C# DropDown smart logic implementation

Posted on 2014-10-20
Last Modified: 2014-11-27
Hi Experts,

I've got a ComboBox in ComboBoxStyle.DropDownList style.

Dropdown selection
This gets dunamically created based on the number of a passed parameter. The letters is an ArrayList of letters which are added to the dropdown

for (int a = i; a < drivesLetters.Count; a++)

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I've been trying to create a method so that when I click on a dropdown list, the letters which are already selected should not show up from the selections. Like on the image above, If I click on M, then M should not show up. If I select V, then V should be selected, not show on the list, M should be added.

I'm not sure how to actually do this. I wonder if I should have two ArrayLists (one holds the letters used in the dropdown, and another one to hold all the unused letters).

I am looking at an exact implementation like this (in javascript) but obviously in C#. I've tried to convert the javascript method to C#, but I was not successful.

Please kindly point me to a good starting point. I've tried so many methods, but no success..

Question by:John Solomon
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LVL 40

Expert Comment

by:Kyle Abrahams
ID: 40395225
You can use the drawItem event.  (

Something like:

  private void ComboBox1_DrawItem(object sender, System.Windows.Forms.DrawItemEventArgs e)
            if (e.Index == ComboBox1.SelectedIndex)
                e.Graphics.FillRectangle(new SolidBrush(Color.White), new Rectangle(2,e.Bounds.Top+2, e.Bounds.Width, 0));

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Author Comment

by:John Solomon
ID: 40395368
Hi Kyle Abrahams, thank you for your answer. However, I am not sure how this method would achieve my goal.
If index is equal to combobox selectedIndex, then redraw a rectangle combobox?.

I might have confused you with my question, and I see that the javascript link was not working.. I was hoping to get something like this combo box selection implementation.

Where a number of comboboxes {combo1, combo2. combo3} are filled with a list of letters {A,B,C,D,E, F, G, H}. In order to prevent a user from choosing the same letter A in combo1 and combo3, if A is already selected in combo1, then it should not show up in combo3 dropdown. Given than letter A is unselected in combo1 and is not selected in any other comboboxes, then letter A should be available in a comboboxes dropdown as selectable.

Thank you very much for your help :)

Author Comment

by:John Solomon
ID: 40395387
Hi Kyle, or an implementation like this. Thanks
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Author Comment

by:John Solomon
ID: 40395409
Or this is a closer implementation that I am looking for javascripcode

I'll try to learn and convert this code into C#. But your help will be greatly appreciated. :) thanks!
LVL 40

Accepted Solution

Kyle Abrahams earned 500 total points
ID: 40395428
Hi John,

Okay, that makes a lot more sense.

What I was proposing was let's say you chose "M" . . . you would not want to see "M" in the same combo box in the values again.  That's not usually the default but I don't question people's intentions.

This should work:

PopulateCombos(ComboBox MychangedCombo)
    string drive1 = combo1.Text;
    string drive2 = combo2.Text;
    string drive3 = combo3.Text;

// populate 1 - do not populate if that's the one that was changed)
   if (MychangedCombo != combo1)
     for (int a = 0; a < drivesLetters.Count; a++)
               if (drivesLetters[a] == drive2 || drivesLetters[a]  == drive3)
                   comboBox1.Text = drive1;
//repeat for 2 && 3  

You can then wire all 3 combo_selectedIndexchanged to:

DrivesChanged(object sender, eventargs e)
   PopulateCombos((ComboBox) sender);

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LVL 33

Expert Comment

ID: 40395788
Without having a VS currently at hands, my approach would be using the MeasureItem event to set the height of the item to 0 and skip it in your own owner draw event.

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