DataGridView: System.InvalidCastException, Value is not a convertible object

Hello there,

is isn't just a General question but I think it fit's best here...

I'm using WinForm and I add custom objects to a datagrid, after that I get
this exception in the Paint routine of the datagrid: 

Unhandled Exception: System.InvalidCastException: Value is not a convertible
object: MyNamespace.MyClass to System.String
  at System.Convert.ToType (System.Object value, System.Type conversionType,
IFormatProvider provider) [0x00000] 
  at System.Convert.ChangeType (System.Object value, System.Type
conversionType, IFormatProvider provider) [0x00000] 
  at System.Windows.Forms.DataGridViewCell.get_FormattedValue () [0x00000] 
  at System.Windows.Forms.DataGridViewRow.PaintCells
(System.Drawing.Graphics graphics, Rectangle clipBounds, Rectangle
rowBounds, Int32 rowIndex, DataGridViewElementStates rowState, Boolean
isFirstDisplayedRow, Boolean isLastVisibleRow, DataGridViewPaintParts
paintParts) [0x00000] 
  at System.Windows.Forms.DataGridViewRow.Paint (System.Drawing.Graphics
graphics, Rectangle clipBounds, Rectangle rowBounds, Int32 rowIndex,
DataGridViewElementStates rowState, Boolean isFirstDisplayedRow, Boolean
isLastVisibleRow) [0x00000] 
  at System.Windows.Forms.DataGridView.OnPaint
(System.Windows.Forms.PaintEventArgs e) [0x00000] 
  at System.Windows.Forms.Control.WmPaint (System.Windows.Forms.Message& m)
[0x00000] 
  at System.Windows.Forms.Control.WndProc (System.Windows.Forms.Message& m)
[0x00000] 
  at System.Windows.Forms.DataGridView.WndProc
(System.Windows.Forms.Message& m) [0x00000] 
  at System.Windows.Forms.Control+ControlWindowTarget.OnMessage
(System.Windows.Forms.Message& m) [0x00000] 
  at System.Windows.Forms.Control+ControlNativeWindow.WndProc
(System.Windows.Forms.Message& m) [0x00000] 
  at System.Windows.Forms.NativeWindow.WndProc (IntPtr hWnd, Msg msg, IntPtr
wParam, IntPtr lParam) [0x00000] 
  at System.Windows.Forms.XplatUIX11.DispatchMessage
(System.Windows.Forms.MSG& msg) [0x00000] 
  at System.Windows.Forms.XplatUI.DispatchMessage (System.Windows.Forms.MSG&
msg) [0x00000] 
  at System.Windows.Forms.Application.RunLoop (Boolean Modal,
System.Windows.Forms.ApplicationContext context) [0x00000] 
  at System.Windows.Forms.Application.Run
(System.Windows.Forms.ApplicationContext context) [0x00000] 
  at System.Windows.Forms.Application.Run (System.Windows.Forms.Form
mainForm) [0x00000] 
  at Alias.Program.Main (System.String[] args) [0x00000]

i never saw such a problem with MS .Net, but I had overloaded the ToString
method to show what I want in the datagrid: 

public override string ToString()
{
  return ("What I want!");                           
}
that worked well with M$... 

now mono makes problem, does anyone of you have a workaround? 

I also implemented the 
public static implicit operator String( MyClass t )
{
 return t.ToString();
} 
to get around the problem, but that doesn't work, too. 
-- 
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