Passing Data between forms

Question

My application reads an array from another system and this

array is holding data that is updated every second using a

timer in the Parent form.

The Parent form has a textbox and a button to open a new

Child Form. When the button is pressed the Child Form is

opened and displays a live value according to the index which

was entered in the text box of the Parent Form.

I have this working fine with the live value being

continually updated on the Child Form using parameters (ie

the Get & Set commands in the Child Form).

My problem is that I want to open multiple Child Forms that

can each display a different live value and be updated every

second.

When I open the second (or third, fourth etc ) Child Form I

have the problem that only the last Child Form opened is

updated with live values. The Child Forms that were opened

before, just show the last values and are not updated.

Can anyone point me in the right direction to get the live

values to update on each Child Form...

I have not copied in my code as have played around with it

quite a lot and would need to clean it up before presenting

to everyone. I was looking for a point in the right direction

rather than being spoon fed code. If needed, I can post the

code...

Many thanks in advance

Sam Hobbs · Answer

Are the child forms MDI child forms or are they separate forms? Since Frogleg did not explain what his code does I don't know if I have an alternative. It sure would help if I had a general idea at least of what it shows. Without spending a lot of time with the details my initial idea would be to create a method in the child form class and also somewhere in your program keep an array or list of all child forms. Then when the data is updated iterate through the list of child forms and call the update method in the child form. I am assuming that each child form knows what index to use in the data to get it's updated data. Would that satisfy the requirements?

Monte · Answer

Please find attached a copy of the project with the problem resolved. This is just for future reference in case it helps someone with the same problem.

Monte · Answer

Just as an overview - I have a Parent form that calls multiple instances of a Child Form. When a Parent Form creates a Child Form a Closed event is added to the Parent for the Child Form. My old Code removed this Closed event (for the Child Form) in the Parent form. However this caused the problem of the Closed event not always firing. The removal of the closed event was to save system resources and be tidy. The problem was resolved by not deleting the closed event for that Child Form when that Child Form is closed. This seems like a bug with the VS2003 system - unless someone knows that this is a feature...! I will add a copy of the project with the project resolved in case someone finds this useful in the future...

Sam Hobbs · Answer

I don't know the details of your code but it sure makes sense that if the Closed event handler is deleted from the list of events to be called then the handler for the Closed event won't be called.

Monte · Answer

I had problems getting the Closed event on the child forms to work correctly - if the first child form to be opened wasnt the last to be closed then the Child Form wasnt recognised as being closed. I posted another topic on this and have now resolved the issue - see my other post on this: http://www.c-sharpcorner.com/Forums/Thread/127395/problem-with-closed-event-of-a-child-form.aspx Basically I had to comment out the bit of code which deletes the closed event handler. Looks like VS2003 gets confused if you try and delete the event handler... Anyway all working now.

Vulpes · Answer

Yeah I should have remembered that the shorthand form: child.Closed -= this.Child_Closed; wasn't introduced until .NET 2.0. Amazing how quickly you forget!

Monte · Answer

Thanks for the info Vulpes. Have got it working OK now. My problem was with getting the event handler to work OK so had to use the following code to make it work. I'm not nitpicking just including the code so others in the future can see what worked. It is due to using an old version of software - Visual Studio 2003. private void ChildForm_Closed(object sender EventArgs e) { Form2 child = (form2)sender; this.child.Closed -= new EventHandler(this.child_Closed); childList.Remove(child); } Thanks also Max for the input - I need to brush up on Delegates and give this a go as a learning exercise.

Max Cervantes · Answer

u can also implement ur own event... it's really easy: define a delegate declare an event in your child form have the parent form to subscribe to that event and fire the event from the child form whenever u want :D

Vulpes · Answer

If I remember correctly the Form.FormClosed event wasn't introduced until VS 2005 (.NET 2.0). Before that you had to use the Form.Closed event which is still there though deprecated in later versions. The revised code will be: private void ChildForm_Closed(object sender EventArgs e) { Form2 child = (Form2)sender; child.Closed -= this.Child_Closed; childList.Remove(child); }

Monte · Answer

I am using VS 2003 and struggling to find a way to check if the child form is closed from within the parent. I am trying to do what Vulpes suggested above but cant get the .FormClosed event to be recognised as per Vulpes code. I am wondering if this wasnt available in VS 2003? The project is the same as described in this post ie one parent form with a button that will open multiple instances of a child form. I am using an arraylist to keep tabs on which Child Forms are open which works fine. The problem is when I close a Child Form I want to be able to delete it from the Array list in the Parent. If the .FormClosed event does not work in VS2003 then any other ideas how to recognise a child form is closed and delete it from the arraylist in the parent? Thanks in advance...

