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

[BrennanConroy]

Announcement: aspnet/Announcements#461

[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

---

Issue metadata

• Issue type: breaking-change

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Thank you for contacting us. Due to a lack of activity on this discussion issue we're closing it in an effort to keep our backlog clean. If you believe there is a concern related to the ASP.NET Core framework, which hasn't been addressed yet, please file a new issue.

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[ghost]

Thank you for contacting us. Due to a lack of activity on this discussion issue we're closing it in an effort to keep our backlog clean. If you believe there is a concern related to the ASP.NET Core framework, which hasn't been addressed yet, please file a new issue.

This issue will be locked after 30 more days of inactivity. If you still wish to discuss this subject after then, please create a new issue!