一、C#中FindByValue函数的用法：
ListItem item = DropDownList1.Items.FindByValue(theme);
根据指定的值("theme")获取 DropDownList 中对应的项。
二、FindControl的使用方法：
Control.FindControl (String)：在当前的命名容器中搜索带指定 id 参数的服务器控件。（有点类似javascript中的getElementById(string)）
简单的例子：
<form id="form1" runat="server">
<div>
<asp:TextBox ID="TextBox1" runat="server">TextBox</asp:TextBox>
<asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
<br />
<asp:Label ID="Label1" runat="server" Text="Label"></asp:Label></div>
</form>

如果需要获得页面中的"TextBox1",代码中可以使用this.TextBox1来引用，这里我们使用FindControl:

protected void Button1_Click(object sender, EventArgs e)
{
//Control c = this.FindControl("TextBox1");
//TextBox tb= (TextBox)c;
//FindControl返回的是一个Control类型的控件，需要强制类型转化成TextBox类型
TextBox tb=(TextBox)this.FindControl("TextBox1");
this.Label1.Text = tb.Text; }

当TextBox1放到其他控件里应该怎么查找呢？

<div>

<asp:Panel ID="Panel1" runat="server" Height="50px" ;125px">
<asp:TextBox ID="TextBox1" runat="server">TextBox</asp:TextBox>
<asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
<asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
</asp:Panel>
</div>

当TextBox1放到Panel里，似乎没什么影响 TextBox tb=(TextBox)this.FindControl("TextBox1")，当查看生存页

面的HTML代码是发现，TextBox的ID并没有改变，所以可以获得TextBox1。

<div>

<div id="Panel1" style="height:50px;;">

<input name="TextBox1" type="text" value="http://www.mamicode.com/TextBoxdsd" id="TextBox1" />
<span id="Label1">TextBoxdsd</span>
<input type="submit" name="Button1" value="http://www.mamicode.com/Button" id="Button1" />

</div>
</div>

当TextBox1放到DataGrid中
<asp:DataGrid ID="dg1" runat="server" OnSelectedIndexChanged="dg1_SelectedIndexChanged">
<Columns>
<asp:TemplateColumn>
<ItemTemplate>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
</ItemTemplate>
</asp:TemplateColumn>
<asp:ButtonColumn CommandName="Select" Text="选择"></asp:ButtonColumn>
</Columns>
</asp:DataGrid>

这时候this.FindControl("TextBox1")==null,无法获得TextBox1,查看生成页面HTML发现，页面有多个
<input name="dg1$ctl02$TextBox1" type="text" id="dg1_ctl02_TextBox1" />
<input name="dg1$ctl03$TextBox1" type="text" id="dg1_ctl03_TextBox1" />
TextBox1隐藏了，给DataGrid添加选择列，通过以下方法获得被选择行的TextBox1

protected void dg1_SelectedIndexChanged(object sender, EventArgs e)
{
Control c = this.dg1.Items[this.dg1.SelectedIndex].FindControl("TextBox1");
//Control c = this.dg1.SelectedItem.FindControl("TextBox1");
TextBox tb = (TextBox)c;
tb.Text = "TextBox";

}

protected void dg1_EditCommand(object source, DataGridCommandEventArgs e)
{
TextBox tb = (TextBox)e.Item.FindControl("TextBox1");
this.Label1.Text = tb.Text.ToString();
}

如果是在DataGrid的页眉和页脚：

((TextBox)this.dg1.Controls[0].Controls[0].FindControl("TextBoxH")).Text = "Head";
((TextBox)this.dg1.Controls[0].Controls[this.dg1.Controls[0].Controls.Count - 1].FindControl

("TextBoxF")).Text = "Footer";

TextBox1在Repeater中

<asp:Repeater ID="Repeater1" runat="server" DataSourceID="SqlDataSource1"

OnItemCommand="Repeater1_ItemCommand">
<ItemTemplate>
<asp:TextBox ID="TextBox1" runat="server" Text=""></asp:TextBox><%#DataBinder.Eval

(Container.DataItem,"ProductName")%><asp:Button ID="btn" OnClick="btn_click" runat="server"

Text="dddd" /><br />
</ItemTemplate>
</asp:Repeater>

通过按钮来获得TextBox1:

protected void btn_click(object sender, EventArgs e)
{
//获得按钮
Button btn = (Button)sender;
TextBox tb = (TextBox)btn.Parent.FindControl("TextBox1");
tb.Text = "Text";
}

或者

foreach (RepeaterItem item in this.Repeater1.Items)
{
((TextBox)item.FindControl("TextBox1")).Text = "Text2";
}

自定义控件里的TextBox1
<%@ Control Language="C#" AutoEventWireup="true" CodeFile="WebUserControl.ascx.cs"
Inherits="WebUserControl" %>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
引用<uc1:WebUserControl ID="WebUserControl1" runat="server" />

获取TextBox1:

((TextBox)this.WebUserControl1.FindControl("TextBox1")).Text = "userc";

模板页访问页面TextBox1

//模板页的TextBox1
TextBox tbM = (TextBox)this.FindControl("TextBox1");
//页面中的TextBox1
TextBox tbC = (TextBox)this.FindControl("ContentPlaceHolder1").FindControl("TextBox1");
tbC.Text = tbM.Text;

页面使用模板页的TextBox1

//模板页的TextBox1
TextBox tbM = (TextBox)Master.FindControl("TextBox1");
//本页面的TextBox1
//错误的方法：TextBox tbC = (TextBox)this.FindControl("TextBox1");
TextBox tbC = (TextBox)Master.FindControl("ContentPlaceHolder1").FindControl("TextBox1");
tbM.Text = tbC.Text.ToString();

C#中FindByValue、FindControl函数的用法详解