Partially Overriding a Virtual Auto-Property in a Child Class

Question

Time for a theoretical question I just ran across.

The following code is valid and compiles:

public class Parent
{
    public virtual object TestProperty { get; set; }
}

public class Child : Parent
{
    private string _testValue = "Hello World!";

    public override object TestProperty
    {
        get { return _testValue; }
    }
}

public class Consumer
{
    Parent p = new Child();

    public Consumer(){ p.TestProperty = 3; }
}

My question is:

Why does C# allow me to partially override the TestProperty auto property in a child when it leads to partially unpredictable behavior? Is there a practical application?

I'm allowed to set the value of TestProperty using the parent's setter (I checked the IL being generated and the setter is still setting the backing object in the parent class) even though value is not accessible to the public.

Answer 1

This behavior is consistent with non-auto-implemented properties in C#. It's always been possible to override only a get or set method for a virtual property. Hence making it impossible to do with an auto-implemented property would create an unnecessary inconsistency.

For example, the following is legal

class A
{
    public virtual int P1
    {
        get { return 42; }
        set { }
    }
}

class B : A
{
    public override int P1
    {
        get { return 18; }
    }
}

Answer 2

Doesn't it make sense for a setter, though? If you partially override only the setter, that could be useful so that you can respond to that event, in addition to calling base.TestProperty = value, without having to bother with a boilerplate override of the getter as well.