How to document old value vs new value in generic function callback

I have a generic callback function like this to notify listeners when a value has changed: 

public Func<T, T, Task> ValueChangedAsync { get; set; }

where the first T should be the old value and second T should be the new value.

I will use this callback like this: 

await ValueChangedAsync(old, newValue);

Can I make users of this function aware of which T is the old value and new value? When using the callback this is what gets implemented by default: 

private Task ValueChangedAsync(string s, string s1)
{
    throw new NotImplementedException();
}

I would really like the "s" to be named "oldValue" and "s1" to be named "newValue" or such.

Jon Skeet
people
quotationmark

You could do this with events and a custom delegate type, where the delegate type documentation can indicate the old value and new value parts. For example:

// As we want a non-void return type, we're not following the event conventions
// anyway... so we might as well have three parameters...
public delegate Task ValueChangedHandler<T>(object source, T oldValue, T newValue);

Then:

public event ValueChangedHandler<string> ValueChanged;

Invoke it as:

ValueChangedHandler<string> handler = ValueChanged;
if (handler != null)
{
    var allHandlers = handler.GetInvocationList();
    foreach (ValueChangedHandler<string> individualHandler in allHandlers)
    {
        await individualHandler(this, oldValue, newValue);
    }
}

people

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