Question

I wrote the following code by serialization and a deserialization of forms and their contents to Delphi

unit SerAndDeser;

interface

  uses Classes,MainForm,ListOfTables,DataOfTable,SerialForms,sysutils,ActiveX, DatabaseClasses, UnloadProcs;

  procedure Ser();
  procedure Deser();
  function GetGUID(): string;
  function DeleteSymbols(inputstr : string) : string;
implementation

  function GetGUID(): string;
  var
  GUID : TGUID;
  begin
  Result := '';
  if CoCreateGuid(GUID) = 0 then
    Result := GUIDToString(GUID);
    Result := StringReplace(Result, '{', '', []);
    Result := StringReplace(Result, '}', '', []);
    Result := StringReplace(Result, '-', '', [rfReplaceAll]);
  end;

  function DeleteSymbols(inputstr : string): string;
  begin
  Result := '';
    Result := StringReplace(inputstr, '-', '', [rfReplaceAll]);
    Result := StringReplace(Result, ' ', '', [rfReplaceAll]);
    Result := StringReplace(Result, '\', '', [rfReplaceAll]);
    Result := StringReplace(Result, '/', '', [rfReplaceAll]);
  end;

  procedure Ser();

    var mForm : MainFormInfo;
        tForm : TableFormInfo;
        lForm : ListFormInfo;
        tempFmtable : TfmTableData;
        i,j : integer;
        MyFileStream : TFileStream;
        MyMemoryStream : TMemoryStream;
        field : TableFieldInfo;
  begin
        try
      mForm := nil;

      mForm := MainFormInfo.Create(
        nil,
        MainWindow.Left,
        MainWindow.Top,
        MainWindow.Height,
        MainWindow.Width,
        MainWindow.partofconnectstring,
        MainWindow.dbname,
        MainWindow.dbfilename);
      mForm.Name := 'MainWindow';

      //table forms
      try
        tForm := nil;
        field := nil;

        for i := 0 to MainWindow.ComponentCount - 1 do
          if (MainWindow.Components[i] is TfmTableData) then
            begin
              tempFmtable := MainWindow.Components[i] as TfmTableData;
              tForm := TableFormInfo.Create(
                mForm,
                tempFmtable.Left,
                tempFmtable.Top,
                tempFmtable.Height,
                tempFmtable.Width,
                tempFmtable.tname);
              tForm.Name := tempFmtable.Name;

              //fields
              for j := 0 to tempFmtable.DBGrid1.Columns.Count - 1 do
              begin
                field := nil;

                field := TableFieldInfo.Create(
                  tForm,
                  tempFmtable.DBGrid1.Columns[j].FieldName,
                  tempFmtable.DBGrid1.Columns[j].Title.Caption,
                  tempFmtable.DBGrid1.Columns[j].Index,
                  tempFmtable.DBGrid1.Columns[j].Visible);
                  field.Name := DeleteSymbols(tempFmtable.DBGrid1.Columns[j].FieldName);{tempFmtable.DBGrid1.Columns[j].FieldName} {+ GetGUID;}
              end;
              //

            end;
      except
        field.Free;
        tForm.Free;
      end;
      //

      //List form
      try
        lForm := nil;
        lForm := ListFormInfo.Create(
          mForm,
          fmListOfTables.Left,
          fmListOfTables.Top,
          fmListOfTables.Height,
          fmListOfTables.Width);
        lForm.Name := 'fmListOfTables';
      except
        lForm.Free;
      end;
      //

      //save
      MyFileStream := nil;
      MyMemoryStream := nil;

      MyFileStream := TFileStream.Create('test.txt', fmCreate);
      MyMemoryStream := TMemoryStream.Create;
      MyMemoryStream.WriteComponent(mForm);
      MyMemoryStream.Position := 0;
      ObjectBinaryToText(MyMemoryStream,MyFileStream);
      MainWindow.Panel1.DockManager.SaveToStream(MyFileStream);
      //

    finally
      mForm.Free;
      MyFileStream.Free;
      MyMemoryStream.Free;
    end;
  end;


  procedure Deser();

