What is the difference between String and string keyword in c#

If you come from another programming background (e.g Java), you may noticed that in C# you're able to use string and String to declare a string variable in your code.

String textA = "Hello";
string textB = "Hello";

And you may asked to yourself, both works anyway but, there's any difference between them? technically no.

string is an alias in C# for System.String.  It can be compared in a case like int and System.Int32, just an integer or just like the bool and Boolean. Both of them are compiled to System.String in IL (Intermediate Language)

string is a reserved word and String is a class name. This means that string cannot be used as a variable name by itself i.e :

StringBuilder String = new StringBuilder(); // Compile succesfully 
StringBuilder string = new StringBuilder(); // Doesn't compile

Therefore comes in handy to use string, the code using string will never break, but there is a slim chance that code using String will. (You can redefine String, but not string).

If you need to refer specifically to the class, is recommendable to use String i.e :

Decimal pricePerOunce = 17.36m; 
String s = String.Format("The current price is {0:C2} per ounce.", pricePerOunce);

string is just one of all the aliases that C# has built in, unlike Java. You can see all the aliases in the following list:

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

As said before, you can use the aliases without even import use System;

Final conclusion

string is a keyword, and you can't use string as an identifier.

String is not a keyword, and you can use it as an identifier:

string String = "I am a string";

The keyword string is an alias for System.String aside from the keyword issue, the two are exactly equivalent, therefore :

typeof(string) == typeof(String) == typeof(System.String)

Notes

• You can use string without import any component, opposite to String as you can't use String without using System; beforehand.
• You can't use string in reflection; you must use String.
• To avoid confusion use one of them consistently. But from best practices perspective when you do variable declaration it's good to use string and when you are using it as a class name then String is preferred.