This repository contains a minimal ASP.NET Core example implementing a "lazy" distributed session middleware that avoids contacting the remote distributed cache unless session data actually exists.

• Only create/store a session id and remote data when values are written.
• Do not query the distributed cache for reads if there was no session cookie on the incoming request.
• Remove session cookie and remote entry when session data is cleared.
• Reuse existing ASP.NET Core primitives where possible (ISession, ISessionFeature, SessionOptions.Cookie.Build).
• Reduce allocations where straightforward by using ValueTask for internal helpers.

• src/CustomSession/LazySession.cs — ISession implementation that defers loading from IDistributedCache when no cookie exists and only writes to cache when modified.
• src/CustomSession/LazySessionMiddleware.cs — middleware that installs an ISessionFeature backed by LazySession, commits changes at the end of the request, and manages the session cookie using SessionOptions.Cookie.Build(HttpContext).
• src/CustomSession/SessionExtensions.cs — UseLazyDistributedSession() extension to register the middleware.
• src/CustomSession/ValueTaskExtensions.cs — small helper utilities for working with ValueTask results.
• src/CustomSession/Program.cs — minimal example app showing how to register the distributed cache and configure SessionOptions.
• src/CustomSession.Tests — xUnit tests that mock IDistributedCache and verify the middleware behavior (no cache calls when no cookie and no access; store when setting values; remove and delete cookie when clearing a session that had a cookie).

• Lazy load behavior: when the request has no session cookie, the session starts empty and never queries the distributed cache. If the application writes to the session, a session id is generated and IDistributedCache.SetAsync is called on commit.
• Cookie management: when the session ends with data, we append a cookie produced by SessionOptions.Cookie.Build(HttpContext). If the request started with a cookie but the session ends empty (cleared or no values), the middleware removes the remote entry and deletes the cookie using the same built options (so path/SameSite/etc. match).
• ValueTask usage: internal helpers use ValueTask to reduce allocations when operations complete synchronously. Public ISession methods still expose Task per interface and convert via AsTask().

1. In Program.cs configure a distributed cache and session options:

builder.Services.AddDistributedMemoryCache(); // or Redis, SQL, etc.
builder.Services.Configure<SessionOptions>(opts => {
    opts.Cookie.Name = ".AspNetCore.Session";
    opts.IdleTimeout = TimeSpan.FromMinutes(20);
});

app.UseLazyDistributedSession();

2. Use HttpContext.Session as usual inside request handlers. The middleware will ensure the session is only materialized in the remote store if you write values.

Run tests from the repository root:

dotnet test src/CustomSession/CustomSession.sln

The tests mock IDistributedCache to verify the following scenarios:

• No cookie and no session access -> no cache calls.
• Setting a session value -> SetAsync called and Set-Cookie header produced.
• Request with incoming cookie, then clearing session -> RemoveAsync called and cookie deleted.

• Replace AddDistributedMemoryCache() with AddStackExchangeRedisCache(...) or a SQL implementation as appropriate.
• The middleware uses IDistributedCache so any registered implementation will be used.

• Add more tests verifying expiry and cookie attributes.
• Support ISession concurrency semantics more closely to the reference implementation (locking/refresh semantics) if needed.
• Consider exposing configuration options for cookie naming/creation behavior via options on the middleware.