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column.ValueType = typeof(double);					

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schnipp
   NpgsqlDataAdapter da = new NpgsqlDataAdapter(sql, conn);
   
                ds_auftraguebersicht.Reset();       
                da.Fill(ds_auftraguebersicht);           
                dt_auftraguebersicht = ds_auftraguebersicht.Tables[0];      
                dataGridView2.DataSource = dt_auftraguebersicht;             
                conn.Close();

                if (Form1.kunde != "mayer")
                {
                    auftrag_dataviewii();
                }
                else
                {
                    dataGridView2.Columns[0].Width = 40;
                    dataGridView2.Columns[1].Width = 70;
                    dataGridView2.Columns[2].Width = 70;  //renummer
                    dataGridView2.Columns[3].Width = 70;
                    dataGridView2.Columns[4].Width = 80;   //tel vorkasse
                    dataGridView2.Columns[4].DefaultCellStyle.Alignment = DataGridViewContentAlignment.MiddleRight;
                    dataGridView2.Columns[5].Width = 80;      //tel 2 zahlungsart
                    dataGridView2.Columns[5].DefaultCellStyle.Alignment = DataGridViewContentAlignment.MiddleRight;
                    dataGridView2.Columns[5].ValueType = typeof(double);  //bringt leider keine veränderung
                    dataGridView2.Columns[6].Width = 100;        //
                    dataGridView2.Columns[7].Width = 130; //bez
schnipp					

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Random rnd = new Random(DateTime.Now.Millisecond);
DataTable table = new DataTable();            
table.Columns.Add(new DataColumn() { ColumnName = "Zahlungsart", DataType = typeof(double) });
dataGridView1.DataSource = Enumerable.Range(1,10).Select(x => { var row = table.NewRow(); row["Zahlungsart"] = rnd.NextDouble(); return row; }).CopyToDataTable<DataRow>();