字符串不可分配给参数类型Excel.XIFixedFormatType

我正在将excel文件转换为pdf文件，但当我传入用于导出的字符串时，我会收到此错误（红色下划线convertFilePath）Argument type string is not assignable to parameter type Microsoft.Office.Interop.Excel.XIFixedFormatType

var convertFileId = Guid.NewGuid();
var convertFilePath = @"c:'temp'" + convertFileId + ".pdf";

public Microsoft.Office.Interop.Excel.Workbook excelWorkbook { get; set; }
    void ExcelToPdf(string convertFilePath)
    {
        Microsoft.Office.Interop.Excel.Application appWord = new Microsoft.Office.Interop.Excel.Application();
        excelWorkbook = appWord.Workbooks.Open(DocumentUNCPath.Text);
        excelWorkbook.ExportAsFixedFormat(convertFilePath, WdExportFormat.wdExportFormatPDF);
        excelWorkbook.Close();
        appWord.Quit();
    }