Adding a Horizontal Scrollbar to a ComboBox using C#

• Download project - 11.88 KB

In our WebCopy application, we decided to update the User Agent configuration to allow selection from a predefined list of common agents, but still allow the user to enter their own custom agent if required.

Rather than use two separate fields, we choose to use a ComboBox in simple mode, which is both a textbox and a listbox in a single control. This mode seems somewhat out of fashion, I think the only place I see it used is in the Font common dialog, virtually unchanged since Windows 3.1.

The problem was immediately apparent however on firing up WebCopy and going to select a user agent - the agent strings can be very long, far longer than the width of the control.

Unfortunately however, the .NET ComboBox doesn't allow you to directly enable horizontal scrolling. So we'll do it the old fashioned way using the Windows API.

In order for a window to support horizontal scrolling, it needs to have the WS_HSCROLL style applied to it. And to setup the horizontal scrollbar, we need to call the SendMessage API with the CB_SETHORIZONTALEXTENT message.

As usual, we'll be starting off by creating a new Component, which we'll inherit from ComboBox.

Traditionally, you would call GetWindowLong and SetWindowLong APIs with the GWL_STYLE or GWL_EXSTYLE flags. However, we can more simply override the CreateParams property of our component and set the new style when the control is created.

protected override CreateParams CreateParams
{
  get
  {
    CreateParams createParams;

    createParams = base.CreateParams;
    createParams.Style |= WS_HSCROLL;

    return createParams;
  }
}

With that done, we can now inform Windows of the size of the horizontal scroll area, and it will automatically add the scrollbar if required. To do this, I'll add two new methods to the component. The first will set the horizontal extent to a given value. The second will calculate the length of the longest piece of text in the control and then set the extent to match.

public void SetHorizontalExtent()
{
  int maxWith;

  maxWith = 0;
  foreach (object item in this.Items)
  {
    Size textSize;

    textSize = TextRenderer.MeasureText(item.ToString(), this.Font);
    if (textSize.Width > maxWith)
      maxWith = textSize.Width;
  }

  this.SetHorizontalExtent(maxWith);
}

public void SetHorizontalExtent(int width)
{
  SendMessage(this.Handle, CB_SETHORIZONTALEXTENT, new IntPtr(width), IntPtr.Zero);
}

The first overload of SetHorizontalExtent iterates through all the items in the control and uses the TextRenderer object to measure the size of the text. Once it has found the largest piece of text, it calls the second overload with the size.

The second overload does the actual work of notifying Windows using the SendMessage call, CB_SETHORIZONTALEXTENT message and the given width. SendMessage takes two configuration parameters per message, but CB_SETHORIZONTALEXTENT only requires one, and so we send 0 for the second.

The above function works with all display modes of the ComboBox.

For completeness, here are the API declarations we are using:

private const int WS_HSCROLL = 0x100000;
private const int CB_SETHORIZONTALEXTENT = 0x015E;

[DllImport("user32.dll")]
private static extern IntPtr SendMessage(IntPtr hWnd, UInt32 msg, 
                                         IntPtr wParam, IntPtr lParam);

As usual, a demonstration project is available from the link at the top of this post.