title: Open an existing Excel file in the current instance
body: <p>I've written an Excel Addin for Office 2013 with C# language.</p>
  
  <p>I know there is a few ways to open an existing Excel file, but all of them open the file in a new instance on Excel.</p>
  
  <p>I have a current instance of Excel and want when somebody clicks on a button, the new file is opened in the current isctance(not a new one)!</p>
  
  <p>for example I use the following piece of code:</p>
  
  <pre><code>Excel.Application excelApp = new Excel.Application();
  excelApp.Visible = true;
  string workbookPath = (@"C:\Downloads\Sample.xlsx");
  Excel.Workbook excelWorkbook = excelApp.Workbooks.Open(workbookPath,
      0, false, 5, "", "", false, Excel.XlPlatform.xlWindows, "",
      true, false, 0, true, false, false);
  </code></pre>
  
  <p>But this code opens the Sample.xlsx in a new instance of excel file.</p>
  
  <p>Any Idea?</p>
  
  <p>Thanks in advance.</p>