KB183218 - How to subclass a dialog item in ATL

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You need to perform the following steps to subclass a control, such as the ComboBox, on the dialog template in an ATL project:

Declare a member of type CContainedWindow in the CDialogImpl-derived class for the Dialog Template as:
```
      CContainedWindow   m_ComboBox;
						
```
Initialize the CContainedWindow member in the constructor of the CDialogImpl-derived class (CMyDialog class) with the appropriate class name as:
```
      CMyDialog::CMyDialog() : m_ComboBox(_T("COMBOBOX"), this, 1)
						
```
Add WM_INITDIALOG and WM_DESTROY message handlers, and an Altered Message Map (ALT_MSG_MAP), to the default Message Map of the CDialogImpl-derived class. The Altered Message Map is used to trap messages that are sent to the subclassed control, as in the following example:
Sample Code
```
BEGIN_MSG_MAP(CMyDialog)
   MESSAGE_HANDLER(WM_INITDIALOG, OnInitDialog)
   MESSAGE_HANDLER(WM_DESTROY, OnDestroy)

ALT_MSG_MAP(1)   // Handlers for the subclassed combo box.
   MESSAGE_HANDLER(WM_RBUTTONDOWN, OnRBtnDown) // Trap a right[ASCII 150]click.
END_MSG_MAP()
						
```
In the WM_INITDIALOG handler of the CDialogImpl-derived class, subclass the ComboBox as:
```
      m_ComboBox.SubclassWindow(::GetDlgItem(m_hWnd,IDC_SUB_COMBOBOX));
						
```
In the WM_DESTROY handler of the CDialogImpl derived class unsubclass the ComboBox as:
```
      m_ComboBox.UnsubclassWindow();
						
```

The previous steps ensure that the ALT_MSG_MAP traps the messages meant for the subclassed control. They also work for a property page in an ATL control instead of a standard dialog box.

How to subclass a dialog item in ATL

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Sample Code