Forms leaving scope?

Question

I am programming an MDI application that allows a user to have multiple views of data. I accomplish this by using a datagrid in a MDI child form to the main form. The user can then have multiple child forms open each having its own data grid. Users began to report lost data when trying to update when multiple forms were open. Running through the debugger I discovered that the datagrid objects were being disposed even though a datagrid form was still open! I am wondering if my code is inherently flawed somehow and my child forms are out of scope? Here's a brief example: in frmMain()... private void mnuViewFinalPhrases_Click(object sender System.EventArgs e) { string [] sfilter = {status = 'F'}; bool [] bcolumns = {true true true true true true false false false false}; frmPhraseView frmFinalView = new frmPhraseView(Final Phrases sfilter bcolumns); frmFinalView.MdiParent = clsGlobal.g_frmMain; frmFinalView.Show(); } I've setup a global class to instantiate a static global form so that I could always refer back to the main from (clsGlogal.g_frmMain) from anywhere in the program. frmPhraseView contains my datagrid. I can create multiple forms this way by creating a new form every time the user clicks this menu item. I guess my question is this: Once the code execution leaves this member function that is mnuViewFinalPhrases_Click will the form I create in this function go out of scope? Do I need to create some kind of static reference to this form to keep it from going out of scope?

dbailiff · Answer

This makes sense but I'm still skeptical. I read on a board elsewhere that the CLR maintains a reference list of every form which supposedly is enough to keep it from going out of scope or more specifically from being collected by the GC. Just like C++ just because I orphan an object (or block of memory) doesn't mean that the system is now free to release or use the memory of the object. If the CLR is maintaining a reference to the form even though I can't access it through the variable frmFinalView I should be able to get a reference to it somehow. I tested this and found that I could enumerate the list of MdiChildren of frmMain and got back the count of windows that were still visible. Which leads me to believe the form has not been disposed which is why the debugger is saying the data grid has been disposed and not the form. Additionally I think I may have discovered the true source of my bug. Each MdiChild has a datagrid that has its own dataview but they all share the same static global data adapater (using ADO.NET). Editing in one form updates the other forms since they are all bound to the same data source. Where I'm having problems is when I use a form to make changes then call the SubmitUpdates method of the data adapter. If I then create a new form bound (allegedly) to the same data source and then make changes in the new form the old form is not updated. If I then call the SubmitUpdates method the database reflects the changes of the new form but the old form is still not updated. Additionally any changes to the old form and then SubmitUpdates does not create any updates in the database. It would seem that the old form's dataview has been disconnected or placed out of scope since the update. I was not aware that calling the SubmitUpdates method of a data adapter would invalidate any current existing data views. Further testing shows me that I don't even need a 2nd data grid in this scenario. Calling SubmitUpdates to a data adapter with only one form means that any updates to that one form will no longer be posted upon a SubmitUpdates call. It's as if any dataviews created pre-SubmitUpdates are no longer valid to the data adapter object. Sorry if this is long-winded but it helped me to sort it out.

Adam Erstelle · Answer

your code line: frmPhraseView frmFinalView = new frmPhraseView(Final Phrases sfilter bcolumns); is what is causing the forms to be lost. you only have 1 pointer to the form which is frmFinalView. Every time the function is launched the form pointer gets changed. Creating a static pointer will not solve your problem. You have two options. 1) the form will have to contain its own variables for connections datasets etc. It will have to handle all of its own events as it will kind of be like an orphan. 2) keep an array list of pointers to each form. You will have to change your code in your click code so that you add a new element to the arraylist which will contain a new form. Both options will require you to do some investigation but hopefully will help to solve your problem.