Answer by Burak Yeniçeri for What is the difference between String and string in C#?

String is the class of string. If you remove System namespace from using statements, you can see that String has gone but string is still here. string is keyword for String. Like
int and Int32
short and Int16
long and Int64
So the keywords are just some words that uses a class. These keywords are specified by C#(so Microsoft, because C# is Microsoft's). Briefly, there's no difference. Using string or String. That doesn't matter. They are same.