What is the difference between mutable and immutable?

Question

Can any one help me in finding the basic difference between mutable and immutable?

Answer 1

Immutable means that once initialized, the state of an object cannot change.

Mutable means it can.

For example - strings in .NET are immutable. Whenever you do an operation on a string (trims, upper casing, etc...) a new string gets created.

In practice, if you want to create an immutable type, you only allow getters on it and do not allow any state changes (so any private field cannot change once the constructor finished running).

Answer 2

A very basic definition would be:

Mutable Objects: When you have a reference to an instance of an object, the contents of that instance can be altered

Immutable Objects: When you have a reference to an instance of an object, the contents of that instance cannot be altered

Answer 3

Immutable means "cannot be modified after it is created".

• An immutable type has a constructor and getters but not setters.
• A mutable type can also have setters.

An example of an immutable type is DateTime. The method AddMinutes does not modify the object - it creates and returns a new DateTime.

Another example is string. For a mutable class similar to string you can use the class StringBuilder.

There is no keyword in C# to declare a type as immutable. Instead you should mark all member fields as readonly to ensure that they can only be set in the constructor. This will prevent you from accidentally modifying one of the fields, breaking the immutability.