C#上传图片前判断上传图片的宽和高

<asp:FileUpload ID="fuTx" runat="server" CSSClass="uploadFile" />

System.Drawing.Image upimage = System.Drawing.Image.FromStream(fuTx.PostedFile.InputStream);int width = upimage.Width;int height = upimage.Height;if (width >= 500 && height >= 500){       fuTx.SaveAs(txipath);//上传文件到ipath这个路径里} else{        //此处提示："头像图片的宽和高不能小于500！"        return;}