ComboBox selection generating list for another comboBox question

[William Winner]

It would have solved some issues if we knew that you were never getting to the SelectedIndexChanged code. I assumed that you had already handle the EditControlShowing event and added the event handler for the SelectedIndexChanged event. Henry gave you code to do this, but I can try to explain what is happening since you seemed a bit confused about it. When you create the DataGridView, you can specify columns and specify which column within a table or a datasource that that column is linked to. You can also set up what type of control the column is edited with. You can choose a simple TextBox, or you can have other types, for instance the ComboBox type. When the user clicks within a cell, the DataGridView then checks that column to see what type of control to use to allow the user to edit the cell. This is called the EditControl. If the column type was selected to be a ComboBox, then it will use a ComboBox that the DataGridView created to allow the user to edit the value. It creates the control once and then re-uses it. The EditControlShowing event is set off when the control is actually shown. This ComboBox is just a standard ComboBox and has all of the same events and properties as a standard ComboBox. One of the arguments passed during this event is e.Control. This is the control that is being used. It won't always be a ComboBox...it will be whatever type of control that is being used for that column. So, when that control is passed, you then need to add a handler to whichever event you are interested in...in this case, the SelectedIndexChanged event. You also want to make sure to remove the handler afterwards so that it doesn't get called more than once. In Henry's case, he removed it before adding a new one which is perfectly reasonable since the user could change the selection more than once without the EditControl changing. This may be a bit rambly, but I hope it helps with your understanding of what's happening. I wish you luck with this, but I'm still going to say that an internet course without feedback and online tutorials isn't going to get you far. Your company may not be able to afford a programmer, and it may not fit the timetable for this particular project, but I would suggest finding an entry level course on programming at your community college or state university. You can generally do that pretty cheaply and having an instructor that you can ask questions of can help a ton.

[Henry Minute]

OK I have looked at your picture and I have to say that I am surprised that you can even reach the keyboard, still let's press on. The

ComboBox cb = e.Control as ComboBox;

line is not instantiating a new ComboBox, it is declaring a variable of the ComboBox type (on the left hand side of the '=') and making it reference (point to) the ComboBox in the cell that you are currently editing. The event that is being handled is the EditingControlShowing event and the DataGridViewEditingControlShowingEventArgs passed to it contain a reference to the editing control being used (e.Control). Because the column we are talking about is a DataGridViewComboBoxColumn, when you start to edit one of its cells, the DataGridView control pops up a real live ComboBox for you to do the edit with (hence the as ComboBox part). (If it were a DataGridViewTextBoxColumn the DGV would give you a TextBox or if it was a DataGridViewCheckBoxColumn the DGV would give you a CheckBox.) So the line is saying to the compiler "Please reserve enough space to hold a reference to a ComboBox which I'll be calling 'cb' and put a copy of the pointer from e.Control in there. Thank you!" (please note that it is a very polite line). Basically it just makes it easier to refer to cb.SelectedIndexChanged for example, rather than:

	private void dataGridView1\_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
	{
		if ((ComboBox)e.Control != null)
		{
			// first remove event handler to keep from attaching multiple:
			((ComboBox)e.Control).SelectedIndexChanged -= cb\_SelectedIndexChanged;
			// now attach the event handler
			((ComboBox)e.Control).SelectedIndexChanged += cb\_SelectedIndexChanged;
		}
	}

which we would have to do otherwise. I hope that you'll agree that the first version is easier to read than this one. If it helps, think of cb as being a 'magic' PhotoCopy of the ComboBox from the cell you are editing. It is magic because any thing you do to the copy also happens to the original. So when the code says to hook up column1DataGridViewTextBoxColumn_SelectedIndexChanged to the copy it is also hooking it up to the real one in your DataGridView When dataGridView1_EditingControlShowing ends, cb is thrown

[mprice214]

Crystal clear. I appreciate you taking the time. Maybe we could trade CAD work for software. :-D

[mprice214]

Thanks William. I always knew you software guys have big hearts. I have three small children and I think my wife might go crazy if I do that now, but it may be worth the risk... :laugh: I suggested to Henry that I should trade CAD work for software..... :laugh: Anyway, I'll be working on this later and will let you know how it goes, but both you and Henry have made things very clear.

