What is the difference between String and string in C#?

String stands for System.String whereas string is an alias in C# for System.String −

For example

string str = "Welcome!";

It’s not essential, but generally String is used when you work with classes.

string str = String.Format("Welcome! {0}!", user);

Since the string is an alias for System. String. The alias for other datatypes are −

Example

object: System.Object string: System.String bool: System.Boolean float: System.Single double: System.Double decimal: System.Decimal byte: System.Byte sbyte: System.SByte short: System.Int16 ushort: System.UInt16 int: System.Int32 uint: System.UInt32 long: System.Int64 ulong: System.UInt64 char: System.Char