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Sets the 32-bit application-specific value associated with the item specified by nItem.
Index of the list item whose data is to be set.
A 32-bit value to be associated with the item.
Nonzero if successful; otherwise 0.
This value is the lParam member of the LVITEM structure, as described in the Windows SDK.
// Set the data of each item to be equal to its index.
for (int i = 0; i < m_myListCtrl.GetItemCount(); i++)