[mprice214]

Almost there.....maybe. I'm having object type issues: I've cleaned things up and have the following:

private void column1DataGridViewComboBox_SelectedIndexChanged(object sender, DataGridViewCellEventArgs e)
{

        string strPrimary = dataGridView1\[e.ColumnIndex, e.RowIndex\].Value.ToString();
        Debug.WriteLine(strPrimary);
        if (strPrimary == "Test1")
        {

            //Build a list 
            var dataSource = new List<Units>();
            dataSource.Add(new Units() { Unit = "blah1" });
            dataSource.Add(new Units() { Unit = "blah2" });
            dataSource.Add(new Units() { Unit = "blah3" });

            //Setup data binding 
            this.column2DataGridViewComboBox.DataSource = dataSource;
            this.column2DataGridViewComboBox.DisplayMember = "Unit";

        }
    }



    private void dataGridView1\_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
    {
        ComboBox cb = e.Control as ComboBox;

        if (cb != null)
        {
            // first remove event handler to keep from attaching multiple:
            cb.SelectedIndexChanged -= column1DataGridViewComboBox\_SelectedIndexChanged;
            // now attach the event handler
            cb.SelectedIndexChanged += column1DataGridViewComboBox\_SelectedIndexChanged;
        }
    }

I get "No overload for 'column1DataGridViewComboBox_SelectedIndexChanged' matches delegate 'System.EventHandler'" However, as you probably know, I can change

private void column1DataGridViewComboBox_SelectedIndexChanged(object sender, DataGridViewCellEventArgs e)

to

private void column1DataGridViewComboBox_SelectedIndexChanged(object sender, EventArgs e)

which fixes that. But I need the DataGridViewCellEventArgs class to access the ColumnIndex and RowIndex properties. Thoughts?

[Henry Minute]

For the SelectedIndexChanged handler, you don't need the Column and Row because each time the handler gets called it is called by the relevant ComboBox. Therefore you can get at the ComboBox by Casting the sender parameter. Here we go again. :-D

private void column1DataGridViewComboBox\_SelectedIndexChanged(object sender, EventArgs e)
{
    ComboBox cb = sender as ComboBox;
    string someString = cb.SelectedItem;
    // OR
    int cbIndex = cb.SelectedIndex;  // and so on
}

or, to do it the longer, more difficult to read way

private void column1DataGridViewComboBox\_SelectedIndexChanged(object sender, EventArgs e)
{
    string someString = ((ComboBox)sender).SelectedItem;
    // OR
    int cbIndex = ((ComboBox)sender).SelectedIndex;  // and so on
}

Good luck! :)

Henry Minute Do not read medical books! You could die of a misprint. - Mark Twain Girl: (staring) "Why do you need an icy cucumber?" “I want to report a fraud. The government is lying to us all.” Why do programmers often confuse Halloween and Christmas? Because 31 Oct = 25 Dec.

[mprice214]

Wait, let me take my earplugs out....Oh yes, now I think I can see :-D

private void dataGridView1_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
{
ComboBox cb = e.Control as ComboBox;
if (cb != null)
{
// first remove event handler to keep from attaching multiple:
cb.SelectedIndexChanged -= column1DataGridViewComboBox_SelectedIndexChanged;
// now attach the event handler
cb.SelectedIndexChanged += column1DataGridViewComboBox_SelectedIndexChanged;
}

Since DataGridViewEditingControlShowingEventArgs is handling the EditingControlShowing event and will be passing the arguments to the SelectedIndexChanged event handle, the SelectedIndexChanged event cannot have a handling class that is trying to control the same thing at the same time (in this case the ComboBox). Do you concur?

[Henry Minute]

mprice214 wrote:

Since DataGridViewEditingControlShowingEventArgs is handling the EditingControlShowing event and will be passing the arguments to the SelectedIndexChanged event handle

I think that you have slightly misunderstood what is happening here. The EditingControlShowing handler does not pass any arguments, it simply tells the ComboBox where to find the code to execute when its SelectedIndex changes and regardless of whatever else is going on it will fire off that code when the index does change. Of course it may not execute it instantly, not even modern computers can really do more than one thing at a time. It has just occurred to me that you are actually asking about the '-=' and '+=' bits. They are like that because the SelectedIndexChanged contains a list of methods that it should execute. If we used the '+=' line only, then each time we did so another instance of the column1DataGridViewComboBox_SelectedIndexChanged would be added to the list meaning that it would execute once after the first time, twice after the second, three times...... You get the idea. So we first '-=' it to remove the handler from the list, before adding it again. Of course we could test to see if it was there already NOTE** this is not real code

if (!cb.SelectedIndexChanged.List.Contains(column1DataGridViewComboBox_SelectedIndexChanged))
{
cb.SelectedIndexChanged += column1DataGridViewComboBox_SelectedIndexChanged;
}

oops forgot to close the tag. but the '-=' then '+=' method is quicker. Hope that this helps. :)

