LambdaImprovements.cs

using System;

var app = new EndpointConventionBuilder();

// Traditional way of defining a function with an attribute
[HttpGet("/")] int GetAnswer() => 42;
app.MapAction((Func<int>)GetAnswer);

// Now, we can remove the type cast:
app.MapAction(GetAnswer);

// We can even add attributes directly to lambdas:
app.MapAction([HttpGet("/")] () => 42);

// ===
// Simulate presence of ASP.NET:

[AttributeUsage(AttributeTargets.Method)]  
public class HttpGetAttribute : Attribute  
{  
    public HttpGetAttribute(string _) { /* ... */ }
} 

interface IEndpointConventionBuilder { }

class EndpointConventionBuilder : IEndpointConventionBuilder { }

static class EndpointConventionBuilderExtensions
{
    public static void MapAction(this IEndpointConventionBuilder builder, Delegate action) { }
}