Creating Alert In Asp.net C#

protected void but1(object sender, EventArgs e)
    {
        System.Text.StringBuilder sb = new System.Text.StringBuilder();
        sb.Append("<script type = 'text/javascript'>");
        sb.Append("window.onload=function(){");
        sb.Append("alert('");
        sb.Append("Message to be displayed");
        sb.Append("')};");
        sb.Append("</script>");
        ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", sb.ToString());
     
    }