Как я мог выбрать папку или файл из веб-приложения asp.net?

РЕДАКТИРОВАТЬ:

Глядя на ваш комментарий, я думаю, вы вместо этого имеете в виду отправку в поток ответов?

 protected void lnbDownloadFile_Click(object sender, EventArgs e)
 {
  String YourFilepath;
  System.IO.FileInfo file = 
  new System.IO.FileInfo(YourFilepath); // full file path on disk
  Response.ClearContent(); // neded to clear previous (if any) written content
  Response.AddHeader("Content-Disposition", "attachment; filename=" + file.Name);
  Response.AddHeader("Content-Length", file.Length.ToString());
  Response.ContentType = "text/plain";
  Response.TransmitFile(file.FullName);
  Response.End();
 }

Это должно отобразить диалоговое окно в браузере, позволяющее пользователю выбрать, где сохранить файл.

Вы хотите использовать элемент управления FileUpload

http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.fileupload.aspx

protected void UploadButton_Click(object sender, EventArgs e)
  {
    // Specify the path on the server to
    // save the uploaded file to.
    String savePath = @"c:\temp\uploads\";

    // Before attempting to perform operations
    // on the file, verify that the FileUpload 
    // control contains a file.
    if (FileUpload1.HasFile)
    {
      // Get the name of the file to upload.
      String fileName = FileUpload1.FileName;

      // Append the name of the file to upload to the path.
      savePath += fileName;


      // Call the SaveAs method to save the 
      // uploaded file to the specified path.
      // This example does not perform all
      // the necessary error checking.               
      // If a file with the same name
      // already exists in the specified path,  
      // the uploaded file overwrites it.
      FileUpload1.SaveAs(savePath);

      // Notify the user of the name of the file
      // was saved under.
      UploadStatusLabel.Text = "Your file was saved as " + fileName;
    }
    else
    {      
      // Notify the user that a file was not uploaded.
      UploadStatusLabel.Text = "You did not specify a file to upload.";
    }

  }