private void button1_Click(object sender, EventArgs e)
{
    CloseExcelWorkbook("TestReport.xlsx");
}

//put the following abbreviation to the "using" block: using Excel = Microsoft.Office.Interop.Excel;
internal void CloseExcelWorkbook(string workbookName)
{
    try 
    {           
        Process[] plist 
 
= Process.GetProcessesByName("Excel", ".");
        if (plist.Length  > 1)
            throw new Exception("More than one Excel process running.");
        else if (plist.Length == 0)
             throw new Exception("No Excel process running.");

        Object obj = Marshal.GetActiveObject("Excel.Application
 
");

        Excel.Application excelAppl = (Excel.Application)obj;

        Excel.Workbooks workbooks = excelAppl.Workbooks;
        foreach (Excel.Workbook wkbk in workbooks )
        {
            if (wkbk.Name == workbookName)
                wkbk.Close();
        }
        //dispose
        //workbooks.Close(); 
 
//this would close all workbooks
        GC.Collect();
        GC.WaitForPendingFinalizers();
        if (workbooks != null)
            System.Runtime.InteropServices.Marshal.FinalReleaseComObject(workbooks);
        //excelAppl.Quit(); //would close the excel application
            System.Runtime.InteropServices.Marshal.FinalReleaseComObject(excelAppl);
        GC.Collect();
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}

注意 如果由於以下Marshal.GetActiveObject("Excel.Application") 出錯.則不要用管理員身份執行即可,(多程序時測試中發現是可以關閉對應的)