Answer by Simon_Weaver for What is the difference between String and string in C#?

string is a reserved word, but String is just a class name. This means that string cannot be used as a variable name by itself.

If for some reason you wanted a variable called string, you'd see only the first of these compiles:

StringBuilder String = new StringBuilder();  // compilesStringBuilder string = new StringBuilder();  // doesn't compile 

If you really want a variable name called string you can use @ as a prefix:

StringBuilder @string = new StringBuilder();

Another critical difference: Stack Overflow highlights them differently.