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Constraint Violated Exception on MediatR Behavior

See original GitHub issue

I have the following classes:

public class GetPostsRequest : IRequest<Envelope<GetPostsResponse>> {
  public Int32 Age { get; set; }
}

public class GetPostsResponse {
  public String Code { get; set; }
  public String Name { get; set; }      
}

Where Envelope is a wrapper class:

public class Envelope<T> {
  public List<T> Result { get; private set; } = new List<T>();  
  public List<Error> Errors { get; private set; } = new List<Error>();
}

Then I created a ValidationBehavior using FluentValidation:

public class ValidationBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, Envelope<TResponse>> where TRequest : IRequest<Envelope<TResponse>> {

  private readonly IEnumerable<IValidator<TRequest>> _validators;

  public ValidationBehavior(IEnumerable<IValidator<TRequest>> validators) {
    _validators = validators;
  }

  public Task<Envelope<TResponse>> Handle(TRequest request, CancellationToken cancellationToken, RequestHandlerDelegate<Envelope<TResponse>> next) {

    ValidationContext context = new ValidationContext(request);

    List<Error> errors = _validators
      .Select(x => x.Validate(context))
      .SelectMany(x => x.Errors)
      .Select(x => new Error(ErrorCode.DataNotValid, x.ErrorMessage, x.PropertyName))
      .ToList();

    if (errors.Any())
      return Task.FromResult<Envelope<TResponse>>(new Envelope<TResponse>(errors));  

    return next();

  }

}

And I registered the ValidationBehavior on the ASP.NET Core application:

services.AddScoped(typeof(IPipelineBehavior<,>), typeof(ValidationBehavior<,>));

When I call the API I get the following error:

An unhandled exception has occurred while executing the request.
System.ArgumentException: GenericArguments[0], 'TRequest', on 'ValidationBehavior`2[TRequest,TResponse]' violates the constraint of type 'TRequest'. 
---> System.TypeLoadException: GenericArguments[0], 'TRequest', on 'ValidationBehavior`2[TRequest,TResponse]' violates the constraint of type parameter 'TRequest'.

Am I missing something?