web.config — session was on by default -->
<sessionState mode="InProc" timeout="20" />

After (Core — explicit opt-in required):

// Program.cs — service registration
builder.Services.AddDistributedMemoryCache(); // Dev only — replace for production
builder.Services.AddSession(options =>
{
    options.IdleTimeout = TimeSpan.FromMinutes(20);
    options.Cookie.HttpOnly = true;
    options.Cookie.IsEssential = true;
});

// Middleware pipeline — order matters: after routing, before endpoints
app.UseSession();

⚠️ Data loss risk: AddDistributedMemoryCache() stores data in-process and loses everything on restart. For production, replace with Redis or SQL Server:

// Redis (recommended for multi-instance deployments)
builder.Services.AddStackExchangeRedisCache(options =>
{
    options.Configuration = builder.Configuration.GetConnectionString("Redis");
});

// SQL Server (when Redis is unavailable)
builder.Services.AddDistributedSqlServerCache(options =>
{
    options.ConnectionString = builder.Configuration.GetConnectionString("SessionDb");
    options.SchemaName = "dbo";
    options.TableName = "Sessions";
});

Step 3: Migrate HttpSessionState to ISession

ASP.NET Core ISession has a fundamentally different API. Session values are byte arrays — there is no automatic object serialization.

Before (Framework):

// Reading — returns object, cast required
var cart = (ShoppingCart)Session["Cart"];
var name = Session["UserName"] as string;

// Writing — any serializable object
Session["Cart"] = cart;
Session["UserName"] = "Alice";

// Removing
Session.Remove("Cart");
Session.Abandon();

After (Core):

// Reading — use typed extension methods
var cart = HttpContext.Session.Get<ShoppingCart>("Cart");
var name = HttpContext.Session.GetString("UserName");

// Writing — must serialize explicitly
HttpContext.Session.Set("Cart", cart);
HttpContext.Session.SetString("UserName", "Alice");

// Removing
HttpContext.Session.Remove("Cart");
HttpContext.Session.Clear();

The built-in ISession only provides GetString/SetString and GetInt32/SetInt32. For complex objects, add a JSON extension helper:

public static class SessionExtensions
{
    public static void Set<T>(this ISession session, string key, T value)
    {
        session.SetString(key, JsonSerializer.Serialize(value));
    }

    public static T? Get<T>(this ISession session, string key)
    {
        var value = session.GetString(key);
        return value is null ? default : JsonSerializer.Deserialize<T>(value);
    }
}

Place this in a shared location (e.g., Extensions/SessionExtensions.cs). All session reads and writes for complex types must use these helpers — without them, Get/Set of objects silently fails.

Key API differences:

| Framework | Core | Notes |

|---|---|---|

| Session["key"] | HttpContext.Session.GetString("key") | No indexer in Core |

| Session["key"] = obj | HttpContext.Session.Set("key", obj) | Requires JSON helper |

| Session.Abandon() | HttpContext.Session.Clear() | Clear removes data but keeps session ID |

| Session.SessionID | HttpContext.Session.Id | Property name change |

| Session.Count | HttpContext.Session.Keys.Count() | Must enumerate keys |

Step 4: Migrate TempData Provider

TempData API surface is compatible between Framework and Core, but the backing store changed. Framework uses session-based TempData by default; Core uses cookie-based TempData.

Cookie TempData (Core default) — no code changes needed if data is small:

// TempData usage stays the same
TempData["Message"] = "Item saved successfully";
var msg = TempData["Message"] as string;

⚠️ Silent truncation risk: Cookie-based TempData is limited to ~4096 bytes total. Large objects stored in TempData will be silently truncated or fail. If TempData stored complex objects in Framework, switch to session-based TempData provider:

// Program.cs — switch to session-based TempData (requires AddSession)
builder.Services.AddControllersWithViews()
    .AddSessionStateTempDataProvider();

TempData in WebAPI controllers: TempData never existed in WebAPI. If Framework code used workarounds (e.g., storing values between redirects in a Web API context), replace with query string parameters, response headers, or a distributed cache lookup.

Step 5: Replace Application State and HttpRuntime.Cache

ASP.NET Core has no HttpApplicationState or HttpRuntime.Cache. Replace with dependency-injected services.

