What is the difference between a mutable and immutable string in C# and VB.NET?

A mutable string can be changed.

An immutable string cannot be changed.

Mutable is the english word for can change. Immutable is the english word for cannot change.

if you alter a String, you are creating a new string object internally. So String is immutable. If you alter a StringBuilder no new Objects are created internally so, StringBuilder is mutable.