asp.net C#在单击按钮时将值从gridview传递到texbox

我有一个简单的gridview，每行都有一个按钮，我想在标签中传递或显示user_full_name_ar的值，我尝试使用如下所示的javascript函数来实现这一点，但它没有显示它显示为null的数据

gridview代码：

<asp:Label ID="Label5" Text='transfer text here' runat ="server" />

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" DataKeyNames="user_name" DataSourceID="SqlDataSource1" Height="100px" Width="383px">
    <Columns>
        <asp:BoundField DataField="user_name" HeaderText="user_name" ReadOnly="True" SortExpression="user_name" />
        <asp:BoundField DataField="user_full_name_ar" HeaderText="user_full_name_ar" SortExpression="user_full_name_ar" />

        <asp:TemplateField HeaderText="user_full_name_ar"  SortExpression="user_full_name_ar">
           <ItemTemplate>
                <asp:Label ID="Label4" Text='<%# Session["lang"].ToString() == "en"? Eval("user_full_name_en") : Eval("user_full_name_ar")  %>' runat ="server" />
           </ItemTemplate>
        </asp:TemplateField>

        <asp:TemplateField>
           <ItemTemplate>
            <asp:Button ID="ButtonSearch" runat="server" text="select"  OnClientClick ="txt();" CommandArgument ='<%# Bind("user_name") %>' />
            </ItemTemplate>
        </asp:TemplateField>

    </Columns>
</asp:GridView>

用于将gridview中label4的值显示为gridview外部label5的javascript函数：

<script  type ="text/javascript" >
    function txt() {

        var t = document.getElementById("Label4");
        document.getElementById("Label5") = t.value;
    }
</script>

函数txt（）{
var t=document.getElementById（“Label4”）；
document.getElementById（“Label5”）=t.value；
}

您可以在gridview中使用CommandField

 <asp:Label ID="Label5" Text='transfer text here' runat ="server" />

        <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" DataKeyNames="user_name" DataSourceID="SqlDataSource1" Height="100px" Width="383px">
    <Columns>
    <asp:CommandField HeaderText="select" SelectText="select " ShowSelectButton="True">
                          </asp:CommandField>
        <asp:BoundField DataField="user_name" HeaderText="user_name" ReadOnly="True" SortExpression="user_name" />
        <asp:BoundField DataField="user_full_name_ar" HeaderText="user_full_name_ar" SortExpression="user_full_name_ar" />

              <asp:TemplateField HeaderText="user_full_name_ar"  SortExpression="user_full_name_ar">
                   <ItemTemplate>
                        <asp:Label ID="Label4" Text='<%# Session["lang"].ToString() == "en"? Eval("user_full_name_en") : Eval("user_full_name_ar")  %>' runat ="server" />
                   </ItemTemplate>
                   </asp:TemplateField>

        <asp:TemplateField>
       <ItemTemplate>
<asp:Button ID="ButtonSearch" runat="server" text="select"  OnClientClick ="txt();" CommandArgument ='<%# Bind("user_name") %>' />
         </ItemTemplate>
         </asp:TemplateField>

    </Columns>
</asp:GridView>

Label5.Text = GridView1.SelectedRow.Cells[2].Text;