无法访问封闭的流？ C＃

Question

我有一个包含来自我的xamarin表单签名板的签名图像的流。我需要将图像上传到服务器但我收到错误：

  

FATAL UNHANDLED EXCEPTION：System.ObjectDisposedException：不能   访问封闭的流

我在PostAsync行中收到异常。

如何访问流？

public async void Upload(object sender, EventArgs e)
{
    signimgurl = await signature.GetImageStreamAsync(SignatureImageFormat.Png);

    image.HeightRequest = 250;

    image.Source = ImageSource.FromStream(() => 
    {
        return signimgurl;
    });

    var _content = new MultipartFormDataContent();

    _content.Add(new StreamContent(signimgurl),
        "\"file\"",
        $"\"{"testsign"}\"");

    var _httpClient = new HttpClient();
    var _uploadServiceBaseAdress = "http://myserver.rocket.com/api/Files/Upload";

    var httpResponesMessage = await _httpClient.PostAsync(_uploadServiceBaseAdress, _content);
    string _imgLocation = await httpResponesMessage.Content.ReadAsStringAsync();
}