    var mForm : MainFormInfo;
        tForm : TableFormInfo;
        lForm : ListFormInfo;
        tempFmtable : TfmTableData;
        i,j : integer;
        MyFileStream : TFileStream;
        MyMemoryStream : TMemoryStream;
        table : TTableSpec;
        descr : string;
        field : TableFieldInfo;


  begin
    try
      //destroy environment
      i := 0;
      while (i <= MainWindow.ComponentCount - 1) do
      begin
        if MainWindow.Components[i] is TfmTableData then
          try
            tempFmTable := nil;
            tempFmTable := MainWindow.Components[i] as TfmTableData;
            tempFmTable.IBQuery1.Close;
            tempFmtable.Free;
          except
            tempFmTable.Free;
          end
        else
          inc(i);
      end;
      fmListOfTables.Free;
      DBSchema.Free;
      //

      //read
      mForm := nil;
      MyFileStream := nil;
      MyMemoryStream := nil;

      mForm := MainFormInfo.Create(nil, -1, -1, -1, -1, MainWindow.partofconnectstring, MainWindow.dbname, MainWindow.dbfilename);

      MyFileStream := TFileStream.Create('test.txt', fmOpenRead);
      MyMemoryStream := TMemoryStream.Create;
      ObjectTextToBinary(MyFileStream,MyMemoryStream);
      MyMemoryStream.Position := 0;
      MyMemoryStream.ReadComponent(mForm);
      //

      //go
      UnloadProcs.ConnectToDatabase(MainWindow.partofconnectstring, MainWindow.SQLConnection1);
      //UnloadProcs.CreateObjs(MainWindow.SQLConnection1, MainForm.DBSchema);
      //fmListOfTables.Show;

      MainWindow.Left := mForm.LeftValue;
      MainWindow.Top := mForm.TopValue;
      MainWindow.Height := mForm.HeightValue;
      MainWindow.Width := mForm.WidthValue;

      //list
      i := 0;
      while i <= mForm.ComponentCount - 1 do
      begin
        if mForm.Components[i] is ListFormInfo then
          try
            lForm := nil;
            lForm := mForm.Components[i] as ListFormInfo;
            fmListOfTables.Left := lForm.LeftValue;
            fmListOfTables.Top := lForm.TopValue;
            fmListOfTables.Height := lForm.HeightValue;
            fmListOfTables.Width := lForm.WidthValue;
            fmListOfTables.Show;
            inc(i);
          finally
            lForm.Free;
          end
        else
          inc(i);
      end;
      //

      //fmListOfTables.Show;
      //tables
      for j := 0 to mForm.ComponentCount - 1 do
        if mForm.Components[j] is TableFormInfo then
          try
            table := nil;
            tempFmtable := nil;
            tForm := nil;

            tForm := mForm.Components[j] as TableFormInfo;

            table := TTableSpec(DBSchema.Tables.FindComponent(tForm.Table));
            tempFmtable := TfmTableData.Create(MainWindow);
            tempFmtable.Name := tForm.Name;
            tempFmtable.tname := tForm.Table;
            //tempFmtable.Caption := Utf8ToAnsi(table.Description);
            tempFmtable.Left := tForm.LeftValue;
            tempFmtable.Top := tForm.TopValue;
            tempFmtable.Height := tForm.HeightValue;
            tempFmtable.Width := tForm.WidthValue;

            tempFmTable.IBQuery1.SQL.Clear;
            tempFmtable.IBQuery1.SQL.Add('select * from ' + table.Name);
            tempFmtable.IBQuery1.Open;

            i := 0;
            while i <= tForm.ComponentCount - 1 do
              if tForm.Components[i] is TableFieldInfo then
              begin
                field := nil;

                field := tForm.Components[i] as TableFieldInfo;
                tempFmtable.DBGrid1.Columns[i].FieldName := field.FieldNameValue;
                tempFmtable.DBGrid1.Columns[i].Title.Caption := field.DescriptionValue;
                tempFmtable.DBGrid1.Columns[i].Index := field.IndexValue;
                tempFmtable.DBGrid1.Columns[i].Visible := field.VisibleValue;
                //tempFmtable.CheckListBox1.Items.Add(field.Description);
                //tempFmtable.CheckListBox1.Checked[i] := field.Visible;
                inc(i);
              end
              else
                inc(i);

