Action does not contain a definition for ShouldThrow.

CustomerValidatorTest.cs

using System;
using Xunit;

namespace CustomerValidator.Tests
{
    public class CustomerValidatorTests
    {
        [Fact]
        public void WhenCustomerIsNull_ThenArgumentNullExceptionIsThrown()
        {
            var validator = new CustomerTDD.CustomerValidator();

            Action action = () => validator.Validate(null);

            action.ShouldThrow<ArgumentNullException>();
        }
    }
}