What is the reccommended pattern for re-using a data contract as an object model in code?

Let's say I own a service and publish a contract like

public class MyObject
{
    public int Foo { get; }

    public string Bar { set; }
}

I imagine that it is not proper have "behavior" in a data contract, things like

public class MyObject
{
    public int Foo { get; }

    public string Bar { set; }

    public void DoSomeAlgorithmWithMyProperties() { … }
}

In other words, data contracts are supposed to be just value bags.

So my question is how I create behavior on a such an object. One way I can see is to just create a separate mirroring object, like

public class MyObjectInternal
{
    public int Foo { get; }

    public string Bar { set; }

    public class MyObjectInternal(int foo, string bar)
    {
        this.Foo = foo;
        this.Bar = bar;
    }

    public void DoSomeAlgorithmWithMyProperties() { … }
}

Another is inheritance

public class MyObjectInternal : MyObject
{
    public class MyObjectInternal(int foo, string bar)
    {
        this.Foo = foo;
        this.Bar = bar;
    }

    public void DoSomeAlgorithmWithMyProperties() { … }
}

and other possibility could be to just completely separate out behavior and data by having separate classes that operation on the values, like

public static class MyObjectAlgorithmDoer
{
     public static void DoSomeAlgorithmWithMyProperties(MyObject myObject) { … }
}