Can't understand delegate function

Can't understand delegate function

• randy.liden

  cgraus, I will read msdn on delegates.
  
  However, rather than burn the book I'll give it to another beginning programmer whom I dislike.
  
  Could you do me a favor and show me such a function and how to associate it with the delegate
  
  dennist685
  
  

• noremy

  Sven, thanks. Maybe the Wrox book isn't the best, but the IDE is lacking as well. Why doesn't it show us
  
  

  Public Function IsGreaterThan(ByVal v1 As Object, ByVal v2 As Object) As Boolean

  If v1 > v2 Then

  Return True

  Else

  Return False

  End If

  End Function

  dennist685
  

  
  
  

• KhalidMirza

  The greaterThan parameter of the DoSort Sub, is a reference (a pointer) to a method that is declared like the Delegate Compare (ie. it has the same paramters and return value).  

  What this comes down to, is that there is a function somehere that is declared somewhat like this:

  Public Function IsGreaterThan(ByVal v1 As Object, ByVal v2 As Object) As Boolean

  If v1 > v2 Then

  Return True

  Else

  Return False

  End If

  End Function

  now the method DoSort will be called like this

  Dim Range As Object() = New Object(3) {20, 12, 33, 4}

  DoSort(Range, AddressOf IsGreaterThan)

  The AddressOf operator creates a procedure delegate instance that points to the method.

  In this case, the greaterThan parameter will 'point' to the IsGreaterThan method and this method will be executed by the greatherThan.Invoke statement.

  This allows you to create multiple functions (with the same signature as the delegate) and pass them as an argument to the Sub DoSort)

  If you would create a function like the one below

  Public Function IsSmallerThan(ByVal v1 As Object, ByVal v2 As Object) As Boolean

  If v1 < v2 Then

  Return True

  Else

  Return False

  End If

  End Function

  You could pass a reference to that pointer to the Sub like this

  Dim Range As Object() = New Object(3) {20, 12, 33, 4}

  DoSort(Range, AddressOf IsSmallerThan)

  This allows you to use the same method (DoSort) to sort the array descending or ascending

  In this scenario, this would be confusing, since the paramter is called 'greaterThan', but as long as the parameter is a Compare delegate (i.e. a Function that accepts 2 object parameters and returns a boolean), you can pass it to the DoSort Sub

  I hope this clarifies it a bit for you

   

  ps. Oh yeah... Listen to cgraus and burn the Wrox book

  
  
  

• Roman Benko

  No idea, I don't have the book so I can't see the module you are talking about.

  If the example works, it should be there somewhere though

  
  
  

• Pete Claar

  cgraus, thank you. I tried to mark your response as an answer three times, but I kept getting sorry an unknown error has occured.
  
  I will read msdn on delegates, thanks for the suggestion.
  
  Could you be so kind as to show me such a function and how to associate it with the delegate
  
  I don't think I'm going to burn the book, however. I may give it to a another beginning programmer who I dislike, however.
  
  dennist685
  
  

• NiklasEngfelt

  I've never seen that syntax before. Wrox books are terrible, IMO, too many listings of code you don't need, not enough info. In this case, you'd need to create a function that matches that signature, then associate it with the delegate, so it's the function that gets called.

  I would read the MSDN on delegates. And possibly burn your Wrox book :-)

  
  
  
• Can't understand delegate function