Henry Minute Do not read medical books! You could die of a misprint. - Mark Twain Girl: (staring) "Why do you need an icy cucumber?" “I want to report a fraud. The government is lying to us all.” Why do programmers often confuse Halloween and Christmas? Because 31 Oct = 25 Dec.

modified on Wednesday, May 19, 2010 10:31 AM

[mprice214]

Henry Minute wrote:

I think that you have slightly misunderstood what is happening here.

That's a surprise. :^) Got it, but also SelectedIndexChanged cannot access the control via DataGridViewCellEventArgs because the control is accessed via EditingControlShowing with the DataGridViewEditingControlShowingEventArgs class, correct?

[Henry Minute]

Ah now I understand.

mprice214 wrote:

also SelectedIndexChanged cannot access the control via DataGridViewCellEventArgs because the control is accessed via EditingControlShowing with the DataGridViewEditingControlShowingEventArgs class, correct?

No, that is not quite right. The reason is that each event handler has its own, internally defined, set of parameters, that you have to adhere to. Most event handlers have the object sender (in fact I can't think of one off the top of my head that doesn't) after that most also have some form of EventArgs and if the internal definition says it has to have a plain old EventArgs, then it will only work with EventArgs. If the definition says it should have an DataGridViewCellEventArgs then it will only work if you declare it with one of those. SelectedIndexChanged has an EventArgs because it doesn't need to pass any additional information. You know who sent it from the sender parameter. The fact that the event has fired means that the SelectedIndex of the sender has changed and that is all the information you need to make use of it DataGridViewEditingControlShowingEventArgs holds additional information about where in the DataGridView (Column, Row) the event was fired from.

Henry Minute Do not read medical books! You could die of a misprint. - Mark Twain Girl: (staring) "Why do you need an icy cucumber?" “I want to report a fraud. The government is lying to us all.” Why do programmers often confuse Halloween and Christmas? Because 31 Oct = 25 Dec.

[mprice214]

I understand. As I continue, I'm sure it will more embedded.

[mprice214]

Guess who. I have the following:

private void column1DataGridViewComboBox_SelectedIndexChanged(object sender, EventArgs e)
{

        ComboBox cb = sender as ComboBox;

     
        string strPrimary = cb.SelectedItem.ToString();
        Debug.WriteLine(strPrimary);

        var dataSource = new List<Units>();

        switch (strPrimary)
        {
            case "1":
                
            //Build a list 
            
            dataSource.Add(new Units() { Unit = "blah1" });
            dataSource.Add(new Units() { Unit = "blah2" });
            dataSource.Add(new Units() { Unit = "blah3" });

            //Setup data binding 
            this.column2DataGridViewComboBox.DataSource = dataSource;
            this.column2DataGridViewComboBox.DisplayMember = "Unit";

            break;

            case "2":

            //Build a list 
            dataSource.Add(new Units() { Unit = "blah4" });
            dataSource.Add(new Units() { Unit = "blah5" });
            dataSource.Add(new Units() { Unit = "blah6" });

            //Setup data binding 
            this.column2DataGridViewComboBox.DataSource = dataSource;
            this.column2DataGridViewComboBox.DisplayMember = "Unit";

            break;

            case "3":

            //Build a list 
            dataSource.Add(new Units() { Unit = "blah7" });
            dataSource.Add(new Units() { Unit = "blah8" });
            dataSource.Add(new Units() { Unit = "blah9" });

            //Setup data binding 
            this.column2DataGridViewComboBox.DataSource = dataSource;
            this.column2DataGridViewComboBox.DisplayMember = "Unit";

            break;

