// TreeCtrlMFC.cpp – simple MFC application with a tree control

#include <afxwin.h>
#include <afxtreectrl.h>

class CTreeView : public CTreeView
{
    DECLARE_MESSAGE_MAP()

public:
    afx_msg void OnNodeSelected(NMHDR* pNMHdr, LRESULT nResult);
};

BEGIN_MESSAGE_MAP(CTreeView, CTreeView)
    ON_NOTIFY(TVN_ITEMSELECTED, 0, &CTreeView::OnNodeSelected)
END_MESSAGE_MAP()

void CTreeView::OnNodeSelected(NMHDR*, LRESULT)
{
    CTreeViewInfo *info = (CTreeViewInfo*)GetMessageExtraArgs();
    HTREEITEM hItem = info->iItem;
    CString text; 
    m_treeCtrl.GetItemText(hItem, text);
    AfxMessageBox(_T("Selected: ") + text);
}

class CMyApp : public CWinApp
{
public:
    virtual BOOL InitInstance()
    {
        CFrameWnd* pFrame = new CFrameWnd;
        pFrame->Create(NULL, _T("Tree Control Example"),
                       WS_OVERLAPPEDWINDOW, CRect(0, 0, 800, 600));
        CTreeView* pView = new CTreeView;
        pView->Create(NULL, NULL, WS_CHILD | WS_VISIBLE,
                      pFrame->m_clientRect);
        pFrame->DockControlBar(pView, afxDT_DockTop);

        // Populate tree
        CTreeNodeInfo node; 
        node.pszText = _T("Root");
        HTREEITEM hRoot = m_treeCtrl.InsertItem(&node, TVI_ROOT, TVI_FIRST);
        node.pszText = _T("Child 1"); m_treeCtrl.InsertItem(&node, hRoot, TVI_FIRST);
        node.pszText = _T("Child 2"); m_treeCtrl.InsertItem(&node, hRoot, TVI_FIRST);
        return TRUE;
    }
};

CMyApp theApp;



Leider falsch obiges Beispiel, Codeprüfung.   

Erneute Fehler, das ist im obigen Code vorhanden und korrekt

**Missing `return TRUE;` after initialization** | The code returns `TRUE`, which is correct. |

**Potential issues in the provided code**

| Issue | Explanation |
|-------|--------------|
| **Class inheritance typo** | `class CTreeView : public CTreeView` – you’re inheriting from a class with the same name. It should be something like `class CMyTreeView : public CTreeView`. |
| **Missing tree control member** | The view uses `m_treeCtrl`, but it isn’t declared anywhere in `CTreeView`. You need to add `CTreeCtrl m_treeCtrl;` (or use the base class’s member if you’re using `CTreeView`). |
| **Message handler signature mismatch** | `OnNodeSelected(NMHDR* pNMHdr, LRESULT nResult)` is correct for `TVN_ITEMSELECTED`, but the body casts `GetMessageExtraArgs()` to `CTreeViewInfo*`. The proper type is `CTreeViewInfo` (without `*`). |
| **Using `pFrame->m_clientRect`** | `CFrameWnd::m_clientRect` is not a public member. You should call `pFrame->GetClientRect(&rect)` and then pass that rectangle to the view’s `Create`. |
| **Wrong `TVI_ROOT` usage** | `TVI_ROOT` is used correctly, but you might want to use `TVI_FIRST` for insertion under root. That’s fine. |
| **Missing `return TRUE;` after initialization** | The code returns `TRUE`, which is correct. |   <---  Fehler!!!!  

**Suggested corrections**

```cpp
class CMyTreeView : public CTreeView
{
    DECLARE_MESSAGE_MAP()
    CTreectrl m_treeCtrl;   // Tree control member

public:
    afx_msg void OnNodeSelected(NMHDR* pNMHdr, LRESULT nResult);
};

BEGIN_MESSAGE_MAP(CMyTreeView, CTreeView)
    ON_NOTIFY(TVN_ITEMSELECTED, 0, &CMyTreeView::OnNodeSelected)
END_MESSAGE_MAP()

void CMyTreeView::OnNodeSelected(NMHDR*, LRESULT)
{
    CTreeViewInfo *info = (CTreeViewInfo*)GetMessageExtraArgs();
    HTREEITEM hItem = info->iItem;
    CString text; 
    m_treeCtrl.GetItemText(hItem, text);
    AfxMessageBox(_T("Selected: ") + text);
}

class CMyApp : public CWinApp
{
public:
    virtual BOOL InitInstance();
    virtual int ExitInstance();
};

CMyApp theApp;


 BOOL  CMyApp::InitInstance()
 {
        CWinApp::InitInstance();

        CFrameWnd* pFrame = new CFrameWnd;
        pFrame->Create(NULL, _T("Tree Control Example"),
                       WS_OVERLAPPEDWINDOW, CRect(0, 0, 800, 600));
        CMyTreeView* pView = new CMyTreeView;
        RECT rc; pFrame->GetClientRect(&rc);
        pView->Create(NULL, NULL, WS_CHILD | WS_VISIBLE,
                      rc);
        pFrame->DockControlBar(pView, afxDT_DockTop);

        // Populate tree
        CTreeNodeInfo node; 
        node.pszText = _T("Root");
        HTREEITEM hRoot = pView->m_treeCtrl.InsertItem(&node, TVI_ROOT, TVI_FIRST);
        node.pszText = _T("Child 1"); pView->m_treeCtrl.InsertItem(&node, hRoot, TVI_FIRST);
        node.pszText = _T("Child 2"); pView->m_treeCtrl.InsertItem(&node, hRoot, TVI_FIRST);

        return TRUE;
 }

 int CMyApp::ExitInstance()
 {
       return CWinApp::ExitInstance();
 }


```

These changes address the key compile‑time errors and make the code functional within an MFC project.