Generate hash code based on object identity in .net

How can I generate a hash code for an object based on its identity.

What I mean is that:

• if object.ReferenceEquals(a, b) == true, then a and b will get the same hash code.
• if object.ReferenceEquals(a, b) == false, then a and b should have a decent chance to get different hash codes even if they are memberwise equal.

What I have is:

class SomeClassThatMakesSenseToCompareByReferenceAndByValue {
    override Equals(object o) {
        return MemberwiseEquals(o);
    }

    override GetHashCode() {
        return MemberwiseGetHashCode();
    }
}

class SomeClassThatNeedsReferenceComparison {
    SomeClassThatMakesSenseToCompareByReferenceAndByValue obj;

    override Equals(object o) {
        return o is SomeClassThatNeedsReferenceComparison && object.ReferenceEquals(this.obj, (o as SomeClassThatNeedsReferenceComparison).obj);
    }

    override GetHashCode() {
        return ?????
    }
}