How to distinguish whether Enter event was raised by keyboard or mouse click?

Question

Is there a way to distinguish whether the Enter event on a control was raised by keyboard (Tab, Shift+Tab) or by direct mouse click?

I need to perform an action only when the user is moving to the control using Tab, but not when the user directly clicks on the control. I have tried to intercept the mouse click directly, but it seems the Enter event is raised before Click.

Answer 1

Instead of tracking the Tab key, you can use the WM_MOUSEACTIVATE message to detect activation of the control with the mouse. You could either sub-class each control type you use and override the WndProc method or use a NativeWindow listener class like the one presented below. Depending on how many types of controls you use, it may be less work and clutter to just sub-class those controls to provide a property that indicates that the control was selected using the mouse. It is your decision to make, but the pattern will be the same.

This code is a slight modification of the example shown in the MS documentation.

public class MouseActivateListener : NativeWindow
{
    private Control parent;

    public MouseActivateListener(Control parent)
    {
        parent.HandleCreated += this.OnHandleCreated;
        parent.HandleDestroyed += this.OnHandleDestroyed;
        parent.Leave += Parent_Leave;
        this.parent = parent;
        if (parent.IsHandleCreated)
        {
            AssignHandle(parent.Handle);
        }
    }

    private void Parent_Leave(object sender, EventArgs e)
    {
        MouseActivated = false;
    }

    private void OnHandleCreated(object sender, EventArgs e)
    {
        AssignHandle(((Form)sender).Handle);
    }

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

    public bool MouseActivated { get; set; }


    [System.Security.Permissions.PermissionSetAttribute(System.Security.Permissions.SecurityAction.Demand, Name = "FullTrust")]
    protected override void WndProc(ref Message m)
    {
        const Int32 WM_MouseActivate = 0x21;

        base.WndProc(ref m);

        if (m.Msg == WM_MouseActivate && m.Result.ToInt32() < 3)
        {
            MouseActivated = true;
        }
    }
}

Example Usage:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private MouseActivateListener textBox1Listener;
    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);
        textBox1Listener = new MouseActivateListener(textBox1);
    }

    private void textBox1_Enter(object sender, EventArgs e)
    {
        if (textBox1Listener.MouseActivated)
        {
            MessageBox.Show("Mouse Enter");
        }
        else
        {
            MessageBox.Show("Tab Enter");
        }
    }
}

Answer 2

You can use the Form.KeyPreview event and store the last key press in a variable. Then in your control's Enter event, check the value of the key that was pressed last. If this is a tab, do whatever you need to:

private Keys lastKeyCode;
private void Form1_KeyDown(object sender, KeyEventArgs e)
{
    this.lastKeyCode = e.KeyCode;
}

Then in the Enter event, check it:

if (lastKeyCode == Keys.Tab)
{
    // Whatever...
}

Answer 3

Intercepting WM_KEYUP and WM_KEYDOWN directly with a message filter to retrieve the state of the Tab key worked. This seems excessive for such a seemingly straightforward task, but apparently the Tab key is suppressed from most windows forms events.

Would be happy to take a cleaner answer, but for now, this is it:

class TabMessageFilter : IMessageFilter
{
    public bool TabState { get; set; }

    public bool PreFilterMessage(ref Message m)
    {
        const int WM_KEYUP = 0x101;
        const int WM_KEYDOWN = 0x100;

        switch (m.Msg)
        {
            case WM_KEYDOWN:
                if ((Keys)m.WParam == Keys.Tab) TabState = true;
                break;
            case WM_KEYUP:
                if ((Keys)m.WParam == Keys.Tab) TabState = false;
                break;
        }

        return false;
    }
}

class MainForm : Form
{
    TabMessageFilter tabFilter;

    public MainForm()
    {
         tabFilter = new TabMessageFilter();
         Application.AddMessageFilter(tabFilter);
    }

    protected override void OnFormClosed(FormClosedEventArgs e)
    {
         Application.RemoveMessageFilter(tabFilter);
         base.OnFormClosed(e);
    }

    void control_Enter(object sender, EventArgs e)
    {
         if (tabFilter.TabState) // do something
         else // do domething else
    }
}