Question

MySQL unexpected AV in Delphi App

Asked by: StevenB

Using the Delphi Direct Access Components for MySQL (http://www.microolap.com)

Can anybody explain why the following (Delphi) code snippet causes an Access Violation when I try to close aQuery1 after I've closed aQuery2, but not if I close aQuery1 first? Better still, can anybody tell me how to stop this from happening (ie how to have two nested queries and close the innermost one first).

procedure TfrmNexusTest.TntButton1Click(Sender: TObject);
var
  dbMySQL : TmySQLDatabase;
  aQuery1 : TMySQLQuery;
  aQuery2 : TMySQLQuery;
begin
  aQuery1 := nil;
  aQuery2 := nil;
  dbMySQL := TmySQLDatabase.Create(nil);
  try
    dbMySQL.Host := 'localhost';
    dbMySQL.UserName := 'root';
    dbMySQL.UserPassword := 'mypassword';
    dbMySQL.DatabaseName := 'mydatabase';
    dbMySQL.Open;

    aQuery1 := TMySQLQuery.Create(nil);
    aQuery1.Database := dbMySQL;
    aQuery2 := TMySQLQuery.Create(nil);
    aQuery2.Database := dbMySQL;

    aQuery1.SQL.Text := 'Select * from table1';
    aQuery1.Open;
    // aQuery1.Close; // If I call aQuery1.Close here then everything is OK

    aQuery2.SQL.Text := 'Select * from table2';
    aQuery2.Open;
    aQuery2.Close;

    aQuery1.Close; // If I call aQuery1.Close here then I get an AccessViolation
  finally
    FreeAndNil(aQuery2);
    FreeAndNil(aQuery1);
    dbMySQL.Close;
    FreeAndNil(dbMySQL);
  end;
end;


Alternatively, are there any better Delphi MySQL components out there that would solve the problem?

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Asked On
2005-06-08 at 20:19:16ID21451898
Tags

accessviolation

,

freeandnil

,

tmysqlquery

Topic

MySQL Server

Participating Experts
2
Points
500
Comments
5

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Answers

 

by: mnasmanPosted on 2005-06-08 at 23:09:20ID: 14177010

Hi

The code seems ok, may the MSql components are the problem

but remove these lines, you are reference to nil and your components still not initialized
  aQuery1 := nil;
  aQuery2 := nil;


try MyDac from crLab, they offer best Direct Connect components for Oracle/SqlServer/MySql for delphi and .Net, there component are great and stable
http://crlab.com/mydac/

and you can run the applicatin with myDac without install the MySql Client, which is very nice feature

also try AnyDac, it's new direct connection components for many db,
http://www.da-soft.com/AnyDACHome.html


Regards,
Mohammed

 

by: StevenBPosted on 2005-06-08 at 23:34:04ID: 14177091

<slightly off topic>
  It is safe Delphi practice to initialise the local variables to nil as I have done so that the call to FreeAndNil is safe (since TObject.Free checks for Self <> nil) should the finally block be entered before the local variable in question has been initialised. Potentially the local variable could be initialised to some random memory address if it is not explicitly assigned a nil value, which would cause an access violation in FreeAndNil.
</slightly off topic>

Thanks for the component suggestions.

Steven

 

by: JeePeeTeePosted on 2005-06-09 at 00:39:01ID: 14177338

Hi,

I use the same components (V2.3.0) and your code works well on my machine.

JeePee

 

by: JeePeeTeePosted on 2005-06-09 at 00:44:52ID: 14177359

Hello again....

The other part of your question... about better components.

I looked at MyDAc from http://crlab.com/mydac/ cause there is no support for record locking within the current set of components I use from http://www.microolap.com - and that sucks... My next MySQL/Delphi project will be on MyDac!

JeePee

 

by: StevenBPosted on 2005-11-13 at 18:41:21ID: 15285401

Just cleaning up my open questions...

20120131-EE-VQP-002

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