The ClientIDMode="Static" setting in ASP.NET forces a server control to render the exact ID value defined in the markup rather than allowing ASP.NET to automatically generate a unique client-side ID. Without this setting, controls inside MasterPages, UserControls, Repeaters, GridViews, or nested containers may render unpredictable IDs such as ctl00_MainContent_txtName, making JavaScript, jQuery, CSS targeting, and AJAX interactions more difficult.

Using ClientIDMode="Static" ensures consistent HTML output, simplifies frontend scripting, and makes element selection more reliable across complex ASP.NET WebForms applications.

Example Without ClientIDMode="Static"

<asp:TextBox ID="txtName" runat="server"></asp:TextBox>
<!-- Possible HTML Output: -->
<input name="ctl00$MainContent$txtName" type="text" id="ctl00_MainContent_txtName">

Example With ClientIDMode="Static"

<asp:TextBox ID="txtName" runat="server" ClientIDMode="Static"></asp:TextBox>
<!-- HTML Output: -->
input name="ctl00$MainContent$txtName" type="text" id="txtName">

Alternatively, if you want this to apply to all controls across your entire application, you can set it globally in your web.config file inside the <system.web> section

<system.web>
  <pages clientIDMode="Static" />
</system.web>