        }
    }

Well, the problem of course as you know is that ComboBox cb points to the second comboBox once I select it. In fact, once I select a value on the first comboBox and then select the second comboBox to select a value, debug.writeline gives me "WindowsFormsApplication1.Form1+Units" (although the 2nd comboBox does get populated with the appropriate list based upon the selection of the first comboBox). If I use an integer instead of strPrimary and set it as cb.SelectedIndex, what I actually get from debug is the index of the selection of the list of the second comboBox (eg if blah5 is selected, I get an integer = 1) followed by an integer

[Henry Minute]

Aaaaand relax! I think I know what is going on here. If you recall, well even if you don't recall actually :) , in my second message to you I had this bit of code: (I've annotated it a bit and modified it v,v slightly)

private void dataGridView1\_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
{
   ComboBox cb;

   // switch (dataGrid.CurrentCell.ColumnIndex)   <===================================== silly! already have column in eventargs
   switch (e.Column)                              //<==============================Use this instead
   {
       case 0:  // use the index for your comboboxcolumn    <============ **V IMPORTANT** make sure that the 0: matches the index for your column
           cb = e.Control as ComboBox;
           if (cb != null)
           {
           // first remove event handler to keep from attaching multiple:
           cb.SelectedIndexChanged -= column1DataGridViewComboBox\_SelectedIndexChanged;
           // now attach the event handler
           cb.SelectedIndexChanged += column1DataGridViewComboBox\_SelectedIndexChanged;
           }
           break;
        // <================================== You do not need the part below since you only need it to apply to the first cbcolumn
      //case 3: // use the index for your second combo
        //   cb = e.Control as ComboBox;
          // if (cb != null)
           //{
           // first remove event handler to keep from attaching multiple:
         //  cb.SelectedIndexChanged -= cb\_SecondSelectedIndexChanged;
           // now attach the event handler
          // cb.SelectedIndexChanged += cb\_SecondSelectedIndexChanged;
           //}
           //break;
   }  
}

If you can absolutely guarantee that you will only want to do this for one ComboBoxColumn you can simplify the code slightly.

private void dataGridView1\_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
{
   ComboBox cb;

   // replace the switch statement by an if statement
   if (e.Column == 0)  //<============ **V IMPORTANT** change the 0 (if necessary) to  match the index for your column
   {
           cb = e.Control as ComboBox;
           if (cb != null)
           {
           // first remove event handler to keep from attaching multiple:
           cb.Selec

[mprice214]

Good morning (or whatever time of day it is to you). DataGridViewEditingControlShowingEventArgs does not contain the "column" method. I do remember your second message and did try to do that yesterday without success. Let me give you both methods together:

private void column1DataGridViewComboBox_SelectedIndexChanged(object sender, EventArgs e)
{

        ComboBox cb = sender as ComboBox;

      
        int intPrimary = cb.SelectedIndex;
        Debug.WriteLine(intPrimary + "SelectedIndexChanged");

        var dataSource = new List<Units>();

        switch (intPrimary)
        {
            case 0:
                
            //Build a list 
            
            dataSource.Add(new Units() { Unit = "blah1" });
            dataSource.Add(new Units() { Unit = "blah2" });
            dataSource.Add(new Units() { Unit = "blah3" });

            //Setup data binding 
            this.column2DataGridViewComboBox.DataSource = dataSource;
            this.column2DataGridViewComboBox.DisplayMember = "Unit";

            break;

            case 1:

            //Build a list 
            dataSource.Add(new Units() { Unit = "blah4" });
            dataSource.Add(new Units() { Unit = "blah5" });
            dataSource.Add(new Units() { Unit = "blah6" });

            //Setup data binding 
            this.column2DataGridViewComboBox.DataSource = dataSource;
            this.column2DataGridViewComboBox.DisplayMember = "Unit";

            break;

            case 2:

            //Build a list 
            dataSource.Add(new Units() { Unit = "blah7" });
            dataSource.Add(new Units() { Unit = "blah8" });
            dataSource.Add(new Units() { Unit = "blah9" });

            //Setup data binding 
            this.column2DataGridViewComboBox.DataSource = dataSource;
            this.column2DataGridViewComboBox.DisplayMember = "Unit";

            break;

        }
    }



    private void dataGridView1\_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
    {
        ComboBox cb;


        Debug.WriteLine(dataGridView1.CurrentCell.ColumnIndex + "EditControlShowing");
        switch (dataGridView1.CurrentCell.ColumnIndex)
        {
            case 0:
                
            cb = e.Contr

[Henry Minute]

mprice214 wrote:

DataGridViewEditingControlShowingEventArgs does not contain the "column" method.