Application State → DI Singleton

Before (Framework):

// Writing — typically in Global.asax Application_Start
HttpContext.Application["SiteSettings"] = LoadSettings();
HttpContext.Application.Lock();
HttpContext.Application["VisitorCount"] = (int)HttpContext.Application["VisitorCount"] + 1;
HttpContext.Application.UnLock();

// Reading — anywhere
var settings = HttpContext.Application["SiteSettings"] as SiteSettings;

After (Core):

// Define a service to hold shared state
public class AppStateService
{
    private int _visitorCount;
    public SiteSettings SiteSettings { get; set; } = new();
    public int IncrementVisitors() => Interlocked.Increment(ref _visitorCount);
}

// Register as singleton in Program.cs
builder.Services.AddSingleton<AppStateService>();

// Inject where needed
public class HomeController : Controller
{
    private readonly AppStateService _appState;
    public HomeController(AppStateService appState) => _appState = appState;
}

Application state with Lock()/UnLock() patterns needs thread-safe replacements — use Interlocked, ConcurrentDictionary, or lock in the singleton service. The Lock()/UnLock() API does not exist in Core.

HttpRuntime.Cache → IMemoryCache

Before (Framework):

HttpRuntime.Cache.Insert("Products", products, null,
    DateTime.Now.AddMinutes(30), Cache.NoSlidingExpiration);
var cached = HttpRuntime.Cache["Products"] as List<Product>;

After (Core):

// Program.cs
builder.Services.AddMemoryCache();

// In controller or service — inject IMemoryCache
public class ProductService
{
    private readonly IMemoryCache _cache;
    public ProductService(IMemoryCache cache) => _cache = cache;

    public List<Product> GetProducts()
    {
        return _cache.GetOrCreate("Products", entry =>
        {
            entry.AbsoluteExpirationRelativeToNow = TimeSpan.FromMinutes(30);
            return LoadProductsFromDb();
        })!;
    }
}

For multi-instance deployments, use IDistributedCache instead of IMemoryCache to avoid cache inconsistencies across instances.

Step 6: Replace Static State with DI Singletons

Static fields holding per-application state are a common pattern in Framework apps. These work in single-instance deployments but break in multi-instance or container environments.

Before (Framework):

public static class AppConfig
{
    public static string ConnectionString { get; set; }
    public static Dictionary<string, object> Settings = new();
}

After (Core):

public class AppConfig
{
    public string ConnectionString { get; set; } = string.Empty;
    public Dictionary<string, object> Settings { get; } = new();
}

// Program.cs
builder.Services.AddSingleton<AppConfig>();

Convert all static field access to constructor injection. If static access is needed in non-DI contexts (e.g., extension methods), expose the service through IServiceProvider at the call site rather than reverting to static state.

Step 7: Verify Serialization and Data Round-Trips

After migrating, verify that data survives a full write-read cycle:

• Session data — Write a complex object to session, read it back, and confirm all properties match. Test after app restart to confirm the distributed cache retains data (in-memory cache will not).
• TempData — Store a value, perform a redirect, and verify the value is available on the destination page. Test with objects that approach the 4096-byte cookie limit if using cookie-based TempData.
• Cache entries — Verify expiration behavior matches the original web.config settings.

Build the project and run existing tests to confirm no regressions.

Troubleshooting

| Symptom | Cause | Fix |

|---|---|---|

| Session is always null | Missing app.UseSession() or AddSession() | Add both to Program.cs |

| Session data lost on restart | Using AddDistributedMemoryCache() in production | Switch to Redis or SQL Server cache |

| TempData silently empty after redirect | Cookie exceeds 4096 bytes | Switch to AddSessionStateTempDataProvider() |

| HttpContext.Application compile error | No equivalent in Core | Replace with DI singleton (Step 5) |

| HttpRuntime.Cache compile error | No equivalent in Core | Replace with IMemoryCache (Step 5) |

| Thread-safety issues with singleton state | Missing synchronization | Use ConcurrentDictionary or Interlocked |

Success Criteria

• Session services registered with AddSession() and UseSession() in Program.cs
• Distributed cache provider configured (not in-memory for production)
• All Session["key"] indexer access replaced with ISession extension methods
• JSON serialization helper created for complex session objects
• TempData provider chosen and configured based on data size requirements
• HttpContext.Application replaced with DI singleton services
• HttpRuntime.Cache replaced with IMemoryCache or IDistributedCache
• Static state fields converted to singleton services with DI
• No System.Web session or cache references remain
• Project builds without errors