[TestMethod]
[ExpectedMissingInputException()]
public void ExecuteTaskThrowsIfInputIsNull()
{
    CallMyMethod(null);
}

...
void CallMyMethod(string str)
{
    if (str==null) throw ArgumentNullException( "str");
}

[AttributeUsage(AttributeTargets.Method, AllowMultiple = false)]
public sealed class ExpectedMissingInputExceptionAttribute : ExpectedExceptionBaseAttribute
{
  public ExpectedMissingInputExceptionAttribute() {}

  protected override void Verify(Exception exception)
  {
    if (!(exception is ArgumentNullException))
    {
      throw new ArgumentException("ArgumentNullException is expected, but actual exception was {0}", exception.GetType().FullName);
    }
  }
}