Cursor.Current vs. this.Cursor

Question

Is there a difference between Cursor.Current and this.Cursor (where this is a WinForm) in .Net? I've always used this.Cursor and have had very good luck with it but I've recently started using CodeRush and just embedded some code in a "Wait Cursor" block and CodeRush used the Cursor.Current property. I've seen on the Internet and at work where other programmers have had some problems with the Cursor.Current property. It just got me to wondering if there is a difference in the two. Thanks in advance.

I did a little test. I have two winforms. I click a button on form1, set the Cursor.Current property to Cursors.WaitCursor and then show form2. The cursor doesn't change on either form. It remains Cursors.Default (pointer) cursor.

If I set this.Cursor to Cursors.WaitCursor in the button click event on form1 and show form2, the wait cursor only shows on form1 and the default cursor is on form2 which is expected. So, I still don't know what Cursor.Current does.

Solution 1

Windows sends the window that contains the mouse cursor the WM_SETCURSOR message, giving it an opportunity to change the cursor shape. A control like TextBox takes advantage of that, changing the cursor into a I-bar. The Control.Cursor property determines what shape will be used.

The Cursor.Current property changes the shape directly, without waiting for a WM_SETCURSOR response. In most cases, that shape is unlikely to survive for long. As soon as the user moves the mouse, WM_SETCURSOR changes it back to Control.Cursor.

The UseWaitCursor property was added in .NET 2.0 to make it easier to display an hourglass. Unfortunately, it doesn't work very well. It requires a WM_SETCURSOR message to change the shape and that won't happen when you set the property to true and then do something that takes a while. Try this code for example:

private void button1_Click(object sender, EventArgs e) {
  this.UseWaitCursor = true;
  System.Threading.Thread.Sleep(3000);
  this.UseWaitCursor = false;
}

The cursor never changes. To whack that into shape, you'll need to use Cursor.Current as well. Here is a little helper class to make it easy:

using System;
using System.Windows.Forms;

public class HourGlass : IDisposable {
  public HourGlass() {
    Enabled = true;
  }
  public void Dispose() {
    Enabled = false;
  }
  public static bool Enabled {
    get { return Application.UseWaitCursor; }
    set {
      if (value == Application.UseWaitCursor) return;
      Application.UseWaitCursor = value;
      Form f = Form.ActiveForm;
      if (f != null && f.Handle != IntPtr.Zero)   // Send WM_SETCURSOR
        SendMessage(f.Handle, 0x20, f.Handle, (IntPtr)1);
    }
  }
  [System.Runtime.InteropServices.DllImport("user32.dll")]
  private static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wp, IntPtr lp);
}

And use it like this:

private void button1_Click(object sender, EventArgs e) {
  using (new HourGlass()) {
    System.Threading.Thread.Sleep(3000);
  }
}

Solution 2

I believe that Cursor.Current is the mouse cursor currently being used (regardless of where it is on the screen), while this.Cursor is the cursor it will be set to, when the mouse passes over your window.

Solution 3

this.Cursor is the cursor that will be used when the mouse is over the window referred to by this. Cursor.Current is the current mouse cursor, which might be different from this.Cursor if the mouse is over a different window.

Solution 4

Actually if you would like to use HourGlass from another thread that will give you back cross-threading exception because you are trying to access f.Handle from different thread than form was originally created. Use GetForegroundWindow() instead from user32.dll.

[DllImport("user32.dll")]
private static extern IntPtr GetForegroundWindow();

and then

public static bool Enabled
{
    get
    {
        return Application.UseWaitCursor;
    }

    set
    {
        if (value == Application.UseWaitCursor)
        {
            return;
        }

        Application.UseWaitCursor = value;
        var handle = GetForegroundWindow();
        SendMessage(handle, 0x20, handle, (IntPtr)1);
    }
}

Solution 5

I have noticed an interesting thing about setting cursors, so I would like to clear some misunderstandings that I myself had before and I hope it may help others too:

When you try to set a form's cursor by using

this.cursor = Cursors.Waitcursor

you actually set the cursor for the control and not the whole form since cursor is property of the Control class.

