Minmal .NET API - Delegate 'RequestDelegate' does not take 0 arguments

[mindingdata]

I'm trying to test out creating a minimal .NET Core API, with a very simple structure. e.g. :

using Microsoft.AspNetCore.Builder;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapGet("/hello", () => "Hello, World!");

app.Run();

I seem to get the following error :

Delegate 'RequestDelegate' does not take 0 arguments

I checked for other issues inside this repo, and the only similar mention was when the cast was removed from the delegate here : #33393

Because previously you had to cast to a Func like so (And this does get my code compiling without errors).

app.MapGet("/", (Func<string>)(() => "Hello World!"));

If I run my code using dotnet run, the code actually runs fine and generates a response. So I'm thinking it could be more about the tooling I'm using, but I'm not sure where.

I have the latest .NET Core 6 - Preview 6 SDK :

.NET SDK (reflecting any global.json):
 Version:   6.0.100-preview.6.21355.2
 Commit:    7f8e0d76c0

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19042
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\6.0.100-preview.6.21355.2\

Host (useful for support):
  Version: 6.0.0-preview.6.21352.12
  Commit:  770d630b28

My VS Code version is :

Version: 1.58.2 (system setup)
Commit: c3f126316369cd610563c75b1b1725e0679adfb3
Date: 2021-07-14T22:10:15.214Z
Electron: 12.0.13
Chrome: 89.0.4389.128
Node.js: 14.16.0
V8: 8.9.255.25-electron.0
OS: Windows_NT x64 10.0.19042

My VS Code C# extension version is 1.23.13

My Visual Studio 2019 is version 16.10.3 (But not preview, which could be the issue)

So again :

• Code will build fine with dotnet build
• Code will run find using dotnet run OR running via Visual Studio
• Code will display an output warning/error in both Visual Studio and VS Code that Delegate 'RequestDelegate' does not take 0 arguments

[DamianEdwards]

Indeed I think the issue is your tooling has an older version of the compiler which doesn't have the improvements to delegate casting. If you try it in the VS2019 preview or VS2022 preview it should work.

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