Question

In my other methods I could do something like this,

    public void Add(T item)
    {
        if (dispatcher.CheckAccess())
        {
            ...
        }
        else
        {
            dispatcher.Invoke(new Action<T>(Add), item);
        }
    }

But how do I invoke a property for a situation like this?

    public T this[int index]
    {
        get
        {
            ...
        }
        set
        {
            if (dispatcher.CheckAccess())
            {
                ...
            }
            else
            {
                dispatcher.Invoke(???, value, index); // <-- problem is here
            }
        }
    }

Answer 1

Edit: The following paragraph no longer applies, as the OP’s question has since been edited.

First off, your second code seems logically incorrect: You apparently want to call the setter, but while you provide a value for index, you don’t provide the actual value (i.e. item). I’ll return to that issue in a second.

---

You could wrap an anonymous delegate or lambda function around the property setter, e.g. (using an anonymous delegate) to make it invokable:

dispatcher.Invoke(
    new Action<T>(  delegate (T item) { this[index] = item; }  ),
    item );

or (using a lambda function available since C# language version 3):

dispatcher.Invoke(
    new Action<T>(  (T item) => { this[index] = item; }  ),
    item );

Note: You create an anonymous delegate or lambda function that accepts one argument (item). The other required argument (index) is taken from the “outer” context. (The term closure comes to mind.) I see this as the only way how you would not have to change your dispatcher to a delegate type that is sometimes invoked with two arguments instead of one.

If this however is not an issue, the Invoke code could change to e.g.:

dispatcher.Invoke(
    new Action<int,T>(  (int index, T item) => { this[index] = item; }  ),
    index, item );