Combobox selected value not getting

Question

This is function that I used to load combobox. I can load combobox but when I try to get selectedvalue of combobox it showing null; I am not getting the actual value.

 

public static DataTable GetComboBoxedDataTable(DataTable oldDataTable, string valueColumn, string textColumn, string topRowValue, string topRowText, ComboBox cmb) { DataTable newDataTable = new DataTable();     newDataTable.Columns.Add(valueColumn);     newDataTable.Columns.Add(textColumn); foreach (DataRow oldDR in oldDataTable.Rows) { DataRow newDR = newDataTable.NewRow();         newDR[0] = oldDR[valueColumn].ToString();         newDR[1] = oldDR[textColumn].ToString();         newDataTable.Rows.InsertAt(newDR, newDataTable.Rows.Count); } // Add your 'Select an item' option at the top DataRow dr = newDataTable.NewRow();     dr[0] = topRowValue;     dr[1] = topRowText;     newDataTable.Rows.InsertAt(dr, 0);      cmb.ValueMember = valueColumn;     cmb.DisplayMember = textColumn; return newDataTable; }

the code where i am in need of combobox.selectedvalue

 

private void cmbroomno_SelectedIndexChanged(object sender, EventArgs e) { try { object[,] ParamArray = new object[,] { { "@RoomID", cmbroomno.SelectedValue } };---code where i need selected value                 DataSet ds = new DataSet();                 ds = DB.ExecuteQuery_SP("SelectGuestDetailsForService", ParamArray);