Question

The problem:

My application requires a user to be able to select multiple entries in a datagrid via a column of checkboxes. The desired behavior is that when you click on a checkbox in the column, it behaves like a normal checkbox, but if you drag over it while the left mouse button is down, its selection state changes to the opposite of what is was before.

What I have tried so far:

I have tried subclassing CheckBox and handling OnMouseEnter, but the first checkbox that is clicked seems to capture the mouse so no other checkboxes fire the OnMouseEnter event.

I have tried implementing a drag-and-drop hack, where the user clicks to select a checkbox, and then drags that checkbox over the others so the others recieve a DragOver event and can switch states. This solution causes the cursor to display as a circle with a slash when not over another checkbox during the drag and drop, which is not acceptable for this application.

What I would like:

I would like a method to implement a checkbox that has the functionality I describe, ideally in a xaml style or subclass that I can reuse, as this functionality is needed in multiple places in my application.

Is there an elegant way to achieve this effect?

Answer 1

I did this in my application, very handy when you have to select, say, 30 checkBoxes.
To do this, i handled the preview mouse event myself : PreviewMouseLeftButtonDown, PreviewMouseMove, PreviewMouseLeftButtonUp.

In PreviewMouseLeftButtonDown : i get the mouse position relative to the control.
In PreviewMouseMove : i draw a rectangle from start to current position if i am far enough from firstPoint. then i iterate in CheckBoxes, see if they intersect with rectangle, and highlight them if so (so the user know whiwh chexboxes will swap)
In PreviewMouseLeftButtonUp : i do the swap for intersecting CheckBoxes.

if it can help you, here’s the code i use. it is not MVVM (:-)) but works fine, it might give you ideas. it is an automatic translation from vb.net code.

To make it work, you need a Canvas on top of your CheckBoxes (=within the same grid cell for instance), with property IsHitTestVisible=”False” .
Within this Canvas, put a Rectangle nammed “SelectionRectangle” with proper fill and stroke, but with 0.0 Opacity.

// '' <summary>
// '' When Left Mouse button is pressed, remember where the mouse move start
// '' </summary>
private void EditedItems_PreviewMouseLeftButtonDown(object sender, System.Windows.Input.MouseButtonEventArgs e) {
    StartPoint = Mouse.GetPosition(this);
}

// '' <summary>
// '' When mouse move, update the highlight of the selected items.
// '' </summary>
private void EditedItems_PreviewMouseMove(object sender, System.Windows.Input.MouseEventArgs e) {
    if ((StartPoint == null)) {
        return;
    }
    PointWhereMouseIs = Mouse.GetPosition(this);
    Rect SelectedRect = new Rect(StartPoint, PointWhereMouseIs);
    if (((SelectedRect.Width < 20) 
                && (SelectedRect.Height < 20))) {
        return;
    }
    //  show the rectangle again
    Canvas.SetLeft(SelectionRectangle, Math.Min(StartPoint.X, PointWhereMouseIs.X));
    Canvas.SetTop(SelectionRectangle, Math.Min(StartPoint.Y, PointWhereMouseIs.Y));
    SelectionRectangle.Width = Math.Abs((PointWhereMouseIs.X - StartPoint.X));
    SelectionRectangle.Height = Math.Abs((PointWhereMouseIs.Y - StartPoint.Y));
    foreach (CheckBox ThisChkBox in EditedItems.Children) {
        object rectBounds = VisualTreeHelper.GetDescendantBounds(ThisChkBox);
        Vector vector = VisualTreeHelper.GetOffset(ThisChkBox);
        rectBounds.Offset(vector);
        if (rectBounds.IntersectsWith(SelectedRect)) {
            ((TextBlock)(ThisChkBox.Content)).Background = Brushes.LightGreen;
        }
        else {
            ((TextBlock)(ThisChkBox.Content)).Background = Brushes.Transparent;
        }
    }
}

// '' <summary>
// '' When Left Mouse button is released, change all CheckBoxes values. (Or do nothing if it is a small move -->
// '' click will be handled in a standard way.)
// '' </summary>
private void EditedItems_PreviewMouseLeftButtonUp(object sender, System.Windows.Input.MouseButtonEventArgs e) {
    PointWhereMouseIs = Mouse.GetPosition(this);
    Rect SelectedRect = new Rect(StartPoint, PointWhereMouseIs);
    StartPoint = null;
    SelectionRectangle.Opacity = 0;
    //  hide the rectangle again
    if (((SelectedRect.Width < 20) 
                && (SelectedRect.Height < 20))) {
        return;
    }
    foreach (CheckBox ThisEditedItem in EditedItems.Children) {
        object rectBounds = VisualTreeHelper.GetDescendantBounds(ThisEditedItem);
        Vector vector = VisualTreeHelper.GetOffset(ThisEditedItem);
        rectBounds.Offset(vector);
        if (rectBounds.IntersectsWith(SelectedRect)) {
            ThisEditedItem.IsChecked = !ThisEditedItem.IsChecked;
        }
        ((TextBlock)(ThisEditedItem.Content)).Background = Brushes.Transparent;
    }
}

Edit : i used that code within a user control. This control takes a list of booleans and a list of strings (caption) as argument, and builds (with a WrapPanel) an array of CheckBoxes having the right caption. And so you can select/unselect with the rectangle, and there are also two buttons to check all/uncheck all. I tried also to keep good column/rows ratio to handle selection of 7 to 200 booleans with a good column/row balance.