interface IFoo
{
    int MyReadOnlyVar { get; }
}


class Foo : IFoo
{
    int MyReadOnlyVar { get; set; }
}


public IFoo GetFoo()
{
    return new Foo { MyReadOnlyVar = 1 };
}

Is the above an acceptable way of implementing a readonly/immutable object? The immutability of IFoo can be broken with a temporary cast to Foo.

In general (non-critical) cases, is hiding functionality through interfaces a common pattern? Or is it considered lazy coding? Or even an anti-pattern?