C#/.NET: Difference between String and string

In C# "string" is an alias for "System.String". So, technically, both declarations are indeed identical, because internally, "string" is defined as "String".

Nevertheless, I recommend using the lowercase declaration always when you need a string object, for example:

string s = "ABC";

However, if you need any class function, you should use the uppercase variant:

string a = "A";
string b = "B";
string c = "C";
string s = String.Concat(a, b, c);

In this example, we are creating 3 string objects (a, b and c) using the lowercase string and we are using the class function String.Concat with the uppercase String in order to store the result into the object s.

Another argument for going this way is, that "string" is a consciously introduced lexical construct as an abstraction that should be used for a default string. Apart from that, "System.String" is only a type. So, if at any time in the future (unlikely) the default string type should be changed, string would not be the same as System.String anymore and if you already have used "string" you would be on the safe side because "string" would be changed to the new default string type at this time and you would not have to change your code.

By the way, the same thing also applies to int ( = System.Int32), long ( = System.Int6), bool ( = System.Boolean), char ( = System.Char), object ( = System.Object) and many other types.
Last update on 2021-01-13 | Created on 2016-08-05