Changing columns do not reflect at runtime

Question

I am having an issue when I change my ColumnDefinitions at runtime. When I do, the columns Width does not reflect immediately. If I resize the current window/usercontrol then the Width does reflect.

I am following MVVM as well, I have a DependencyProperty that allows me to change them as needed on the Grid.

Currently here is what I am using and does work if I resize the window and or control the Grid is on...

public static void ColumnSpacerChanged(
            DependencyObject obj, DependencyPropertyChangedEventArgs e)
        {
            if (!(obj is Grid) || ((Grid)obj).ColumnDefinitions.Count == 0 || (int)e.NewValue == (int)e.OldValue)
                return;

            Grid grid = (Grid)obj;
            for (int i = 0; i < grid.ColumnDefinitions.Count; i++)
            {
                if(grid.ColumnDefinitions[i].Tag != null && grid.ColumnDefinitions[i].Tag.ToString() == COLUMN_SPACER)
                {
                    grid.ColumnDefinitions[i] = new ColumnDefinition() { Width = new GridLength((int)e.NewValue, GridUnitType.Pixel), Tag = COLUMN_SPACER };
                }
            }

            grid.UpdateLayout(); // Tried this, but doesn't work

        }

I have tried the following and none work.

 grid.UpdateLayout();
 grid.Refresh; // (off the parent)

Is there something I am missing when I change the ColumnDefinitions during run-time they immediately do not reflect?

Answer 1

I found the solution with some help from @Mike Strobel....

InvalidateArrange()

This needs to be called to invalidate the arrange state (layout) for the element. After this invalidation the element will then have its layout updated and reflected asynchronously.