Impossible to modify ValidationProblemDetails when validation in Minimal APIs is triggered

[mirekholec]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When an error occurs in Minimal APIs, the CustomizeProblemDetails method is always executed, allowing customization of the ProblemDetails response. For example, this applies when an endpoint is not found or when Results.BadRequest is returned from an endpoint. However, if validation is triggered in Minimal APIs and an error occurs, the CustomizeProblemDetails method is not executed, and the error response structure cannot be customized.

builder.Services.AddProblemDetails(x=> x.CustomizeProblemDetails = (c) =>
{
    c.ProblemDetails = new ProblemDetails(){Title = "My Error"};
});

Expected Behavior

Once validation in Minimal API fails, the CustomizeProblemDetails method should be invoked so that we can modify the error structure like this:

builder.Services.AddProblemDetails(x=> x.CustomizeProblemDetails = (c) =>
{
    if (c.HttpContext.Response.StatusCode == 400)
    {
        // customize the ProblemDetails for 400 Bad Request
        c.ProblemDetails = new ValidationProblemDetails();
    }
});

Steps To Reproduce

1. Create new project (dotnet new webapi -minimal) with .NET SDK 10 prev5
2. Add Minimal APIs validation (builder.services.AddValidation())
3. Create new endpoint app.MapPost("/test", (TestEntity data,) => { }
4. Create entity TestEntity with some validation attributes
5. Register ProblemDetails together with UseExceptionHandler and UseStatusCodePages

Example:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddProblemDetails(x=> x.CustomizeProblemDetails = (c) =>
{
     // it is not fired
});

builder.Services.AddValidation();

var app = builder.Build();

app.UseExceptionHandler();
app.UseStatusCodePages();

app.MapPost("/test", (TestEntity test) =>
{
    return Results.Ok();
});

app.Run();


public class TestEntity
{
    [Required]
    public string Title { get; set; }
}

Exceptions (if any)

No response

.NET Version

10.0.100-preview.5.25277.114

Anything else?

No response

[mirekholec]

Motivation for fixing this issue:

1. Property names generated in ValidationProblemDetails use PascalCase, even though the standard naming convention is camelCase. Any serializer settings is also ignored.

2. ValidationProblemDetails returns the Errors property as an object, but in typical development scenarios, it is more practical to return Errors as an array of error objects.

While the current behavior is not ideal, changing it directly would break backward compatibility. So the only viable solution for developers is to customize the ProblemDetails response. This is currently not possible because CustomizeProblemDetails is not invoked when Minimal APIs validation fails.

[marcominerva]

Hi @mirekholec,

The integration of validation with IProblemDetailsService (which allows the use of CustomizeProblemDetails) has been added in PR #62066. It will be available in .NET 10 Preview 6.

[captainsafia]

@marcominerva Any interest in opening a PR on the sample app (https://github.com/captainsafia/minapi-validation-support) to demonstrate the feature?

[marcominerva]

Yes, of course 😄 Here it is: captainsafia/minapi-validation-support#2

[mirekholec]

@marcominerva Great work! I'll give it a try and I am looking forward to showing it to the community in a video.