Windows ComboBox Intercept WM_COMMAND Notifications

Question

Updated to reflect a more functional description of my request.

Functionally I need a read only ComboBox that does not directly inherit from the System.Windows.Forms.ComboBox class. I need the text box portion to be read only, so the user can copy out text that was entered into the box but not change it. And this is not a case of using DropDown vs DropDownList mode.

Using this article and some experimentation, I have the text box portion taken care of but still need to prevent changes from the drop down component due to technical reasons listed below. I still need one of the following:

1. Disable the drop down button on a ComboBox - to prevent it from opening
2. Prevent the drop down from opening in a ComboBox when clicked
3. Prevent the selection of an item from the drop down from changing the selection in the control (possibly by intercepting and ignoring the clicked item notification message)

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I'm using the KryptonToolkit for visual prettiness in my application. For the KryptonComboBox control, it has an internal, extended System.Windows.Forms.ComboBox class that intercepts the draw calls. It exposes access to the ComboBox object, so I can get access to it's Handle.

I'm using the A Complete Read Only ComboBox code as a base to create a read only capable version of the KryptonComboBox. Instead of using the KryptonComboBox.Handle in the SendMessage() calls, you have to use the KryptonComboBox.ComboBox.Handle to get the actual handle to the Windows control. By doing that I'm able to set the read only mode on the text box part of the ComboBox correctly.

BUT! Since the base class of the extended control is KryptonComboBox and not ComboBox, when the WndProc() is called it's for messages of the KryptonComboBox. So I am unable to prevent the control from acting (changing the selection) on the clicked drop down item when it is in read only mode.

How do I intercept (and potentially ignore) WM_COMMAND messages from an existing ComboBox control. Specifically I want to have a message 273 chain ignored under certain circumstances. Is there a way to functionally do the same thing to an "external" ComboBox control that I could from an inherited control? Since I have the Handle for the ComboBox, could I use something like SetWindowsHookEx() to intercept the messages?

Answer 1

So I found a solution that works for "plugging into" the command messages for the control and being able to interrupt them. It involves using the System.Windows.Forms.NativeWindow class to create a listener to the ComboBox which is (from my understanding) called subclassing.

Listener Class:

private class ComboBoxListener : NativeWindow, IDisposable {
    private ComboBox m_oComboBox;

    private EventHandler m_oComboBox_HandleCreated;
    private EventHandler m_oComboBox_HandleDestroyed;

    public ComboBoxListener(ComboBox oComboBox) {
        // Save the combobox and assign the handle if it's already created
        m_oComboBox = oComboBox;
        if (m_oComboBox.IsHandleCreated) {
            AssignHandle(m_oComboBox.Handle);
        }

        // Subscribe to the handle events of the combobox
        m_oComboBox.HandleCreated += m_oComboBox_HandleCreated = new EventHandler(ComboBox_HandleCreated);
        m_oComboBox.HandleDestroyed += m_oComboBox_HandleDestroyed = new EventHandler(ComboBox_HandleDestroyed);
    }

    ~ComboBoxListener() {
        Dispose(false);
    }

    public void Dispose() {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    private void Dispose(bool bDisposing) {
        if (bDisposing && (m_oComboBox != null) && !m_oComboBox.IsDisposed) {
            // Unsubscribe from the combo box
            m_oComboBox.HandleCreated -= m_oComboBox_HandleCreated;
            m_oComboBox.HandleDestroyed -= m_oComboBox_HandleDestroyed;
        }
        m_oComboBox = null;

        // If a handle is currently assigned, release it
        if (this.Handle != IntPtr.Zero) {
            ReleaseHandle();
        }
    }

    private void ComboBox_HandleCreated(object sender, EventArgs e) {
        AssignHandle(m_oComboBox.Handle);
    }

    private void ComboBox_HandleDestroyed(object sender, EventArgs e) {
        ReleaseHandle();
    }

    protected override void WndProc(ref Message m) {
        // Do checks for whether to ignore the message
        // If ignoring, just call return and skip the base.WndProc(ref m) call
        base.WndProc(ref m);
    }
}

Then in the constructor for the deriving class from KryptonComboBox I added:

m_oComboBoxListener = new ComboBoxListener(this);

And added a cleanup in the overridden Dispose(bool disposing):

// Dispose the listener
if (disposing && (m_oComboBoxListener != null)) {
    m_oComboBoxListener.Dispose();
}
m_oComboBoxListener = null;