Help I am getting an error CS0079 in my code and I don't know why.

Question

I am trying to make some flexible UI that creates a set number of buttons based on a list. I have done this before and had no problems. However, when I try to do it again using almost the exact same code I am getting a strange error.

The code I used the first time.

 foreach (ChoiceButton choiceButton in buttons)
     {
       // Instantiate button
       GameObject newButton = Instantiate(choiceButtonPrefab, choiceButtonPosStart, Quaternion.identity, choiceButtonParent.transform);
 
       // Set button position
       Vector3 choiceButtonPos = choiceButtonPosStart;
       choiceButtonPos.x += _x * choiceButtonPosOffsetX;
       newButton.transform.localPosition = choiceButtonPos;
       _x++;
 
       // Set button text
       newButton.GetComponentInChildren<TextMeshProUGUI>().text = choiceButton.label;
 
       // Add button event listener
       newButton.GetComponent<Button>().onClick.AddListener(() => onClickChoice(choiceButton.newSceneID));
     }

The code giving me the error.

 foreach(string projectPath in projectPaths)
     {
       // Instantiate button
       GameObject projectButton = Instantiate(projectButtonsPreFab, projectButtonsPosStart, Quaternion.identity, projectButtonsParent.transform);
 
       // Set button position
       Vector3 projectButtonsPos = projectButtonsPosStart;
       projectButtonsPos.y += y * projectButtonsPosOffsetY;
       projectButton.transform.localPosition = projectButtonsPos;
       y++;
 
       // Set button text
       string buttonText = projectPath.Remove(0,16);
       projectButton.GetComponentInChildren<TextMeshProUGUI>().text = buttonText;
 
       //Add button event listener
       projectButton.GetComponent<Button>().onClick.AddListener(() => UpdateUISceneButtons(projectPath));
     }

For some reason the last line of the code

 projectButton.GetComponent<Button>().onClick.AddListener(() => UpdateUISceneButtons(projectPath));

is giving me this error.

The event 'Button.onClick' can only appear on the left hand side of += or -=

And yet I did almost the exact same thing in the first script and it had no problem with it. So why is it throwing an error now?

I have been trying to find the answer online and yet can't find anything.

Any help would be much appreciated.

Answer 1

Answer by exploringunity · Jun 10, 2020 at 06:15 PM

Check your imports. My guess is that in your first script you are using IMGUI -- that is, you have this line of code in your script:

 using UnityEngine.UI;

and in the second one you are using UIElements -- that is, you have this line of code in your script instead of the one above:

 using UnityEngine.UIElements;

The reason for the compilation errors is that onClick is a different type for IMGUI buttons vs UIElements buttons. UnityEngine.UI.Button uses onClick.AddListener(() => YourFunction()) like you have, but UnityEngine.UIElements.Button used to use the C# event style onClick += YourFunction -- actually though, that property is obsolete on UIElements buttons, and you should use clicked += YourFunction instead. Hope this helps!

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Help I am getting an error CS0079 in my code and I don't know why.

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