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