If a DataGrid.DataSource contains multiple sources of data, you should set the DataGrid.DataMember to one of the sources. For example, if the DataGrid.DataSource is a System.Data.DataSet or System.Data.DataViewManager that contains three tables named Customers, Orders, and OrderDetails, you must specify one of the tables to bind to. If the System.Data.DataSet or System.Data.DataViewManager contains only one System.Data.DataTable, you should set the DataGrid.DataMember to the System.Data.DataTable.TableName of that System.Data.DataTable.

If the DataGrid.DataSource is set to a System.Data.DataSet that contains System.Data.DataRelation objects, parent tables will appear with a plus sign (+) in each row header. Clicking the plus sign causes a node to appear that contains links to child tables. For example, if a System.Data.DataSet contains two System.Data.DataTable objects named Customers and Orders, setting the DataGrid.DataMember to the Customers table causes the System.Windows.Forms.DataGrid to display a parent table with a plus sign visible on each row header. If the DataGrid.DataMember is set to Orders, however, the row headers will be blank.

If the DataGrid.DataSource is a System.Data.DataTable, System.Data.DataView, collection, or array, setting the DataGrid.DataMember property throws an exception.