DataTable定義キー列

workTable.PrimaryKey = new DataColumn[] {workTable.Columns["CustID"]};

// Or

DataColumn[] columns = new DataColumn[1];
columns[0] = workTable.Columns["CustID"];
workTable.PrimaryKey = columns;

workTable.PrimaryKey = new DataColumn[] {workTable.Columns["CustLName"], 
                                         workTable.Columns["CustFName"]};

// Or

DataColumn[] keyColumn = new DataColumn[2];
keyColumn[0] = workTable.Columns["CustLName"];
keyColumn[1] = workTable.Columns["CustFName"];
workTable.PrimaryKey = keyColumn;