String is immutable in C# that would mean you couldn’t modify it after it is created. It creates a new object of string type in memory if you will perform any operation.
string str1 = "Welcome!"; // creates a new string instance str1 += "Hello"; str1 += "World”;
StringBuilder is mutable in C#. This means that if an operation is performed on the string, it will not create a new instance every time. With that, it will not create new space in memory, unlike Strings.
StringBuilder str1 = new StringBuilder(""); str1.Append("Welcome!"); str1.Append("Hello World!"); string str2 = str1.ToString();