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

Mutable and immutable are English words meaning "can change" and "cannot change" respectively. The meaning of the words is the same in the IT context; i.e.

• a mutable string can be changed, and
• an immutable string cannot be changed.

The meanings of these words are the same in C# / .NET as in other programming languages / environments, though (obviously) the names of the types may differ, as may other details.

For the record:

• String is the standard C# / .Net immutable string type
• StringBuilder is the standard C# / .Net mutable string type

To "effect a change" on a string represented as a C# String, you actually create a new String object. The original String is not changed ... because it is unchangeable.

In most cases it is better to use String because it is easier reason about them; e.g. you don't need to consider the possibility that some other thread might "change my string".

However, when you need to construct or modify a string using a sequence of operations, it may be more efficient to use a StringBuilder. An example is when you are concatenating many string fragments to form a large string:

• If you do this as a sequence of String concatenations, you copy O(N^2) characters, where N is the number of component strings.
• If use a StringBuilder you only copy O(N) characters.

And finally, for those people who assert that a StringBuilder is not a string because it is not immutable, the Microsoft documentation describes StringBuilder thus:

"Represents a mutable string of characters. This class cannot be inherited."

String is immutable

i.e. strings cannot be altered. When you alter a string (by adding to it for example), you are actually creating a new string.

But StringBuilder is not immutable (rather, it is mutable)

so if you have to alter a string many times, such as multiple concatenations, then use StringBuilder.