Generic dictionary with weakreference value

Question

Hi,

I am trying to bring generic weak reference dictionary in c# 3.0. I have created a class called WeakDictionary<TKey, TValue> which implements IDictionary<TKey,TValue>. This class has a private member as,
private Dictionary<TKey, WeakReference> weakDictionary = new Dictionary<TKey, WeakReference>();
The other methods like Add,Remove, Clear are working fine for the weakDictionary. But I could not work out the TryGetValue method. It shows the error InvalidCastException was unhandled - Unable to cast object of type 'System.WeakReference' to type 'System.String'. on the line

value = (TValue)weakReference.Target;

I don't know how to retrieve the value. Please help me.

CODE

public bool TryGetValue(TKey key, out TValue value) {
    WeakReference weakReference;
    if (this.weakDictionary.TryGetValue(key, out weakReference) && weakReference.IsAlive) {
        value = (TValue)weakReference.Target;
        return true;
    } else {
        weakDictionary.Remove(key);
        value = default(TValue);
        return false;
    }
}

Thank you.