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Add clarifications about Control.Invoke behavior when an EventHandler is used #9317

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Sep 28, 2024
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Add clarifications about Control.Invoke behavior when an EventHandler is used #9317

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add clarifications about Control.Invoke behavior when an EventHandler…
tbolon Sep 21, 2023
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tbolon Sep 23, 2023
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7 changes: 6 additions & 1 deletion xml/System.Windows.Forms/Control.xml
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Expand Up @@ -7899,7 +7899,12 @@ MyControl.Font = New Font(MyControl.Font, _
> [!NOTE]
> In addition to the <xref:System.Windows.Forms.Control.InvokeRequired%2A> property, there are four methods on a control that are thread safe: <xref:System.Windows.Forms.Control.Invoke%2A>, <xref:System.Windows.Forms.Control.BeginInvoke%2A>, <xref:System.Windows.Forms.Control.EndInvoke%2A>, and <xref:System.Windows.Forms.Control.CreateGraphics%2A> if the handle for the control has already been created. Calling <xref:System.Windows.Forms.Control.CreateGraphics%2A> before the control's handle has been created on a background thread can cause illegal cross thread calls. For all other method calls, you should use one of the invoke methods to marshal the call to the control's thread.

The delegate can be an instance of <xref:System.EventHandler>, in which case the sender parameter will contain this control, and the event parameter will contain <xref:System.EventArgs.Empty?displayProperty=nameWithType>. The delegate can also be an instance of <xref:System.Windows.Forms.MethodInvoker>, or any other delegate that takes a void parameter list. A call to an <xref:System.EventHandler> or <xref:System.Windows.Forms.MethodInvoker> delegate will be faster than a call to another type of delegate.
The delegate can be an instance of <xref:System.EventHandler>, in which case the parameters will depend on the `args` value:
- If no parameters are passed, the sender parameter will contain this control and the event parameter will contain <xref:System.EventArgs.Empty?displayProperty=nameWithType>.
- When a single parameter is passed, the sender parameter will contain the first args element and the event parameter will contain <xref:System.EventArgs.Empty?displayProperty=nameWithType>.
- If more than one parameter is passed, the sender parameter will contain the first element from `args`, and the <xref:System.EventArgs> parameter will contain the second element.

A call to an <xref:System.EventHandler> or <xref:System.Windows.Forms.MethodInvoker> delegate will be faster than a call to another type of delegate.

> [!NOTE]
> An exception might be thrown if the thread that should process the message is no longer active.
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