            {for i := 0 to table.Fields.ComponentCount - 1 do
            begin
              descr := Utf8ToAnsi(((table.Fields.Components[i]) as TFieldSpec).Description);
              tempFmtable.CheckListBox1.Items.Add(descr);
              tempFmtable.DBGrid1.Columns[i].Title.Caption := descr;
              tempFmtable.CheckListBox1.Checked[i] := true;
            end; }

            tempFmtable.Show;


          except
            tempFmtable.Free;
            tForm.Free;
            table.Free;
          end;
      //

      //dock
      MainWindow.Panel1.DockManager.BeginUpdate;
      MainWindow.Panel1.DockManager.LoadFromStream(MyFileStream);
      MainWindow.Panel1.DockManager.ResetBounds(TRUE);
      MainWindow.Panel1.DockManager.EndUpdate;


      //
    finally
      MyFileStream.Free;
      MyMemoryStream.Free;
    end;

  end;

end.

When debugging I found out that gives out

'Project GUI.exe raised exception class $C0000005 with message 'access violation at 0x00821dae: read of address 0x00000044'. 

exception at line

tempFmtable.IBQuery1.SQL.Add('select * from ' + table.Name);

in the following block of a code from a code is higher

  //tables
  for j := 0 to mForm.ComponentCount - 1 do
    if mForm.Components[j] is TableFormInfo then
      try
        table := nil;
        tempFmtable := nil;
        tForm := nil;

        tForm := mForm.Components[j] as TableFormInfo;

        table := TTableSpec(DBSchema.Tables.FindComponent(tForm.Table));
        tempFmtable := TfmTableData.Create(MainWindow);
        tempFmtable.Name := tForm.Name;
        tempFmtable.tname := tForm.Table;
        //tempFmtable.Caption := Utf8ToAnsi(table.Description);
        tempFmtable.Left := tForm.LeftValue;
        tempFmtable.Top := tForm.TopValue;
        tempFmtable.Height := tForm.HeightValue;
        tempFmtable.Width := tForm.WidthValue;

        tempFmTable.IBQuery1.SQL.Clear;
        tempFmtable.IBQuery1.SQL.Add('select * from ' + table.Name);
        tempFmtable.IBQuery1.Open;

        i := 0;
        while i <= tForm.ComponentCount - 1 do
          if tForm.Components[i] is TableFieldInfo then
          begin
            field := nil;
            //tempFmtable.Show;
            field := tForm.Components[i] as TableFieldInfo;
            tempFmtable.DBGrid1.Columns[i].FieldName := field.FieldNameValue;
            tempFmtable.DBGrid1.Columns[i].Title.Caption := field.DescriptionValue;
            tempFmtable.DBGrid1.Columns[i].Index := field.IndexValue;
            tempFmtable.DBGrid1.Columns[i].Visible := field.VisibleValue;
            //tempFmtable.CheckListBox1.Items.Add(field.Description);
            //tempFmtable.CheckListBox1.Checked[i] := field.Visible;
            //tempFmtable.Show;
            inc(i);
          end
          else
            inc(i);

        {for i := 0 to table.Fields.ComponentCount - 1 do
        begin
          descr := Utf8ToAnsi(((table.Fields.Components[i]) as TFieldSpec).Description);
          tempFmtable.CheckListBox1.Items.Add(descr);
          tempFmtable.DBGrid1.Columns[i].Title.Caption := descr;
          tempFmtable.CheckListBox1.Checked[i] := true;
        end; }

        tempFmtable.Show;


      except
        tempFmtable.Free;
        tForm.Free;
        table.Free;
      end;

Answer 1

The line that fails is:

tempFmtable.IBQuery1.SQL.Add('select * from ' + table.Name);

Before this line runs you assigned table like this:

table := TTableSpec(DBSchema.Tables.FindComponent(tForm.Table));

It would seem that FindComponent returned nil.

As I’m sure you know, you have to have a valid object instance in order to call methods, access fields and properties etc. Your code should check for this condition and respond accordingly.

You can see all this quite readily in the debugger. The debugger will break on that line and when you inspect the variables it will tell you that table is nil. Then you have to look at the code and work out why. I recommend that you spend some time improving your debugging skills and learning how to use the tools available.