#26 – Null Literal
July 13, 2010 3 Comments
The keyword null represents a null literal. A null value indicates that a reference points to no object.
Examples:
object o1 = null;
if (o1 == null) Console.WriteLine("Yup, it's null");
string s1 = null; // Strings can also be null
int n1 = s1.Length; // Throws NullReferenceException, since s1 doesn't point to anything
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2,000 Things You Should Know About C# 
String.IsNullOrEmpty() is quite useful, if I’m not wrong on the function name.
object.ReferenceEqual(obj, null) is also useful.
http://onezero.org/blog/archives/45