Document validation support for minimal APIs

[captainsafia]

In .NET 10 Preivew 3, we're introducing support for minimal API validation support.

I've created a sample repo and some notes on the implementation to serve as source material here: https://github.com/captainsafia/minapi-validation-support.

One thing to note is that this feature has a user-facing component for developers who are building minimal APIs and a framework component for developers who are building frameworks or their own validation libraries.

I think our first priority is developers building APIs since that's where the biggest product gap it exists.

[guardrex]

@captainsafia ... I'll move this one as well. Dan would like to track each preview's work items on one issue for visibility to make sure nothing falls through the cracks.

[captainsafia]

@guardrex Sure -- I surfaced this here since we'll want to track documenting this permanently as part of the minimal API docs before we GA. This is in addition to the point-in-time stuff for Preview 3 what's new.

[Rick-Anderson]

@wadepickett use Ctrl+Alt+G on the appropriate doc to add meta-data to this issue.
Update the bottom of this comment with 2 check boxes, this issue and the WN.3 issue.

[wuuer]

How to implement custom validation error response?

[diegosasw]

I gave it a try with preview 4. Validation only kicks in when endpoints are in the same assembly. Is there any workaround for minimal endpoints which are in a different project and properly identified by openApi and showing in swagger UI, etc.?

Example:
Assuming service.AddValidation() and <InterceptorsNamespaces>$(InterceptorsNamespaces);Microsoft.AspNetCore.Http.Validation.Generated</InterceptorsNamespaces> in main project's csproj.

When having this in main project (where Program.cs is) it works as expected.

public class SampleRequest
{
    [Required]
    public string? Reference { get; init; }
}

app.MapPost("/sample", (
        [FromBody] SampleRequest sampleRequest,
        CancellationToken cancellationToken)
        => TypedResults.Ok(sampleRequest));

When moving endpoint to another assembly, for example with an extension method, validation does not kick in.
In Program.cs

app.MapSampleEndpoint();

In another project

public static class SampleEndpoint
{
    public static void MapSampleEndpoint(this IEndpointRouteBuilder app)
    {
        app
            .MapPost("/sample", (
                    [FromBody] SampleRequest sampleRequest,
                    CancellationToken cancellationToken)
                => TypedResults.Ok(sampleRequest));
    }
}