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

String stands for System.String and it is a .NET Framework type. string is an alias in the C# language for System.String. Both of them are compiled to System.String in IL (Intermediate Language), so there is no difference. At execution time there is no difference. As far as guidelines, it's recommended to use string any time you're referring to an object.

Example:

string place = "Hello World";

It's generally recommended to use String if you need to refer specifically to the class.

Example:

var place = 'World!'
string message = String.Format("Hello {0}!", place);

Similarly for other datatypes,

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