I am trying to download a file from database but its giving me a null reference error, I am new to C# if somebody can please tell me where I am making a mistake that would be a great help.

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Here is my Code and its first line is giving a null reference error:
int Contribution_ID = int.Parse((sender as LinkButton).CommandArgument);

C#

protected void gridContributions_RowCommand(object sender, GridViewCommandEventArgs e)
  {


      int Sale_ID = int.Parse((sender as LinkButton).CommandArgument);
          byte[] bytes;
          string fileName, contentType;
          string ConnectionString= ConfigurationManager.ConnectionStrings["MyDBConnectionString"].ConnectionString;
          using (SqlConnection con = new SqlConnection(ConnectionString))
          {
              using (SqlCommand cmd = new SqlCommand())
              {
                  cmd.CommandText = "select RecieptFileName from Sales where Contribution_ID=@Contribution_ID";
                  cmd.Parameters.AddWithValue("@Sale_ID", Sale_ID);
                  cmd.Connection = con;
                  con.Open();
                  using (SqlDataReader sdr = cmd.ExecuteReader())
                  {
                      sdr.Read();
                      bytes = (byte[])sdr["RecieptFileName"];
                     // contentType = sdr["ContentType"].ToString();
                      fileName = sdr["ReciepFiletName"].ToString();
                  }
                  con.Close();
              }

          }
          Response.Clear();
          Response.Buffer = true;
          Response.Charset = "";
          Response.Cache.SetCacheability(HttpCacheability.NoCache);
         // Response.ContentType = contentType;
          Response.AppendHeader("Content-Disposition", "attachment; RecieptFileName=" + RecieptFileName);
          Response.BinaryWrite((byte[]) bytes);
          Response.Flush();
          Response.End();







  }

Posted 8-Jun-14 18:32pm

ray thomsan

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Sergey Alexandrovich Kryukov · Answer 1 · 2014-06-08T19:13:00

You did not find out what exactly was null. For example, you sender could be not a LinkButton; and than sender as LinkButton would return null. Otherwise, CommandArgument could be null. You did not face parse exception, but your code could throw it if the argument of the int.Parse would be some string not representing integer in correct format.

Not to worry. This is one of the very easiest cases to detect and fix. It simply means that some member/variable of some reference type is dereferenced by using and of its instance (non-static) members, which requires this member/variable to be non-null, but in fact it appears to be null. Simply execute it under debugger, it will stop the execution where the exception is thrown. Put a break point on that line, restart the application and come to this point again. Evaluate all references involved in next line and see which one is null while it needs to be not null. After you figure this out, fix the code: either make sure the member/variable is properly initialized to a non-null reference, or check it for null and, in case of null, do something else.

Please see also: want to display next record on button click. but got an error in if condition of next record function "object reference not set to an instance of an object"[^].

Sometimes, you cannot do it under debugger, by one or another reason. One really nasty case is when the problem is only manifested if software is built when debug information is not available. In this case, you have to use the harder way. First, you need to make sure that you never block propagation of exceptions by handling them silently (this is a crime of developers against themselves, yet very usual). The you need to catch absolutely all exceptions on the very top stack frame of each thread. You can do it if you handle the exceptions of the type System.Exception. In the handler, you need to log all the exception information, especially the System.Exception.StackTrace:
http://msdn.microsoft.com/en-us/library/system.exception.aspx[^],
http://msdn.microsoft.com/en-us/library/system.exception.stacktrace.aspx[^].

The stack trace is just a string showing the full path of exception propagation from the throw statement to the handler. By reading it, you can always find ends. For logging, it's the best (in most cases) to use the class System.Diagnostics.EventLog:
http://msdn.microsoft.com/en-us/library/system.diagnostics.eventlog.aspx[^].

Good luck,

—SA

Pratik65 · Answer 2 · 2014-06-08T19:40:00

I had used this to solve my problem too , so u can also get some idea from this

C#

string cmd_str;
           SqlConnection Cn_Obj = new SqlConnection(ConnString);
           SqlDataAdapter Da_Obj = new SqlDataAdapter();
           SqlCommand Cm_Obj = new SqlCommand();
           DataSet Ds_Obj = new DataSet();

           cmd_str = "SELECT * FROM attachmenttemp WHERE code =" + myKey;
           Da_Obj = new SqlDataAdapter(cmd_str, Cn_Obj);
           Cn_Obj.Open();
           if (Ds_Obj.Tables.Contains("a1")) Ds_Obj.Tables.Remove("a1");
           Da_Obj.Fill(Ds_Obj, "a1");

           Cm_Obj = new SqlCommand(cmd_str, Cn_Obj);
           SqlDataReader reader = Cm_Obj.ExecuteReader();
           if (reader.Read() && reader != null)
           {
               Byte[] bytes;
               bytes = Encoding.UTF8.GetBytes(String.Empty);
               bytes = (Byte[])reader["data"];
               FileStream fs = new FileStream(@Target_Path + Req_NO + @"\" + Ds_Obj.Tables["a1"].Rows[0]["FileName"].ToString(),
                   FileMode.OpenOrCreate);
               fs.Write(bytes, 0, bytes.Length);
               fs.Close();
           }
           Cn_Obj.Close();

I am trying to download a file from database but its giving me a null reference error, I am new to C# if somebody can please tell me where I am making a mistake that would be a great help.

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