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I am having a problem getting simple text values to display in the CExtTreeGridWnd class. I started with the code in the downloadable test_tree_grid_idea project, which allows individual tree elements to be added anywhere in the tree. I have been trying to add code to set some simple text in one of the elements.
First I called GridCellGet() to attempt to get the cell reference. From there, I was going to call TextSet.

cell = ((CMainFrame *)m_pMainWnd)->m_wndTree.GridCellGet(0,0);
cell->TextSet(_T("ABC"));

The problem is that "cell" always gets a NULL pointer from GridCellGet() in the first statement. I cannot get any of the tree cells to display anything. When I change the first line to call ... .GridCellGet(0,0,0,-1), which accesses the header, it works. I need to understand more about the Prof-UIS framework so I can set values in the main part of the tree itself. -- Thanks.

There is a CExtTreeGridWnd::ItemGetCell() method, which you should use to instantiate grid cells inside the tree grid. By default, each grid cell in each tree row is NULL. You can specify the desired cell type by using RUNTIME_CLASS(...):

CExtTreeGridWnd * pTreeGridWnd = . . .
HTREEITEM hti = . . .
LONG nColNo = . . .
CExtGridCell * pCell = pTreeGridWnd->ItemGetCell( hti, nColNo, 0, RUNTIME_CLASS(CExtGridCellString) );
      ASSERT_VALID( pCell );
      pCell->TextSet( _T("Hello world!") );

The code above instantiates a CExtGridCellString grid cell at the position indicated by column nColNo in tree row hti.

The CExtGridWnd::GridCellGet() method should be used with the CExtGridWnd plain grid control and with top/bottom header parts of the CExtTreeGridWnd tree grid. The data cells of tree grid and header cells on left or right should be accessed using the CExtTreeGridWnd::ItemGetCell() method.