You can ty painting an error icon yourself. Here is some simple code
that displays an error unless you type "abc" in a cell.

       Rectangle errorRect = Rectangle.Empty;
       void dataGridView1_CellValidating(object sender,
DataGridViewCellValidatingEventArgs e)
       {
           if (e.FormattedValue.Equals("abc"))
           {
               e.Cancel = true;
               errorRect =
dataGridView1.GetCellDisplayRectangle(dataGridView1.CurrentCell.ColumnIndex,
dataGridView1.CurrentCell.RowIndex, true);
               errorRect.X = errorRect.X - 20;
               errorRect.Width = 20;
               dataGridView1.Refresh();
           }
           else
           {
               errorRect = Rectangle.Empty;
               dataGridView1.Refresh();
           }
       }

       void dataGridView1_CellPainting(object sender,
DataGridViewCellPaintingEventArgs e)
       {
           if (!errorRect.IsEmpty)
           {
               e.Paint(e.ClipBounds, DataGridViewPaintParts.All);
               e.Graphics.DrawIcon(SystemIcons.Error,  errorRect);
               e.Handled = true;
           }
       }

===============
Clay Burch
Syncfusion, Inc.

Hi Patrik,

I performed a test on this issue and saw the same thing as you did. The
reason why the error icon of a DataGridViewCell is not visible when the
DataGridViewCell is in edit mode is that the editing control hosted in the
DataGridViewCell covers the error icon.

I have tried to diminish the width of the hosted editing control, but
without success. Then I tried to use a ErrorProvider and set error text for
the hosted editing control. However, it turned out that the error icon of
the ErrorProvider didn't appear, because the hosted editing control seemed
as if it is docked in the DataGridViewCell.

In fact what you want is a notification to the user if there's an error
during cell validation. I think a solution is to use balloon ToolTip to
show error message.

The following is a sample. It requires you to add a ToolTip component on
the form and set the IsBalloon property of the ToolTip to true.

void dataGridView1_CellValidating(object sender,
DataGridViewCellValidatingEventArgs e)
       {
           if (e.FormattedValue.ToString() != "123")
           {
               if (this.dataGridView1.IsCurrentCellInEditMode)
               {
                   // To ensure the stem of the balloon ToolTip to point
to the target control for the first time, we need to call the ToolTip.Show
method twice. This is a known bug of ToolTip.
                   this.toolTip1.Show("",
this.dataGridView1.EditingControl);
                   this.toolTip1.Show("must be 123",
this.dataGridView1.EditingControl);
                   e.Cancel = true;
               }
           }
           else
           {
               if (this.dataGridView1.IsCurrentCellInEditMode)
               {
                   this.toolTip1.Show("",
this.dataGridView1.EditingControl);
               }
           }
       }      

Hope this helps.
If you have any question, please feel free to let me know.

Sincerely,
Linda Liu
Microsoft Online Community Support

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Thank you Linda and ClayB for your answers!

/Patrik