MFC Edit control produces debug assert on first action

Question

MFC Edit control produces debug assert on first action

Bryan Kelly 486

Windows 11, Visual Studio 17.13.3, C++, MFC, dialog app

Question: In the dialog constructor, add code for an Edit Control to clear it and get a popup dialog stating: Debug Assertion Failed!

Details

I solicited Bing.com and ChatGPT (right name?) for example code and followed it. Found a learn.microosft.com web page and followed it. They give the same advice. Evidently there is something I don’t recognize because both produced the same result.

Create an MFC project with C++, add a second dialog, and get that dialog to open, build, run, be sure that works. In the second dialog, drop an Edit control and add code in the constructor to clear it. At the bottom of the h file VS automatically added this.

class C_Show_Numbers : public CDialog

{

``DECLARE_DYNAMIC(C_Show_Numbers)

…

public:

``CEdit ec_show_numbers_3;

``afx_msg void OnEnChangeShowNumbers3();

};

In the dot cpp file I added this

C_Show_Numbers::C_Show_Numbers(CWnd* pParent /*=nullptr*/)

``: CDialog(IDD_SHOW_NUMBERS, pParent)

{`` ``ec_show_numbers_3.Clear();

``ec_show_numbers_3.SetWindowTextW(_T("third text"));`` ``}

The result is the dialog shown below.

edit_contol_assert_01 So what did I miss in the instructions?

David Lowndes 2,640 Reputation points MVP

2025-04-26T21:12:31.6766667+00:00

The underlying Windows controls have yet to be created in a CDialog constructor.
Do such initialisation in the dialog's OnInitDialog handler.
Bryan Kelly 486 Reputation points

2025-04-27T03:54:40.2766667+00:00

Answer looks good, but, what is the initialization code needed?

From another thread I found:

            CEdit* pEditCtrl = (CEdit*)this->GetDlgItem(IDC_EDIT1);

            ``if (pEditCtrl)

            ``{

                        ``pEditCtrl->SetWindowText(newValue);

            ``}

But that creates a pointer forcing the use of that pointer. The object already exists. What is the code to initialize it? Maybe even better, where can I find a description of the class and the initialization code? I did a search and found a description of CEdit Class here: CEdit Class | Microsoft Learn But it creates a new instance and a pointer. I already have an instance in the MFC app. I just need to initialize it and don’t recognize that code.
David Lowndes 2,640 Reputation points MVP

2025-04-27T09:52:25.45+00:00

That is code you could use if you haven't mapped a control variable to the underlying control (which you may have done from what you showed - though I can't be sure).

If you already have a control member variable then you can use it directly in OnInitDialog as you had tried to do in the constructor.

Have a read about the MFC dialog framework here.
Bryan Kelly 486 Reputation points

2025-04-29T03:55:07.24+00:00

I followed the link, and several threads that it led to. I could not recognize the code to initialize the Edit Control.

So simplify a bit. Create an MFC project with a dialog, build, and run it. OK.

Use the Toolbox and add an Edit Control. Use the default ID of IDC_EDIT1. Build, run, the dialog is present. It has been initialized to some degree.

The edit control appears to be CEdit. With that control in my project, search the entire project for CEdit. Not found. How is this created if the class is not mentioned? Maybe an including class is used, but what is that class?

Search the solution for IDC_EDIT1. It is declared in Resource.h. Looks like it is placed in MFCButton4.rc. But not found anywhere else.

So, I guess, back to the original question. Please show the code needed to initialize the edit control and write text “123” to the control. Preferably in the constructor. So the app starts with that text displayed.

Thank you in advance for your time and patience.
David Lowndes 2,640 Reputation points MVP

2025-04-29T11:01:28.9533333+00:00
If you want to attach text to an edit control in an MFC dialog box, you can do this:

Right click on the edit control in the dialog editor and choose Add Variable...

In the Add Control Variable dialog, change the Category from Control to Value. Give your variable (now a CString) a name and proceed to create it.

In your program's InitInstance (where your dialog class is instantiated in a simple dialog application), prior to calling DoModal, set your text member variable, something like this:

CMFCApplication1Dlg dlg; m_pMainWnd = &dlg; dlg.m_strText = "Hello world"; INT_PTR nResponse = dlg.DoModal();

Build and run your program, you should see the text in your dialog's edit control.

Note that you can only do this for values, if you map an MFC control class to the Window control on the dialog you need to do the operations in the dialog's OnInitDialog handler - where the underlying Windows control has been created and the MFC mapping to your control class variable has taken place.

Have a look at this documentation too.
Bryan Kelly 486 Reputation points

2025-05-02T04:07:18.3433333+00:00

David Lowndes, your answer is a bit over my head. I am at home, alone, no one to confer with, and I just want to do some simple stuff with some dialogs. Once I create an MFC project and add one Edit Control, Please show me how to write text to that edit control. Maybe do it in the constructor and I can get it from there.

Thank you.
David Lowndes 2,640 Reputation points MVP

2025-05-02T09:09:45.06+00:00

Hi Bryan,

I tried to give you the key steps necessary in my prior reply. Short of hand holding you personally through the process I suspect you need to find a modern tutorial on getting started with VS & MFC.

Microsoft had the excellent MFC Scribble Tutorial, but that's for a very old version of Visual C++, so I think that may just add to any confusion now.

Unfortunately, I've not found anything that provides the detail that I think you'd need that the old Scribble tutorial covered.

