Difference between String and string

1. String stands for System.String and it is a .NET Framework type.
2. string is a type in C#. System.String is a type in the CLR.
3. string is an alias for System.String in the C# language. Both of them are compiled to System.String in IL (Intermediate Language),
4. So there is no difference. Choose what you like and use that.
5. For code in C#, its prefer to use string as it's a C# type alias and well-known by C# programmers.

6. 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.

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

7. If you really want a variable name called 'string' you can use @ as a prefix :
StringBuilder @string = new StringBuilder(); this applies to string also.

8. So String is not a keyword and it can be used as Identifier whereas string is a keyword and cannot be used as Identifier. And in function point of view both are same.
9. You can't use String without using System; but string can use without that namespace.
10. Static functions such as String.Format, String.Join, String.Concat, String.isNullOrWhitespace etc... are from String.

I can say the same about (int, System.Int32) etc..