How can I get the array index of a given object in C# with control arrays?

Question

I am dynamically adding a bunch of controls to a form. Each control calls the same method, and in that method I need to know the array index of the the control that performed the action.

CheckBox[] myCB = new CheckBox[100];
int i;
for (i = 0; i < 100; i++)
{
    myCB[i] = new CheckBox();
    myCB[i].Text = "Clicky!";
    myCB[i].Click += new System.EventHandler(dynamicbutton_Click);
    tableLayoutPanel1.Controls.Add(myCB[i]);
}

private void dynamicbutton_Click(Object sender, System.EventArgs e)
{
    label1.Text = sender.???array index property???.ToString();
}

So if I click myCB[42] label1 will read "42" Of course, if there is an easier way to handle dynamic controls I'd appreciate pointers.

score 5 · Answer 1 · answered Jun 09 '09 at 17:50

5

private void dynamicbutton_Click(Object sender, System.EventArgs e)
{
    label1.Text = Array.IndexOf(myCB, (CheckBox)sender).ToString();
}

answered Jun 09 '09 at 17:50

SLaks

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2

This is the simplest answer, although not efficient for large number of checkboxes. For better performance it's probably better to use the Tag property of the checkbox. – rein Jun 09 '09 at 17:54
1

This is the correct answer, but I think that Jon and Rein are right - tags are a better general solution for this problem than finding the index. – Adam Davis Jun 10 '09 at 12:26

score 3 · Answer 2 · answered Jun 09 '09 at 17:48

3

Control's should have a Tag property. Maybe you can attach the index to the Tag. You will incur boxing though...

answered Jun 09 '09 at 17:48

BFree

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If he's explicitly looking for the index rather than an arbitrary ID/indicator, then this probably isn't the best way. – Noldorin Jun 09 '09 at 17:55
1

Well he did explicitly state: "Of course, if there's an easier way to handle dynamic controls I'd appreciate pointers...." Finding the index each time is horribly inefficient IMO. – Jon Skeet Jun 09 '09 at 17:57
Given that the code is being executed on a button click, and that there are only 100 to check through, efficiency is hardly going to be a big issue! – Noldorin Jun 09 '09 at 17:59
In particular, it associates the information with the object itself, which I view as a cleaner solution. – Jon Skeet Jun 09 '09 at 18:19

score 3 · Answer 3 · edited Jun 09 '09 at 21:37

3

int j = i;
myCB[i].Click += delegate(object sender, EventArgs e) {
 // here you can use "j"

};

edited Jun 09 '09 at 21:37

Vinko Vrsalovic

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answered Jun 09 '09 at 17:52

John Saunders

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Ooh, I like the use of closures to do this - that's pretty novel. – Noldorin Jun 09 '09 at 18:00
2

you could use i, but i would be 100. You would want to declare a new variable "int j = i; then use j within the delegate. – Timothy Carter Jun 09 '09 at 18:09
1

To do this, you MUST declare a separate variable inside the loop and assign it to i. `i` itself will be equal to 100 in all of the delegates, which is not what you want. – SLaks Jun 09 '09 at 18:10
Downvoted until the use of "i" is changed to reflect SLaks comment. See my answer for an example of how to do this safely. – Jon Skeet Jun 09 '09 at 18:19
@Jon: do I get my 2 points back even though someone else did the edit? – John Saunders Jun 09 '09 at 23:40

Jon Skeet · Accepted Answer · 2009-06-09T18:32:21.513

2

One obvious solution would be to set the tag:

CheckBox[] myCB = new CheckBox[100];
for (int i = 0; i < myCB.Length; i++)
{
    myCB[i] = new CheckBox();
    myCB[i].Text = "Clicky!";
    myCB[i].Click += new System.EventHandler(dynamicbutton_Click);
    myCB[i].Tag = i;
    tableLayoutPanel1.Controls.Add(myCB[i]);
}

Then:

private void dynamicbutton_Click(Object sender, System.EventArgs e)
{
    Control control = (Control) sender;
    label1.Text = sender.Tag.ToString();
}

Another alternative is to capture the information in the event handler, most simply using a lambda expression or anonymous method:

CheckBox[] myCB = new CheckBox[100];
for (int i = 0; i < myCB.Length; i++)
{
    int index = i; // This is very important, as otherwise i will
                  // be captured for all of them
    myCB[i] = new CheckBox();
    myCB[i].Text = "Clicky!";
    myCB[i].Click += (s, e) => label1.Text = index.ToString();
    tableLayoutPanel1.Controls.Add(myCB[i]);
}

or for more complicated behaviour:

CheckBox[] myCB = new CheckBox[100];
for (int i = 0; i < myCB.Length; i++)
{
    int index= i; // This is very important, as otherwise i will
                  // be captured for all of them
    myCB[i] = new CheckBox();
    myCB[i].Text = "Clicky!";
    myCB[i].Click += (s, e) => DoSomethingComplicated(index, s, e);
    tableLayoutPanel1.Controls.Add(myCB[i]);
}

(where you declare DoSomethingComplicated appropriately).

edited Jun 09 '09 at 18:32

answered Jun 09 '09 at 17:50

Jon Skeet

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Why do you need to declare a new local variable, aren't you just assigning a value which is changing each time? I mean, index.ToString() returns a new string instance, doesn't it? – Vinko Vrsalovic Jun 09 '09 at 18:02
3

Without the new local variable, the "i" variable is captured: not the value, but the variable itself. That means when you clicked on the label, it would set the value to 100 every time. Try it! This is a well-known issue with captured variables and loops. – Jon Skeet Jun 09 '09 at 18:18
1

This is a feature, not a bug. See http://en.wikipedia.org/wiki/Closure_(computer_science) – SLaks Jun 09 '09 at 18:58
I'd say it's a feature in the "for" loop, but a flaw in language design for the "foreach" loop: it certainly *looks* like a new variable is being created in each declaration of the loop with foreach. (See http://stackoverflow.com/questions/969729 for more discussion on this.) – Jon Skeet Jun 09 '09 at 18:59
@SLaks, it has nothing to do directly with closures, but to whether a loop creates a new local variable on each iteration or it reuses a (n implicit) variable declared just outside the loop. Closures will manifest the same 'problem' if you use a while and a variable declared outside. – Vinko Vrsalovic Jun 09 '09 at 21:36
@Vinko: It absolutely has to do with closures. It's only because it's a closure that the *variable* is captured rather than just its value at the point of the delegate creation. – Jon Skeet Jun 10 '09 at 12:28

How can I get the array index of a given object in C# with control arrays?

4 Answers4