从Base64C下载响应PDF#

HttpResponseMessage result = new HttpResponseMessage();
MemoryStream stream = new MemoryStream();
try {
    var base64 = "...";
    byte[] pdfBytes = Convert.FromBase64String(base64);

    stream = new MemoryStream(pdfBytes);

    var resultPDF = Encoding.UTF8.GetString(stream.ToArray());

    result.StatusCode = HttpStatusCode.OK;
    result.Content = new StringContent(resultPDF);
    result.Content.Headers.ContentType = new MediaTypeHeaderValue("application/pdf");
    result.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment") { FileName = "BuildingDetail.pdf" };

} catch (Exception e) {
    stream.Close();
    result.StatusCode = HttpStatusCode.InternalServerError;
    result.ReasonPhrase = e.Message;// "Error occured while exporting csv file!";
} finally {
    stream.Close();
}

当我试图测试出API时,它可以下载,但PDF是空的。

我也试着复制 base64 decode and download 网站似乎运作良好,所以使 不是问题所在。