Max Cervantes · Answer

here have look at this http://msdn.microsoft.com/en-us/library/ee817669.aspx if not clear enough just google for observer pattern

Monte · Answer

Thankyou Vulpes for the nod in the direction of ArrayLists to store the array of child forms and index numbers. Yes I am constricted to VS2003 as I am adding to an existing system which is in VS2003. I have seen a few things along the way which would have been more easily achieved in VS2010... I have changed over to ArrayLists and it works fine (as before) but without the need to specify a max number of child forms - hence the code should be more efficient on memory. Thanks also to Max for the info on Delegates. Yes I went down this route before and got the data exchange to work to the most recent child form opened only. I will have a play with Delegates using your information and try to get the updates to work across all child forms... Many thanks again for the info.

Max Cervantes · Answer

I'm not quite skilled with the .net version ur using but if u say that u were trying delegates u were in the right track. U should have a look at the oberver pattern. This is exactly the kind of scenario it's made for. First thing u would need to do it's a delegate signature that accepts an array. Delegate void updateData(Array data); Then u set an event in ur parent form. event updateData OnUpdate. Next u need to create a method that conforms to the delegate signature in every child form. I would create a base form that implements it and then just subclass it. class baseForm:Form{ .... public virtual void DataUpdate(Array data){} } Now u just need to override the method in every form and select the data u want that form to display. Or u could use a single form and just pass the constructor the index parameter like this: class ChildForm: Form{ int index; public ChildForm(int DataToDisplayIndex){ index = DataToDisplayIndex; } public void DataUpdate(Array data){ this.textbox1.text = data[index].ToString(); } } Next u should subscribe to the parent form event like this: ChildForm child = new ChildForm(selectedDataIndex); OnUpdate += child.DataUpdate; After that it's just a matter of firing the event every time u receive new data: Array data = getNewestData(); if(OnUpdate != null){ OnUpdate.Invoke(data); } and presto! ur childForms will update automatically. don't forget to adda an event handler to the child form FormClosed event. Just like vulped explained before. ... child.FormClosed += this.Child_FormClosed; ... private void ChildForm_FormClosed(object sender FormClosedEventArgs e) { ChildForm child = (ChildForm)sender; child.FormClosed -= this.Child_FormClosed; OnUpdate-= child.DataUpdate; } Hope this help ya this was my first post ever! ;) Greetings from mexico!

Vulpes · Answer

Monte I notice that you are using VS 2003 which pre-dates the introduction of generics in C#. However there is still no need to use a fixed array. You can create a dynamic list of forms using an ArrayList which works like this: private ArrayList formList = new ArrayList(); // add form in some method - you can add as many as you like Form2 f2 = new Form2(); formList.Add(f2); // retrieve form at index 0 in some other method Form2 f2 = (Form2)formList[0]; The cast is necessary because the ArrayList stores everything internally as type object.

Mahesh Chand · Answer

Nice Monte. This is what community is all about. Learn and once you know help others and return the favor. Cheers!

Monte · Answer

Thanks for all the help and advice from everyone. I have got it working doing what Sam suggested by way of creating an array of Forms and an array of index numbers. When a new Child form is requested to be opened then a copy is saved in the array of forms along with the index in the array of indexes. When a timer counts down then the parameters in each Child form are updated by looping through the array of forms and updating values using the array of indexes and using properties (get/set) to update fields. The only negative is that I have to define the array of forms to be a fixed number of forms long. Hence (in my unexperienced mind) it is not very efficient in memory and there is a max number of forms that can be opened. I didnt use the MDI method as I didnt understand that so well. When I get spare time I will read up on MDI and have a play. I have uploaded the final (ish) project so anyone else trying to do the same thing in the future can see what I did. It is a Visual Studio 2003 project and gets information from another system called IMACS - so you may need to comment out the references to this to get it to compile. No exception handling has been added yet. Thanks again for all the help - maybe one day I will be able to help others and return the favour.

Frogleg · Answer

Here's two ways of passing data between forms Just post any questions

Sam Hobbs · Answer

Yes there is an easier way to do it; easier than what has been shown so far. Note that there is not a button labeled third in the parent; at least I don't see it. Do you mean bttnSend? I will attempt to describe what I suggest. First note that you are using MDI. Your main form is a MDI container and the child forms have the parent as a MDI parent. Many developers think that MDI means multiple windows but it means multiple documents with a window for each document. In your application you do have data that is unique to each child so that means you are using MDI appropriately. To use MDI in the manner it was intended to be used it would be more appropriate for the data to be in objects separate from the main form but that is a relatively minor detail. If however your actual requirements are for more than one item to be shown in the child forms then perhaps it would be better to not put the data in the main form. I think that .Net control classes are designed well in the manner that there is a lot of flexibility in the objects that can be used for the items that controls provide a UI for. So what that means for you is that you can create a class for objects of the combobox control. You can create instances of that class and add them to the combobox. One member of the class can be a reference to a child form. So then when a combobox item is selected the item will know what child form to update without doing a lookup. When the update button is clicked the button handler will get the item selected in the comboboxand then just use the child form reference in the object. The problem is that I am not sure that is relevant to your final solution. I get the impression that your prototype solution is not relevant to your actual requirements.

