Response.Redirect into a new window

The only way to open a new window is for it to be initiated on the client side, whether it be through script or clicking on a link.
So the solution always proposed to this problem is to instead write out some script that opens the window, rather than using Response.Redirect:

<script type="text/javascript"> 
    window.open("default.aspx");
</script>

Now the helper method which is used for redirecting to new page.
It's easy enough to write a little helper that abstracts the details away from us... while we're at it, we might as well add target and window Features parameters. If we're going to open the new window with script, why not let you use all of the window.open parameters? For example, with window Features you can specify whether the new window should have a menu bar, and what its width and height are.

 public static class ResponseHelper {
    public static void Redirect(string url, string target, string windowFeatures) {
        HttpContext context = HttpContext.Current;
 
        if ((String.IsNullOrEmpty(target) ||
            target.Equals("_self", StringComparison.OrdinalIgnoreCase)) &&
            String.IsNullOrEmpty(windowFeatures)) {
 
            context.Response.Redirect(url);
        }
        else {
            Page page = (Page)context.Handler;
            if (page == null) {
                throw new InvalidOperationException(
                    "Cannot redirect to new window outside Page context.");
            }
            url = page.ResolveClientUrl(url);
 
            string script;
            if (!String.IsNullOrEmpty(windowFeatures)) {
                script = @"window.open(""{0}"", ""{1}"", ""{2}"");";
            }
            else {
                script = @"window.open(""{0}"", ""{1}"");";
            }
 
            script = String.Format(script, url, target, windowFeatures);
            ScriptManager.RegisterStartupScript(page,typeof(Page),"Redirect",script,true);
        }
    }
}

Now you just call ResponseHelper.Redirect, and it figures out how to honor your wishes. If you don't specify a target or you specify the target to be "_self", then you must mean to redirect within the current window, so a regular Response.Redirect occurs. If you specify a different target, like "_blank", or if you specify window features, then you want to redirect to a new window, and we write out the appropriate script.

Finally you call it in following manner

ResponseHelper.Redirect("popup.aspx", "_blank", "menubar=0,width=100,height=100");