C# Minimal API ValidationFilter w/ FluentValidations

ValidationFilter.cs

using FluentValidation;

public sealed class ValidationFilter<T> : IEndpointFilter
    where T : class
{
    public async ValueTask<object?> InvokeAsync(EndpointFilterInvocationContext context, EndpointFilterDelegate next)
    {
        if (context.Arguments.FirstOrDefault(x => x?.GetType() == typeof(T)) is not { } argument)
            return Results.Problem($"Missing argument type. Expected to validate {typeof(T).Name}.");
        
        var validationType = typeof(ValidationContext<>).MakeGenericType(typeof(T));
        var validationContext = Activator.CreateInstance(validationType, argument) as IValidationContext;
        
        var validator = context.HttpContext.RequestServices.GetRequiredService<IValidator<T>>();

        if (await validator.ValidateAsync(validationContext) is { IsValid: false } validation)
            return Results.ValidationProblem(validation.ToDictionary());
        
        return await next(context);
    }
}

public static class ValidationFilterExtensions
{
    public static RouteHandlerBuilder AddValidationFilter<T>(this RouteHandlerBuilder builder)
        where T : class
    {
        return builder.AddEndpointFilter<ValidationFilter<T>>();
    }
}