Vulpes · Answer

The reason it's not working at present is because the variable ChildForm will always refer to the last child form opened. As Sam said earlier you need to create a list of child forms opened. You'll also need to keep track of any closures. I don't know whether they are all instances of Form2 but I'll assume for now they are: private List childList = new List (); // private field for the list private void button3_Click(object sender System.EventArgs e) { Form2 child = new Form2(); child.ValueFromParent= this.masterTextBox.Text; child.FormClosed += this.Child_FormClosed; child.Show(); childList.Add(child); } // add this handler manually private void ChildForm_FormClosed(object sender FormClosedEventArgs e) { Form2 child = (Form2)sender; child.FormClosed -= this.Child_FormClosed; childList.Remove(child); } When you update the textboxes on each form you'll then need to use code such as this: foreach(Form2 child in childList) { child.ValueFromParent = whatever; }

Monte · Answer

Thanks Frogleg for the solution you provided - I have run it and see it works. I think I can adjust it to suit my requirements. I think you must have written this solution especially for me - so thanks very much for the time and effort. I am still wondering if there is a simple solution- as a relative newcomer to C# I figured there was something that I was missing like a tick box to say keep all ChildForms live etc etc. Thanks for your thoughts as well Sam - I am not using MDI Child forms. I can see that I should have added my code so people can see what I mean. I have created a simple project with two forms - Parent and Child. Forgetting Arrays of data that I mentioned before all I want to be able to do is keep creating Child Forms by event button3_Click. Then when I change data in a textbox in the Parent Form I want this change to be seen in the single textboxes on all the Child Forms. At present only the last ChildForm that I opened will be updated. Once I get this working then I can use the method on the proper project using arrays of data and a single data item to be passed to each child form using a timer. I am assuming that there is a simple fix due to something that I dont know about in C#. I have looked at lots of forums on this but struggle to see an answer to this particular problem. I have also uploaded the file - which is in Visual Studio 2003 - the button labelled third on the parent is the one that I am interested in. The others are just other things that I have tried such as Delegates. Many Thanks again in advance. ***************************** *****PARENT FORM************** *****NB Some code stripped out**** ***************************** using System; using System.Drawing; using System.Collections; using System.ComponentModel; using System.Windows.Forms; using System.Data; namespace FormToForm { /// /// Summary description for Form1. /// public class Form1 : System.Windows.Forms.Form { private System.Windows.Forms.Label label1; private System.Windows.Forms.Button button1; private System.Windows.Forms.Button button2; public System.Windows.Forms.TextBox masterTextBox; /// /// Required designer variable. /// private System.ComponentModel.Container components = null; private System.Windows.Forms.Button button3; private System.Windows.Forms.Button button4; private System.Timers.Timer timer1; //Added manually for the comms to work public Form2 ChildForm; public Form1() { // // Required for Windows Form Designer support // InitializeComponent(); // // TODO: Add any constructor code after InitializeComponent call // } /// /// The main entry point for the application. /// [STAThread] static void Main() { Application.Run(new Form1()); } private void button3_Click(object sender System.EventArgs e) { this.ChildForm = new Form2(); this.ChildForm.ValueFromParent= this.masterTextBox.Text; this.ChildForm.Show(); } private void masterTextBox_TextChanged_1(object sender System.EventArgs e) { if (this.ChildForm !=null) this.ChildForm.ValueFromParent = this.masterTextBox.Text; } } } ****************************** *****CHILD FORM**************** *****NB Some code stripped out***** ****************************** using System; using System.Drawing; using System.Collections; using System.ComponentModel; using System.Windows.Forms; namespace FormToForm { /// /// Summary description for Form2. /// public class Form2 : System.Windows.Forms.Form { private System.Windows.Forms.Label label2; private System.Windows.Forms.Button button1; public System.Windows.Forms.TextBox secondaryTextBox; /// /// Required designer variable. /// private System.ComponentModel.Container components = null; public Form2(string initialValue) { // // Required for Windows Form Designer support // InitializeComponent(); ValueFromParent = initialValue; // // TODO: Add any constructor code after InitializeComponent call // } public Form2() { // // Required for Windows Form Designer support // InitializeComponent(); // // TODO: Add any constructor code after InitializeComponent call // } public string ValueFromParent { set { this.secondaryTextBox.Text = value; } } } }

Frogleg · Answer

Try this