Question

Assuming I have the following delegate

public delegate void ControlInitializer(Control control);

Is there a way when specifying the delegate to specify what type of control the input parameter is? e.g.

Instead of

ControlInitializer one = c => ((TextBox)c).Text = "Init Value"
ControlInitializer two = c => ((DropDownList)c).SelectedValue= "-1"

Can I do something like

ControlInitializer one = (TextBox c) => c.Text = "Init Value"
ControlInitializer two = (DropDownList c) => c.SelectedValue= "-1"

As Textbox is a sub class of Control in this case?

Update: I also need to store these 2 ControlInitialiser delegates in a e.g.

Dictionary<string, ControlInitializer>

will specifying

Dictionary<string, ControlInitializer<Control>>

Work in this case as I can’t seem to get it to work.

Thanks in advance.

Answer 1

You can make the delegate generic:

public delegate void ControlInitializer<TControl>(TControl control)
    where TControl : Control;

And then use it like this:

ControlInitializer<TextBox>      one = c => c.Text = "Init Value";
ControlInitializer<DropDownList> two = c => c.SelectedValue = "-1";

---

I guess your aiming for something like this:

var init = new Init();

init.RegisterInitializer<TextBox>(c => c.Text = "Init Value");
init.RegisterInitializer<DropDownList>(c => c.SelectValue = "-1");

foreach (var c in Controls)
{
    init.ApplyInitializer(c);
}

That’s a bit difficult due to the reasons mentioned in David B‘s answer. What you can do, however, is hide the type cast behind an abstraction, like this:

public class Init
{
    private readonly Dictionary<Type, Action<Control>> initializers;
    ...

    public void RegisterInitializer<TControl>(Action<TControl> i)
        where T Control : Control
    {
        initializers.Add(typeof(TControl), c => i((TControl)c));
    }

    public void ApplyInitializer(Control c)
    {
       initializers[c.GetType()](c);
    }
}