When targeting .NET Framework 2.0 or newer, use the IsNullOrEmpty method. Otherwise, use the Length == comparison whenever possible.

Comparing strings using the String.Length property or the String.IsNullOrEmpty method is significantly faster than using Equals. This is because Equals executes significantly more MSIL instructions than either IsNullOrEmpty or the number of instructions executed to retrieve the Length property value and compare it to zero./

You should be aware that Equals and Length == 0 behave differently for null strings. If you try to get the value of the Length property on a null string, the common language runtime throws a System.NullReferenceException. If you perform a comparison between a null string and the empty string, the common language runtime does not throw an exception; the comparison returns false. Testing for null does not significantly affect the relative performance of these two approaches.