Have you considered using C# & WinForms instead? That provides a higher level abstraction that I think you'd find more intuitive.

1 answer

Your answer

David Lowndes 2,640 Reputation points MVP

2025-04-26T21:12:31.6766667+00:00

The underlying Windows controls have yet to be created in a CDialog constructor.
Do such initialisation in the dialog's OnInitDialog handler.
Bryan Kelly 486 Reputation points

2025-04-27T03:54:40.2766667+00:00

Answer looks good, but, what is the initialization code needed?

From another thread I found:

CEdit* pEditCtrl = (CEdit*)this->GetDlgItem(IDC_EDIT1);

``if (pEditCtrl)

``{

``pEditCtrl->SetWindowText(newValue);

``}

But that creates a pointer forcing the use of that pointer. The object already exists. What is the code to initialize it? Maybe even better, where can I find a description of the class and the initialization code? I did a search and found a description of CEdit Class here: CEdit Class | Microsoft Learn But it creates a new instance and a pointer. I already have an instance in the MFC app. I just need to initialize it and don’t recognize that code.
David Lowndes 2,640 Reputation points MVP

2025-04-27T09:52:25.45+00:00

That is code you could use if you haven't mapped a control variable to the underlying control (which you may have done from what you showed - though I can't be sure).

If you already have a control member variable then you can use it directly in OnInitDialog as you had tried to do in the constructor.

Have a read about the MFC dialog framework here.
Bryan Kelly 486 Reputation points

2025-04-29T03:55:07.24+00:00

I followed the link, and several threads that it led to. I could not recognize the code to initialize the Edit Control.

So simplify a bit. Create an MFC project with a dialog, build, and run it. OK.

Use the Toolbox and add an Edit Control. Use the default ID of IDC_EDIT1. Build, run, the dialog is present. It has been initialized to some degree.

The edit control appears to be CEdit. With that control in my project, search the entire project for CEdit. Not found. How is this created if the class is not mentioned? Maybe an including class is used, but what is that class?

Search the solution for IDC_EDIT1. It is declared in Resource.h. Looks like it is placed in MFCButton4.rc. But not found anywhere else.

So, I guess, back to the original question. Please show the code needed to initialize the edit control and write text “123” to the control. Preferably in the constructor. So the app starts with that text displayed.

Thank you in advance for your time and patience.
David Lowndes 2,640 Reputation points MVP

2025-04-29T11:01:28.9533333+00:00

If you want to attach text to an edit control in an MFC dialog box, you can do this:

Right click on the edit control in the dialog editor and choose Add Variable...

In the Add Control Variable dialog, change the Category from Control to Value. Give your variable (now a CString) a name and proceed to create it.

In your program's InitInstance (where your dialog class is instantiated in a simple dialog application), prior to calling DoModal, set your text member variable, something like this:

CMFCApplication1Dlg dlg; m_pMainWnd = &dlg; dlg.m_strText = "Hello world"; INT_PTR nResponse = dlg.DoModal();

Build and run your program, you should see the text in your dialog's edit control.

Note that you can only do this for values, if you map an MFC control class to the Window control on the dialog you need to do the operations in the dialog's OnInitDialog handler - where the underlying Windows control has been created and the MFC mapping to your control class variable has taken place.

Have a look at this documentation too.
Bryan Kelly 486 Reputation points

2025-05-02T04:07:18.3433333+00:00

David Lowndes, your answer is a bit over my head. I am at home, alone, no one to confer with, and I just want to do some simple stuff with some dialogs. Once I create an MFC project and add one Edit Control, Please show me how to write text to that edit control. Maybe do it in the constructor and I can get it from there.

Thank you.
David Lowndes 2,640 Reputation points MVP

2025-05-02T09:09:45.06+00:00

Hi Bryan,

I tried to give you the key steps necessary in my prior reply. Short of hand holding you personally through the process I suspect you need to find a modern tutorial on getting started with VS & MFC.

Microsoft had the excellent MFC Scribble Tutorial, but that's for a very old version of Visual C++, so I think that may just add to any confusion now.

Unfortunately, I've not found anything that provides the detail that I think you'd need that the old Scribble tutorial covered.

Have you considered using C# & WinForms instead? That provides a higher level abstraction that I think you'd find more intuitive.

Answer 1

Visual Studio 2022, Version 17.13.3, just in case

I discovered this quite by accident and am surprised that no one mentioned it.

Create an MFC project in C++. I suspect this will work for other languages.

Drag and edit control onto the dialog. Right click, Add Variable, name it, take the defaults.

VS adds to the dialog class, the class created with Dlg postfixed to the project name.

Go to the dialog class, scroll down to method OnInitDialog() and find the comment just above the line: return TRUE;

Put the cursor at the end of the comment line and press enter. On my computer, within a few seconds, VS displayed this line of code:

cv_edit_01.SetWindowTextW(_T("Hello World!"));

It was displayed with a gray shadow text. Press tab to accept the prompt and insert the suggested code. Do it a few times for more suggestions.

cv_edit_01 is the name of my variable. Build and run, the dialog has that text.

Complications: My bing.com searches and other search results either would not show what I requested, or showed some lines of code that did not make sense. (To me anyway)

I had done the search: microsoft class description for cedit class

It provided much information, but not how to prepare it for use. After finding out how to get started with it, the Microsoft web page about the class is great.

I suspect that many people already know this, but wonder why no one mentioned it.

Oh well. Got it now.

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MFC Edit control produces debug assert on first action

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