在NPOI workbook.write之后,MemoryStream似乎已关闭？

我正在使用NPOI将DataTable转换为ASP.NET Web API项目中的Excel.

但我从回应中得不到任何东西.这是我的代码:

public HttpResponseMessage GetExcelFromDataTable(DataTable dt)
{
    IWorkbook workbook = new XSSFWorkbook(); // create *.xlsx file, use HSSFWorkbook() for creating *.xls file.
    ISheet sheet1 = workbook.CreateSheet();
    IRow row1 = sheet1.CreateRow(0);
    for (int i = 0; dt.Columns.Count > i; i++)
    {
        row1.CreateCell(i).SetCellValue(dt.Columns[i].ColumnName);
    }

    for (int i = 0; dt.Rows.Count > i; i++)
    {
        IRow row = sheet1.CreateRow(i + 1);
        for (int j = 0; dt.Columns.Count > j; j++)
        {
            row.CreateCell(j).SetCellValue(dt.Rows[i][j].ToString());
        }
    }

    MemoryStream ms = new MemoryStream();
    workbook.Write(ms);
    HttpResponseMessage result = new HttpResponseMessage(HttpStatusCode.OK);
    result.Content = new StreamContent(ms);
    result.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
    result.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment");
    result.Content.Headers.ContentDisposition.FileName = string.Format("{0}.xlsx", dt.TableName);
    return result;
}

我设置了一个断点来检查ms.Lengthafter workbook.Write(ms),但它返回一个异常:System.ObjectDisposedException.

我哪里做错了？

此问题的另一个解决方法是......不使用多个MemoryStream对象.

创建一个NpoiMemoryStream继承MemoryStream并覆盖该Close方法的类:

public class NpoiMemoryStream : MemoryStream
{
    public NpoiMemoryStream()
    {
        // We always want to close streams by default to
        // force the developer to make the conscious decision
        // to disable it.  Then, they're more apt to remember
        // to re-enable it.  The last thing you want is to
        // enable memory leaks by default.  ;-)
        AllowClose = true;
    }

    public bool AllowClose { get; set; }

    public override void Close()
    {
        if (AllowClose)
            base.Close();
    }
}

然后,像这样使用该流:

var ms = new NpoiMemoryStream();
ms.AllowClose = false;
workbook.Write(ms);
ms.Flush();
ms.Seek(0, SeekOrigin.Begin);
ms.AllowClose = true;

在flush和seek之间的某个时刻,NPOI将尝试关闭流,但由于我们覆盖Close()并且AllowClose标志为false,我们可以保持流打开.然后,设置AllowClose回true,正常的处理机制可以关闭它.

不要误会我的意思......这仍然是一个不应该被实现的黑客......但从内存使用的角度来看它有点清晰.

作为ALUN在上面的陈述,也这个问题,你可以喂流进入另一个MemoryStream的:

...
MemoryStream ms = new MemoryStream();
using(MemoryStream tempStream = new MemoryStream)
{
    workbook.Write(tempStream);
    var byteArray = tempStream.ToArray();
    ms.Write(byteArray, 0, byteArray.Length);
    HttpResponseMessage result = new HttpResponseMessage(HttpStatusCode.OK);
    result.Content = new StreamContent(ms);
    result.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");
    result.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment");
    result.Content.Headers.ContentDisposition.FileName = string.Format("{0}.xlsx", dt.TableName);
    return result;
}

不得不这样做有一点代码味道.但是,这仅在输出.xlsx文件时才有必要,因为第三方库涉及处理流的方式.