判断DataTable里面数据是否有重复数据

今天在做一个权限管理模块里面，有关权限分配的时候，涉及到重复权限的判断，故以此文记之。

//添加
        void btnAdd_Click(object sender, EventArgs e)
        {
            if (_dt == null) return;
            //把选中的权限添加到权限树上  
            //把选中的权限添加到权限树上  
            DataRow row, newRow;
            foreach (TreeNode itemp in TreeViewAllMenu.Nodes)
            {
                if (itemp.Checked) //判断是否选中
                {                                       
                    row = (DataRow)itemp.Tag;
                    newRow = _dt.NewRow();
                    newRow["Nodeid"] = row["NodeId"];
                    newRow["ParentNode"] = row["ParentNode"];
                    newRow["NodeName"] = row["NodeName"];
                    newRow["Isendnode"] = row["Isendnode"];
                    //权限重复判断 
                    string str = "NodeName='" + row["NodeName"] + "'";
                    //模糊查询 //string str=" id like 's%'";
                    DataRow[] drws = _dt.Select(str);
                    if (drws.Length > 0)
                    {
                        XtraMessageBox.Show("不能重复添加权限！","系统提示：");
                        continue; 
                    }                    
                    
                    _dt.Rows.Add(newRow);
                    foreach (TreeNode itemch in itemp.Nodes)
                    {
                        if (itemch.Checked)
                        {
                            row = (DataRow)itemch.Tag;
                            newRow = _dt.NewRow();
                            newRow["Nodeid"] = row["Nodeid"];
                            newRow["ParentNode"] = row["ParentNode"];
                            newRow["NodeName"] = row["NodeName"];
                            newRow["Isendnode"] = row["Isendnode"];
                            _dt.Rows.Add(newRow);
                            foreach (TreeNode itemMenu in itemch.Nodes)
                            {
                                if (itemMenu.Checked)
                                {
                                    row = (DataRow)itemMenu.Tag;
                                    newRow = _dt.NewRow();
                                    newRow["Nodeid"] = row["Nodeid"];
                                    newRow["ParentNode"] = row["ParentNode"];
                                    newRow["NodeName"] = row["NodeName"];
                                    newRow["Isendnode"] = row["Isendnode"];
                                    _dt.Rows.Add(newRow);
                                }
                            }
                        }
                    }
                }
            }
            TreeViewBinding(_dt, TreeViewAlready);
        }