:-D That's what you get for trying to code off the top of your head. Good to see that you were smart enough to use the correct form.:thumbsup: Once again I think I have worked out what is happening. In order to save on system resources, dear old M$ are using the same ComboBox for all ComboBoxCells and just hiding it, rather than destroying it, when not needed. Because it is just hidden, the link to the SelectedIndexChanged never gets deleted, which is why it occurs for column2 as well. After very little thought I see three ways to handle the problem: 1. in the SelectedIndexChanged handler test for the column index before anything else and bomb out (a technical term :) ) if not the one we want.

	private void column1DataGridViewComboBox\_SelectedIndexChanged(object sender, EventArgs e)
	{
		if (dataGridView1.CurrentCell.ColumnIndex != 0)   // <=========== Bombing out construct
		{
			return;
		}

		ComboBox cb = sender as ComboBox;

		int intPrimary = cb.SelectedIndex;
		Debug.WriteLine(intPrimary + "SelectedIndexChanged");

                    ................................
                    ................................
                    ................................
                    ................................
             }

2. in the EditingControlShowing handler remove the link when we don't find the column we want.

    private void dataGridView1\_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
    {
        ComboBox cb;


        Debug.WriteLine(dataGridView1.CurrentCell.ColumnIndex + "EditControlShowing");
        switch (dataGridView1.CurrentCell.ColumnIndex)
        {
            case 0:
                
            cb = e.Control as ComboBox;
            if (cb != null)
            {
                // first remove event handler to keep from attaching multiple:
                cb.SelectedIndexChanged -= column1DataGridViewComboBox\_SelectedIndexChanged;
                // now attach the event handler
                cb.SelectedIndexChanged += column1DataGridViewComboBox\_SelectedIndexChanged;
            }
            break;
            default:

[mprice214]

Thanks for your help on this. I was able to become much more familiar with the event handlers. I began focusing on DataGridView FAQ Appendix A, 18 that you mentioned a bit ago. I was trying to get that going and was still having some issues and then came across http://social.msdn.microsoft.com/forums/en-US/winformsdatacontrols/thread/6f6c2632-afd7-4fe9-8bf3-c2c8c08d1a31/[^] Anyway, I'll have to tweak it a bit to keep users from pulling down the second comboBox first (which will throw and exception), but it's exactly what I need. Anyway, you taking the time really helped me out to understand some "basic" things that will help me get through the rest of this project. Thanks again.

public partial class Form1 : Form
{
DataTable tblPrimary, tblSecondary;
BindingSource primaryBS, filteredSecondaryBS, unfilteredSecondaryBS;

    public Form1()
    {

        tblPrimary = new DataTable("Primary");
        tblPrimary.Columns.Add("ID", typeof(int));
        tblPrimary.Columns.Add("Name", typeof(string));

        tblSecondary = new DataTable("Secondary");
        tblSecondary.Columns.Add("ID", typeof(int));
        tblSecondary.Columns.Add("subID", typeof(int));
        tblSecondary.Columns.Add("Name", typeof(string));

        tblPrimary.Rows.Add(new object\[\] { 0, "Force" });
        tblPrimary.Rows.Add(new object\[\] { 1, "Torque" });
        tblPrimary.Rows.Add(new object\[\] { 2, "Pressure" });

        tblSecondary.Rows.Add(new object\[\] { 0, 0, "lb" });
        tblSecondary.Rows.Add(new object\[\] { 1, 0, "N" });
        tblSecondary.Rows.Add(new object\[\] { 2, 0, "oz" });
        tblSecondary.Rows.Add(new object\[\] { 3, 1, "in-lb" });
        tblSecondary.Rows.Add(new object\[\] { 4, 1, "ft-lb" });
        tblSecondary.Rows.Add(new object\[\] { 5, 1, "N-m" });
        tblSecondary.Rows.Add(new object\[\] { 6, 2, "PSI" });
        tblSecondary.Rows.Add(new object\[\] { 7, 2, "Pa" });
        tblSecondary.Rows.Add(new object\[\] { 8, 2, "bar" });
        
        InitializeComponent();

        primaryBS = new BindingSource();
        primaryBS.DataSource = tblPrimary;