VB.NET Why is this subroutine declared this way?

VB.NET 2010, .NET 4

I have a basic question: I have a subroutine that I found somewhere online declared thusly:

Public Sub MyFunction(Of T As Control)(ByVal Control As T, ByVal Action As Action(Of T)) ...

I'm wondering about the (Of T As Control) part of the declaration after the sub's name. I see that T is used later in specifying the type of Control and in Action(Of T), but why is it done this way instead of just doing:

Public Sub MyFunction(ByVal Control As Control, ByVal Action As Action(Of Control)) ...

What does that part after the sub's name mean? What is its purpose? Thanks a lot and sorry for my ignorance.

Solution

(Of T) is a generic type parameter, adding As Control constrains the type of T to inherit from Control. You could write the method the second way, but you'd probably end up having to cast the Control to whatever inherited type, within the lambda expression in the Action, or in the body of MyFunction. Generics allow you to avoid that.

For example:

Sub Main()
    Dim form As New Form()

    Dim textBox As New TextBox
    Dim listBox As New ListBox

    MyFunction(textBox, Sub(c) c.Text = "Hello")
    MyFunction(listBox, Sub(c) c.Items.Add("Hello"))

    MyFunction2(textBox, Sub(c) c.Text = "Hello")
    MyFunction2(listBox, Sub(c) CType(c, ListBox).Items.Add("Hello"))


End Sub

Public Sub MyFunction(Of T As Control)(ByVal Control As T, ByVal Action As Action(Of T))
    Action(Control)
End Sub

Public Sub MyFunction2(ByVal Control As Control, ByVal Action As Action(Of Control))
    Action(Control)
End Sub

It doesn't look too valuable in trivial cases, but it's invaluable for more complex cases.