Apparently this is, like, the problem for the ages or something, but noone on this planet has come up with a way to automatically size the columns in a DataGrid.  Sure, there's all sorts of pyscho ways that involve measuring the length of strings in DataSets with the Graphics context, yada yada.  But, since I'm binding strongly-typed Object Collections to DataGrids in WinForm apps, that doesn't work for me (and it's a little over the top, IMHO).

So, I thought about it like this:

• If you double click on the little splitter between columns they will autosize.
• Therefore, the code to autosize has been written for me; no need to measure strings, etc.
• How do I force a double click? No, wait, wrongheadedness, how do I call whatever THEY call when a double click happens?
• So, I reflectored into DataGrid.OnMouseDown and saw their custom HitTest calls a private ColAutoResize.  Ah, bingo.

If you're going to 'sin' do it with style - do it with Reflection.

private void dgLogging_DataSourceChanged(object sender, System.EventArgs e)
        {
            try
            {
                Type t = dgLogging.GetType();
                MethodInfo m = t.GetMethod("ColAutoResize",BindingFlags.NonPublic);

                for (int i = dgLogging.FirstVisibleColumn; (i < dgLogging.VisibleColumnCount); i++)
                {
                    m.Invoke(dgLogging, new object[]{i});
                }
            }
            catch (Exception ex)
            {
                System.Diagnostics.Trace.Write("Failed Resizing Columns: " + ex.ToString());
            }
        }