Question

Most examples when talking about automatically generated properties talk about “simple” value types such as strings. But what if you’d like to access a field of such a value type that is generated automatically in the IL to back up an “automatic property”?

The compiler won’t allow to do this: “Cannot modify the return value of ‘Position’ because it is not a variable”. I understand why we cannot modify this return value, but how would we then access these fields?

Say we have

class A
{
    Vector2 Position { get; set; }

    public void Foo()
    {
        Position.X = 10.0f;    // Not allowed!
    }
}

How do I access and set the field X of the Vector2 instance within class A?

Answer 1

Accessing reference types that are automatically generated from properties

Your problem is that Vector2 is not a reference type; it is a value type. When you access the property Position a copy is returned, so you are attempting to mutate a temporary. In this situation you need to set a completely new value:

Position = new Vector2(10.0, Position.Y);

You could also create a private field in this case and not use an automatic property:

class A
{
    Vector2 _position;
    Vector2 Position 
    { 
        get { return _position; } 
        set { _position = value; }
    }

    public void Foo()
    {
        _position.X = 10.0f;  
    }
}

Documentation: http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.vector2.aspx