[Solved] DropDownList SelectedIndexChanged event is not firing in ASP.Net

I want to automatically Enabled Textbox when selected Index 0 in DropDownList.

<asp:DropDownList ID="DropDownList1" runat="server" OnSelectedIndexChanged="OnSelectedIndexChanged">

I used the previous code, but it does not work and if I write it in a command, it works

protected void OnSelectedIndexChanged(object sender, EventArgs e)
{
    if (DropDownList1.SelectedIndex == 0)
    {
        TextBox2.Enabled = false;
    }
    else
    {
        TextBox2.Enabled = true;
    }
}