Question

我有一个小问题..我正在使用blob上传我的文件，我也可以在电影或mp3时下载它们，但是当我尝试下载照片时我总是有这个例外...可以有人告诉我为什么？

Unable to cast object of type 'System.DBNull' to type 'System.Byte[]'.

Description: An unhandled exception occurred during the execution of the current
web request. Please review the stack trace for more information about the error 
and where it originated in the code. 

Exception Details: System.InvalidCastException: Unable to cast object of type 
'System.DBNull' to type 'System.Byte[]'.

Source Error: 

 An unhandled exception was generated during the execution of the current web requt.
 Information regarding the origin and location of the exception can be identified using
 the exception stack trace below.

感谢您的帮助。

    This is the code to retrieve them . 

       string filename = Request["file"].ToString();
        var conString = ConfigurationManager.ConnectionStrings["LocalSqlServer"];
        string strConnString = conString.ConnectionString;
        SqlConnection dbConnection = new SqlConnection(strConnString);
        dynamic queryString = ("SELECT Data,ContentType FROM Files WHERE Name = '" + filename + "'");
        SqlCommand theCommand = new SqlCommand(queryString, dbConnection);
        dbConnection.Open();
        SqlDataReader reader = theCommand.ExecuteReader();

        if (reader.Read() && reader != null)
        {

            Byte[] bytes;
            bytes = Encoding.UTF8.GetBytes(String.Empty);
            bytes = (Byte[])reader["Data"];
                            string contentType = reader["ContentType"].ToString();
            Response.ContentType = contentType;
            Response.AddHeader("content-disposition", "attachment;  filename=" + filename + "");
            Response.BinaryWrite(bytes);
            reader.Close();
        }

        dbConnection.Close();

上传它们：

   dbConnection.Open();
                        string queryString = "INSERT INTO Files (Name,Path,UserUpload,Date,Data,ContentType,Size) VALUES (@Name,@Path,@UserUpload,@Date,@Data,@ContentType,@Size);" + "SELECT CAST(scope_identity() AS int)";
                        SqlCommand theCommand = new SqlCommand(queryString, dbConnection);
                        theCommand.Parameters.AddWithValue("@Name", FileUpload1.FileName);
                        theCommand.Parameters.AddWithValue("@Path", GetTheCurrentDirectory(MyTreeView.SelectedNode));
                        theCommand.Parameters.AddWithValue("@UserUpload", Request.Cookies["UserSettings"]["UserName"]);
                        theCommand.Parameters.AddWithValue("@Date", DateTime.Now);
                        theCommand.Parameters.AddWithValue("@Data", FileUpload1.FileBytes);
                        theCommand.Parameters.AddWithValue("@ContentType", FileUpload1.PostedFile.ContentType);
                        theCommand.Parameters.AddWithValue("@Size", FileUpload1.PostedFile.ContentLength);

                        int newFid = (Int32)theCommand.ExecuteScalar();

                        dynamic queryStringFolder = "INSERT INTO FILES_FOLDERS (File_Id,Folder_Id) VALUES (@FileId,@FolderId);";
                        theCommand = new SqlCommand(queryStringFolder, dbConnection);
                        theCommand.Parameters.AddWithValue("@FileId", newFid);
                        theCommand.Parameters.AddWithValue("@FolderId", MyTreeView.SelectedValue);
                        theCommand.ExecuteNonQuery();

Answer 1

听起来您尝试检索的blob是NULL。如果这是预期的条件，则在尝试转换为字节数组之前，需要检查DBNull。

System.Byte，上传图片

1 个答案: