using (var openFileDialog1 = new OpenFileDialog()) { openFileDialog1.Reset(); if (!string.IsNullOrEmpty(ExcelFilePath)) {
Also, to set the default extension you should set FilterIndex property instead of DefaultExt. see: https://stackoverflow.com/a/6104319/381082
Here's a good article on the OpenFileDialog in C#: http://www.c-sharpcorner.com/uploadfile/mahesh/openfiledialog-in-C-Sharp/