Controls in Windows Forms are bound to a specific thread and are not thread safe. Therefore, if you are calling a control's method from a different thread, you must use one of the control's invoke methods to marshal the call to the proper thread. This property can be used to determine if you must call an invoke method, which can be useful if you do not know what thread owns a control.

If the control's handle does not yet exist, Control.InvokeRequired searches up the control's parent chain until it finds a control or form that does have a window handle. If no appropriate handle can be found, the Control.InvokeRequired method returns false.

This means that Control.InvokeRequired can return false if Control.Invoke(Delegate) is not required (the call occurs on the same thread), or if the control was created on a different thread but the control's handle has not yet been created.

In the case where the control's handle has not yet been created, you should not simply call properties, methods, or events on the control. This might cause the control's handle to be created on the background thread, isolating the control on a thread without a message pump and making the application unstable.

You can protect against this case by also checking the value of Control.IsHandleCreated when Control.InvokeRequired returns false on a background thread. If the control handle has not yet been created, you must wait until it has been created before calling Control.Invoke(Delegate) or Control.BeginInvoke(Delegate). Typically, this happens only if a background thread is created in the constructor of the primary form for the application (as in Application.Run(new MainForm()), before the form has been shown or Application.Run has been called.

One solution is to wait until the form's handle has been created before starting the background thread. Either force handle creation by calling the Control.Handle property, or wait until the Form.Load event to start the background process.

An even better solution is to use the SynchronizationContext returned by System.Threading.SynchronizationContext rather than a control for cross-thread marshaling.

For more information about multithreaded Windows Forms controls, see How to: Use a Background Thread to Search for Files and How to: Make Thread-Safe Calls to Windows Forms Controls.