If you've used the the Silverlight 2 DataGrid, you've no doubt seen this:

I really hate when this happens!  I've seen several forum discussions where folks were looking to get rid of this nastiness, so I know I'm not the only one losing sleep over it.  I haven't come across a solution in anything I've seen and so now that I have one, I thought I'd share it. 

I ran into this a few weeks back and tried most of the obvious things with defining the DataGridColumns myself rather than auto-generating them.  In these defined DataGridColumns, I tried setting the Width of the last column to "*" just like you would a standard Grid.ColumnDefinition if you wanted to have it take up the remainder of the space.  This did nothing more than invite my buddy, AG_E_PARSER_BAD_PROPERTY, to show up...again.  It appears that star-sizing isn't yet implemented for DataGridColumn derivatives. 

This morning I was finally able to grab a few minutes to dive into DataGrid and find out what I could do about this.  I decided to create an ExtendedDataGrid and add a LastColumnFill DependencyProperty similar to way the DockPanel has a LastChildFill. Most of the work is happening in this method which is called if the LastColumnFill is true:

private void AdjustLastColumnWidth(Size finalSize)
{
    // get the Vertical ScrollBar
    ScrollBar scrollBar = this.GetTemplateChild("VerticalScrollbar") as ScrollBar;

    // compute the width to allow for the scrollbar based on it's visibility.
    double scrollBarWidthAllowance = (scrollBar != null && scrollBar.Visibility == Visibility.Visible) ? scrollBar.ActualWidth + 2 : 2;

    // compute the width of all the columns excluding the last one
    var widthOfAllButLastColumn = this.Columns
                   .TakeWhile(c => c != Columns.Last() &&
                                            c.Visibility == Visibility.Visible)
                   .Sum(c => c.ActualWidth);

     // set the last column width    
     this.Columns.Last().Width = new DataGridLength(finalSize.Width - widthOfAllButLastColumn - scrollBarWidthAllowance);
}

This is the result:

Much better!  The live demo shows it with and without a vertical scrollbar.  Note that it also handles auto-generated DataGridColumns if that's your thang.

Here's the live demo.

Here's the code.