The difference between System.Text.StringBuilder and string or System.String

System.String and string keywords are the same, and you can use whichever naming convention you prefer. Essentially, string keyword is an alias for System.String class. String objects are immutable (i.e. they cannot be changed after they have been created) whereas StringBuilder is mutable(i.e. they can be changed after they have been created).

All of the String methods and C# operator that seems to modify a string actually return the results in a new string object. StringBuilder offers better performance over the System.String or string because it is mutable.

Example

string or System.String - In this example, string str1 and str2 are concatenated by "+=" operator to combine the contents in a single string.

string str1 = "First Name ";
string str2 = "Last Name";

// Concatenate str1 and str2. This actually creates a new string object 
// and stores it in str1, releasing the reference to the original object.
str1 += str2
System.Console.WriteLine(str1);

// Output: First Name Last Name

System.Text.StringBuilder - In this example, I am creating an object of StringBuilder class and modifying the value of it.

StringBuilder sb = new StringBuilder();
sb.Append("First Name");
sb.Append(" Last Name");

System.Console.WriteLine(sb.ToString());
// Output: First Name Last Name