Assertions.cs

using System;
using System.Linq;
using MediatR;
using Should;
using static Testing;

public static class Assertions
{
    public static void ShouldValidate<TResponse>(this IRequest<TResponse> message)
    {
        var validator = Validator(message);

        validator.ShouldNotBeNull($"There is no validator for {message.GetType()} messages");

        var result = validator.Validate(message);

        var indentedErrorMessages = result
            .Errors
            .OrderBy(x => x.ErrorMessage)
            .Select(x => "    " + x.ErrorMessage)
            .ToArray();

        var actual = String.Join(Environment.NewLine, indentedErrorMessages);

        result.IsValid.ShouldBeTrue(
            $"Expected no validation errors, but found {result.Errors.Count}:{Environment.NewLine}{actual}");
    }

    public static void ShouldNotValidate<TResponse>(this IRequest<TResponse> message, params string[] expectedErrors)
    {
        var validator = Validator(message);

        validator.ShouldNotBeNull($"There is no validator for {message.GetType()} messages");

        var result = validator.Validate(message);

        result.IsValid.ShouldBeFalse("Expected validation errors, but the message passed validation.");

        var actual = result.Errors
            .OrderBy(x => x.ErrorMessage)
            .Select(x => x.ErrorMessage)
            .ToArray();

        actual.ShouldEqual(expectedErrors.OrderBy(x => x).ToArray());
    }
}