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I'm trying to access an AutoCompleteBox on one of my screens. I can see that FindControl() has located the control when I do var testControl = FindControl("MyControl");

However, when I attempt to cast this to the type of control it is supposed to be so I can manipulate it, the result is null.

This is what I'm doing:

System.Windows.Controls.AutoCompleteBox testBox = new System.Windows.Controls.AutoCompleteBox();
testBox = testControl as System.Windows.Controls.AutoCompleteBox;

testBox will be null.

It definitely says the control is an AutoCompleteBox on the screen, I'm not sure what I'm doing wrong. Can anyone help?

EDIT: Thanks to Yann, I was able to resolve this with the following code:

this.FindControl("MyControl").ControlAvailable += (p, e) =>
        {
            //For every  use I can just cast like ((System.Windows.Controls.AutoCompleteBox)e.Control)
        };
Interminable
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2 Answers2

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If as returns null, you're trying to cast to the wrong type. I oother words, testControl is not of type AutoCompleteBox.

Put a breakpoint on that second line and see what type testControl really is at runtime.

Roy Dictus
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  • A '[Microsoft.LightSwitch.Presentation.Implementation.ContentItemProxy' apparently. I have no idea what this is, the control is most definitely an AutoCompleteBox in the screen, and it says it is in the Design View. – Interminable Oct 25 '12 at 12:00
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The object you get from FindControl is just a just proxy object, as you've discovered. The way to get at the real control is done in two steps:

  1. Add code to the screen's Created method (the control is not guaranteed to be available until the screen's Created method runs).
  2. Then add a handler to the proxy's ControlAvailable method.
Private Sub ScreensName_Created

      FindControl("ControlsName"). AddressOf ControlsName_ControlAvailable

End Sub

Private Sub ControlsName_ControlAvailable(sender as Object, e as ControlAvailableEventArgs)

    'do whatever you want in here
    'you can cast e.Control to whatever is the type of the underlying Silverlight control.

End Sub

Of course, you need to replace "ScreensName" & "ControlsName" with your own names.

(For some reason, I wasn't able to sucessfully format the entire text of two methods as code)

casperOne
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Yann Duran
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  • OP has C# tag. Since your code is in VB, the formatting code got confused. I submitted an edit with a [prettify tag](http://meta.stackexchange.com/questions/63800/interface-options-for-specifying-language-prettify) to override the formatting. +1 great answer. – embedded.kyle Oct 26 '12 at 12:32
  • I actually saw the answer before it was reformatted but have only just got around to replying, sorry. I don't really code in VB, however I was able to understand your code. For anyone interested, I believe I have managed to work out the C# equivalent (it seems to work at any rate), I've edited it in to the original post. – Interminable Oct 26 '12 at 14:00