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Middleware: New Use overload

  • Article

A new overload of app.Use has been introduced. If you call app.Use but never call the next middleware, you'll now get compiler error CS0121:

The call is ambiguous between the following methods or properties: 'UseExtensions.Use(IApplicationBuilder, Func<HttpContext, Func, Task>)' and 'UseExtensions.Use(IApplicationBuilder, Func<HttpContext, RequestDelegate, Task>)'

To resolve the error, use app.Run instead of app.Use.

For discussion, see GitHub issue dotnet/aspnetcore#32020.

Version introduced

ASP.NET Core 6.0

Old behavior

app.Use(async (context, next) =>
{
    await next();
});

or

app.Use(async (context, next) =>
{
    await SomeAsyncWork();
    // next not called...
});

New behavior

You can now pass context to the next delegate:

app.Use(async (context, next) =>
{
    await next(context);
});

Use app.Run when your middleware never calls next:

app.Run(async (context) =>
{
    await SomeAsyncWork();
    // next never called
});

Reason for change

The previous Use method allocates two objects per request. The new overload avoids these allocations with a small change to how you invoke the next middleware.

If you get a compile error, it means you are calling app.Use without using the next delegate. Switch to app.Run to fix the error.

Affected APIs

None.