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>