Question

I am trying to write a quick snippet to demonstrate the difference between an immutable and mutable type.
Does this code seem to right to you all?

class MutableTypeExample
{
    private string _test; //members are not readonly
    public string Test
    {
        get { return _test; }
        set { _test = value; } //class is mutable because it can be modified after being created
    }

    public MutableTypeExample(string test)
    {
        _test = test;
    }

    public void MakeTestFoo()
    {
        this.Test = "FOO!";
    }
}

class ImmutableTypeExample
{
    private readonly string _test; //all members are readonly
    public string Test
    {
        get { return _test; } //no set allowed
    }

    public ImmutableTypeExample(string test) //immutable means you can only set members in the consutrctor. Once the object is instantiated it cannot be altered
    {
        _test = test;
    }

    public ImmutableTypeExample MakeTestFoo()
    {
        //this.Test = "FOO!"; //not allowed because it is readonly
        return new ImmutableTypeExample("FOO!");
    }
}

Answer 1

Yup, that looks reasonable.

However, I would also talk about “leaky” mutability. For example:

public class AppearsImmutableButIsntDeeplyImmutable
{
    private readonly StringBuilder builder = new StringBuilder();
    public StringBuilder Builder { get { return builder; } }
}

I can’t change which builder an instance appears on, but I can do:

value.Builder.Append("hello");

It would be worth you reading Eric Lippert’s blog post on kinds of immutability – and indeed all the rest of the posts in the series.