Question

   what is the difference among these 

   1. string str=string.Empty;
   2. string str="";
   3. string str=null;   

and is there any different way to use these statements…

thanks
saj

Answer 1

string.Empty and "" both end up as references to a “real” string object which happens to be empty. So you can call methods on it, use its Length property etc.

The null reference is a value which doesn’t refer to any object. If you try to dereference null, you’ll get a NullReferenceException.

As to whether you choose "" or string.Empty – that’s really a matter of personal preference. I tend to choose "", but others find string.Empty more readable. Note that while "" is a constant expression, string.Empty isn’t, so you can’t use the latter in case statements.

EDIT: Note that both of the other answers present at the time of this edit imply that null is not a value, or that the variable hasn’t been assigned a value. This is incorrect. null is a value just like any other – it’s just that it doesn’t refer to any object. In particular, assigning null to a variable will overwrite any previous value, and if it’s a local variable that assignment means the variable has been “definitely assigned”. For example, compare these two snippets

string s;
Console.WriteLine(s); // Compile-time error: s isn't definitely assigned

string s = null;
Console.WriteLine(s); // Fine: s has the value null