在c#中打开文件路径

我在winform打开绘图时遇到了麻烦。我得到的错误说NullReferenceException未处理，并突出显示路径。任何帮助都是感激的。由于

private void button2_Click(object sender, EventArgs e)
    {
        //Open Solidworks Drawing
        ModelDoc2 swModel = default(ModelDoc2);
        DocumentSpecification swDocSpecification = default(DocumentSpecification);
        string sName = null;
        long longstatus = 0;
        long longwarnings = 0;
        // Drawing document path and name         
        swDocSpecification = (DocumentSpecification)swApp.GetOpenDocSpec("C:''location''????.slddrw");//File Location
        sName = swDocSpecification.FileName;
        // Sheet name         
        swDocSpecification.SheetName = "BOM"; //Open to the BOM sheet
        swDocSpecification.DocumentType = (int)swDocumentTypes_e.swDocDRAWING;
        swDocSpecification.ReadOnly = true;
        swDocSpecification.Silent = false;
        // Open the specified sheet in the specified drawing document         
        swModel = swApp.OpenDoc7(swDocSpecification);
        longstatus = swDocSpecification.Error;
        longwarnings = swDocSpecification.Warning;
    }