onclick event problem

I have a login page and when I run my application I keep getting this errors about the buttons " does not contain defination for btnLogin_Click " even though I have this on my aspx page

<asp:Button ID="btnLogin" runat="server" Text="Login"
                        onclick="btnLogin_Click"  />

when I take out Onclick event from aspx page and did this on cs code behind like this it is fine
 protected override void OnInit(EventArgs e)
    {
        base.OnInit(e);
       
        btnLogin.Click += new EventHandler(btnLogin_Click);
}

I want to know why aspx page onclick is giving problem like this? 
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