What is Difference Between String and string in C# ??

string is an alias in C# for System.String. So technically, there is no difference. It's like int vs. System.Int32.

As far as guidelines, I think it's generally recommended to use string any time you're referring to an object.

e.g.

string place = "world";

Likewise, I think it's generally recommended to use String if you need to refer specifically to the class.

e.g.

string greet = String.Format("Hello {0}!", place);

• string is an alias for the String class in .NET framework.
• Every datatype in Dot net has alias. e.g.- bool and Boolean.
• To use the String class, we need to import the System namespace, while we don’t have to import the namespace for using the string keyword. 
• It is not mandate but as a general practice is while declaring variable string is used and for using classes String is used.

String.Format("{0,10:G}: {0,10:X}", value);