C#, ASP.NET, Internet Explorer 7, Cannot evaluate expression because a thread is stopped at a point where garbage collection is impossible, possibly because the code is optimized.

protected void btnUpload_Click(object sender, EventArgs e)
        {
            if (radUpload.UploadedFiles.Count > 0)
            {
                foreach (Telerik.Web.UI.UploadedFile file in radUpload.UploadedFiles)
                {
                    Workbook workBook = excelApp.Workbooks.Open(file.FileName, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
 
                    //here is the procesing of the workbook without pasing it as a parameter
                    Worksheet sheet = (Worksheet)workBook.Sheets[1];
                    Range excelRange = sheet.UsedRange;
                    object[,] valueArray = (object[,])excelRange.get_Value(XlRangeValueDataType.xlRangeValueDefault);
                    School school = ProcessObjects(valueArray);
 
                    workBook.Close(false, file.FileName, null);
                    Marshal.ReleaseComObject(workBook);
 
                    bool insert = false;
                    SmaakLessenDataContext db = new SmaakLessenDataContext();
 
                    foreach (SchoolTonen schoolTonen in GetSchoolQueryable())
                    {
                        if (Convert.ToDateTime(schoolTonen.InschrijfDatum) == Convert.ToDateTime(school.InschrijfDatum))
                        {
                            insert = true;
                            if (schoolTonen.BezoekAdres == school.BezoekAdres)
                            {
                                insert = false;
                                break;
                            }
                        }
                    }
                    try
                    {
                        if (insert)
                        {
                            //insert data into the database
                        }
                    }
                    catch (DataException ex)
                    {
                        string s = ex.Message;
                    }
                }
            }
        }
 
private School ProcessObjects(object[,] valueArray)
        {
            School school = new School();
 
            for (int row = 2; row <= valueArray.GetLength(0); row++)
            {
                school.Naam = (string)valueArray[row, 1];
                school.BezoekAdres = (string)valueArray[row, 2];
                school.BezoekPostcode = (string)valueArray[row, 3];
                school.BezoekPlaats = (string)valueArray[row, 4];
                school.PostAdres = (string)valueArray[row, 5];
                school.PostPostcode = (string)valueArray[row, 6];
                school.PostPlaats = (string)valueArray[row, 7];
                school.Telefoonnummer = (string)valueArray[row, 8];
                school.Email = (string)valueArray[row, 9];
                school.Website = (string)valueArray[row, 10];
                .
                .
                .
            }
            return school;
        }
                                              

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