How to populate datagridview with datasource

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C#

hi,
I have a datagridview bound to a database-table. the dgv shows data of an item,
where the item could be everything.
when adding a new item the dgv's DataSource has 0 rows, the dgv just shows the columns (correctly).
to add data to the item, I use

Item_CellMouseDown

to add the items id
to the new row, because the id-column is not visible in dgv.

below a two versions of the method and none works correctly.

code A :

private void Item_CellMouseDown(object sender, DataGridViewCellMouseEventArgs e)
{
    int row = e.RowIndex;

    DataRow dr;
    if (row == SourceTable.Rows.Count) {
       dr = SourceTable.NewRow();
       SourceTable.Rows.Add(dr);

       dr["ID"] = this.ID;
    }
}

code B :

private void Item_CellMouseDown(object sender, DataGridViewCellMouseEventArgs e)
{
    int row = e.RowIndex;

    object  id = Item["Id",row].Value;
    if (id == null || id.ToString() == string.Empty) {
       Item["Id",row].Value = this.ID;
    }
}

What I have tried:

code A : Item shows three rows, SourceTable has two rows

code B : Item shows one row, SourceTable has 0 rows

however it should be Item shows two rows, SourceTable has 1 row

what am I missing or what am I doing wrong here ?

thanks in advance
franz