Excel阅读提供[system .__ comobject]作为回报

Question

在我的WPF应用程序中，我试图使用Microsoft.Office.Interop.Excel读取excel文件。它工作正常。但突然间它给出了[system .__ comobject]而不是excel对象。我不知道现在发生了什么。你的帮助将节省我的一整天。谢谢。我的代码如下，

 public class ReadExcelDataModel
{
    Excel.Application excelApp = new Excel.Application();
    Excel.Workbook workbook;
    Excel.Worksheet worksheet;
    Excel.Range range;
    private int row = 0;

public ResponceModel ReadWorksheet(string FileName, string FilePath, ResponceModel ResponceResult)
    {
        Error error = new Error();
        //workbook = Excel.Workbook;
        //worksheet = new Excel.Worksheet();
        object misValue = System.Reflection.Missing.Value;
        List<Task<ResponceModel>> TaskList = new List<Task<ResponceModel>>();
        try
        {
            string excelFilePath = string.Format("{0}\\{1}", FilePath, FileName);
            workbook =  excelApp.Workbooks.Open(excelFilePath);
            for (int i = 1; i <= workbook.Sheets.Count; i++)
            {
                worksheet = (Excel.Worksheet)workbook.Sheets[i];
                ResponceResult.SheetName = worksheet.Name;
                ResponceResult.dataTable.Add(SwitchTheTables(worksheet, ResponceResult).TempDataTable);
            }
            workbook.Close(true, Missing.Value, Missing.Value);
            excelApp.Quit();
            ResponceResult.Responce = "OK";
            return ResponceResult;
        }
        catch
        {
            error.ErrorCode = 405;
            error.ErrorDescription = string.Format("The worksheet name not found.");
            ResponceResult.Responce = "Error";
            ResponceResult.ErrorList.Add(error);
            return ResponceResult;
        }

    }
}

workbook = excelApp.Workbooks.Open（excelFilePath）;在这里工作簿中将[system .__ comobject]作为值。以前它得到了excel对象。