Can't pass a variable as method parameter?

Question

OK, this is a tricky one for me...

I'm trying to add a OnClick listener to my buttons:

 int i = 0;
             foreach (var optionString in optionsCollection.options)
             {
 
                 testButtons[i].onClick.AddListener(() => SetOption(i));
           
                 i++;       
             }

But for some reason when I click the button I get this error:

ArgumentOutOfRangeException: Argument is out of range. Parameter name: index System.Collections.Generic.List`1[System.Collections.Generic.KeyValuePair`2[System.String,System.String]].get_Item (Int32 index) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/List.cs:633) Yarn.VirtualMachine.m_0 (Int32 selectedOption) Yarn.Unity.PopulateUI.SetOption (Int32 selectedOption) (at Assets/Scripts/PopulateUI.cs:116) Yarn.Unity.PopulateUI+cIterator5.<>m_0 () (at Assets/Scripts/PopulateUI.cs:93) UnityEngine.Events.InvokableCall.Invoke (System.Object[] args) ...

But if I go like this:

    int i = 0;
             foreach (var optionString in optionsCollection.options)
             {
 
                 if (i == 0)
                 {
                     testButtons[i].onClick.AddListener(() => SetOption(0));
                 }else
                 {
                     testButtons[i].onClick.AddListener(() => SetOption(1));
                 }
                 i++;       
             }

...it works! My question is why I can't pas the int i as a parameter of the function SetOption? Why if I hardcode the number it works?

Thank you

Answer 1

Answer by NoseKills · Sep 21, 2016 at 08:04 PM

There's no "problem" with what you are doing here, except that this code works probably a bit differently than what one would easily expect.

In C#, when you write a closure like that, it is not the value of i that gets captured in the anonymous method, but it's the variable itself. This almost makes it seem like the fundamentals of value types vs. reference types gets broken.

After you finish that loop, i has the value optionsCollection.options.Count (or .Length) and whenever you finally click your buttons, all of them call SetOption with that same value.

If you want to prevent this, you have to pass to the closure an integer that you don't change anywhere later.

 int i = 0;
 foreach (var optionString in optionsCollection.options)
 {
     var temp = i;
      testButtons[i].onClick.AddListener(() => SetOption(temp));
      i++;       
 }

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Rodlaiz · Sep 22, 2016 at 12:03 PM 0

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Hello @Nose$$anonymous$$ills

Thank you very much for your answer.

I've tried your solution and I don't get the error anymore :)

Still, I don't know why but it doesn't work. Both buttons are getting the same value (1, ins$$anonymous$$d of 0 and 1 each)

I don't believe is an instance problem because with the "if" solution the buttons works just fine. I've debugged the temp var before assigning it to the method and the values are correct (0 and 1)

Can't pass a variable as method parameter?

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