New Use middleware overload, can cause compiler errors if you don't call next

[BrennanConroy]

[New Use middleware overload, can cause compiler errors if you don't call next]

A new overload of app.Use has been introduced. If you were previously using app.Use but never calling the next middleware, you will now get a compiler error (CS0121). If that's the case you should use app.Run instead of app.Use.

Version introduced

ASP.NET Core 6.0-preview4

Old behavior

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

or

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

New behavior

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

Notice that you can now pass context to the next delegate.

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

Use app.Run when your middleware never calls next, otherwise you will get a compile 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>)'

Reason for change

The previous Use method would allocate 2 objects per request. The new overload avoids these allocations with a small change to how you invoke the next middleware.

Recommended action

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.

Category

ASP.NET

Affected APIs

Not detectable via API analysis

Discussion issue dotnet/aspnetcore#32020

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Issue metadata

• Issue type: breaking-change