Also of course the cursor will only be changed to the given cursor when the mouse is actually over the actual control (explicitly the form's area)

As Hans Passant has already stated that:

Windows sends the window that contains the mouse cursor the WM_SETCURSOR message, giving it an opportunity to change the cursor shape

I don't know if windows sends messages directly to controls or if the form relays those messages to it's child controls based on mouse position, I'd most likely guess on the first method since when i fetched the messages with overriding WndProc of the form control, when i was over the textbox for example, the form didn't process any messages. (please someone give clarity on this)

Basically my suggestion would be to reside from using this.cursor also and stick to this.usewaitcursor, since that changes the cursor property to waitcursor for all child controls.

The problem with this is also the same as with the application level Application.usewaitcursor, while you are not over the form/forms with your cursor no WM_SETCURSOR message is being sent by windows, so if you start a time consuming synchronous operation before moving your mouse over the form's area, the form can only process such message when the time consuming synchronous operation finishes.

(I would not suggest running time consuming tasks in the UI thread at all, mainly this is what is causing the issue here)

I made a little improvement on Hans Passant's answer, so the hourglass can be either set on application level or form level, also avoiding InvalidOperationException from cross threaded operation calls:

using System;
using System.Windows.Forms;

public class HourGlass : IDisposable
{
    public static bool ApplicationEnabled
    {
        get{ return Application.UseWaitCursor; }
        set
        {
            Form activeFrom = Form.ActiveForm;
            if (activeFrom == null || ApplicationEnabled == value) return;
            if (ApplicationEnabled == value)return;
            Application.UseWaitCursor = (bool)value;

            if (activeFrom.InvokeRequired)
            {
                activeFrom.BeginInvoke(new Action(() =>
                {
                    if (activeFrom.Handle != IntPtr.Zero)
                    SendMessage(activeFrom.Handle, 0x20, activeFrom.Handle, (IntPtr)1); // Send WM_SETCURSOR
                }));
            }
            else
            {
                if (activeFrom.Handle != IntPtr.Zero)
                SendMessage(activeFrom.Handle, 0x20, activeFrom.Handle, (IntPtr)1); // Send WM_SETCURSOR
            }
        }
    }

    private Form f;

    public HourGlass() 
    {
        this.f = Form.ActiveForm;

        if (f == null)
        {
            throw new ArgumentException();
        }
        Enabled = true;
    }

    public HourGlass(bool enabled)
    {
        this.f = Form.ActiveForm;

        if (f == null)
        {
            throw new ArgumentException();
        }
        Enabled = enabled;
    }

    public HourGlass(Form f, bool enabled)
    {
        this.f = f;

        if (f == null)
        {
            throw new ArgumentException();
        }
        Enabled = enabled;
    }

    public HourGlass(Form f)
    {
        this.f = f;

        if (f == null)
        {
            throw new ArgumentException();
        }

        Enabled = true;
    }

    public void Dispose()
    {
        Enabled = false;
    }

    public bool Enabled
    {
        get { return f.UseWaitCursor; }
        set
        {
            if (f == null || Enabled == value) return;
            if (Application.UseWaitCursor == true && value == false) return;

            f.UseWaitCursor = (bool)value;

            if(f.InvokeRequired)
            {
                f.BeginInvoke(new Action(()=>
                {
                    if (f.Handle != IntPtr.Zero)
                    SendMessage(f.Handle, 0x20, f.Handle, (IntPtr)1); // Send WM_SETCURSOR
                }));
            }
            else
            {
                if (f.Handle != IntPtr.Zero)
                SendMessage(f.Handle, 0x20, f.Handle, (IntPtr)1); // Send WM_SETCURSOR
            }
        }
    }

    [System.Runtime.InteropServices.DllImport("user32.dll")]
    private static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wp, IntPtr lp);
}

To use it on application level:

try
{
  HourGlass.ApplicationEnabled = true;
  //time consuming synchronous task
}
finally
{
  HourGlass.ApplicationEnabled = false;
}

For using it on form level you can either use for the current active form:

using (new HourGlass())
{
  //time consuming synchronous task
}

or you can initialize a local variable in the form like this:

public readonly HourGlass hourglass;

public Form1()
{
    InitializeComponent();
    hourglass = new HourGlass(this, false);
}

and use it later in a try catch finally block

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