public void CreateLetterHeadAsync_WhenLetterHeadIsNull_ThrowsArgumentNullException() { ICommonRepository sut = CreateSut(); ArgumentNullException result = Assert.ThrowsAsync <ArgumentNullException>(async() => await sut.CreateLetterHeadAsync(null)); // ReSharper disable PossibleNullReferenceException Assert.That(result.ParamName, Is.EqualTo("letterHead")); // ReSharper restore PossibleNullReferenceException }