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how to add icon to the CExtPropertyCategory ?thank you

First of all, you should use a CExtPropertyGridCtrl-derived class:

class CMyPropertyGridCtrl : public CExtPropertyGridCtrl
{
public:
    virtual bool OnPgcCreateGrids()
    {
        ASSERT_VALID( this );
        try
        {
            /////////////////////////////////////////////////////////
            // 1) CREATE CATEGORIZED GRID WINDOW
            CMyCategorizedTreeGrid * pGridCategorized =
                new CMyCategorizedTreeGrid( this );
            pGridCategorized->m_bAutoDeleteWindow = true;
            if( ! pGridCategorized->Create(
                    this,
                    __EXT_MFC_ID_PROPERTY_GRID_CATEGORIZED,
                    true
                    )
                )
            {
                ASSERT( FALSE );
                throw __EXT_MFC_ID_PROPERTY_GRID_CATEGORIZED;
            }
            ///////////////////////////////////////////////////////// 
            // 2) INIT ONE ICON INSIDE THE CATEGORIZED GRID WINDOW
            HICON hIcon = (HICON)::LoadImage(
                ::AfxGetInstanceHandle(), MAKEINTRESOURCE(IDR_MAINFRAME),
                IMAGE_ICON, 16, 16, 0 );
            CExtCmdIcon icon;
            icon.AssignFromHICON( hIcon, false );
            pGridCategorized->m_nIconIndex =
                pGridCategorized->GridIconInsert( &icon );
            ///////////////////////////////////////////////////////// 
            // 3) CREATE SORTED GRID WINDOW
            CExtPropertyGridWndSorted * pGridSorted =
                new CExtPropertyGridWndSorted( this );
            pGridSorted->m_bAutoDeleteWindow = true;
            if( ! pGridSorted->Create(
                    this,
                    __EXT_MFC_ID_PROPERTY_GRID_SORTED
                    )
                )
            {
                ASSERT( FALSE );
                throw __EXT_MFC_ID_PROPERTY_GRID_SORTED;
            }
        }
        catch( ... )
        {
            return false;
        }
        return true;
    }
};

The overridden OnPgcCreateGrids() virtual method is similar to the original one, but it creates the CMyCategorizedTreeGrid window instead of that created by the default CExtPropertyGridWndCategorized window. Besides, the method loads and inserts the icon into the categorized grid window. The CMyCategorizedTreeGrid window is required for adding icons to property category rows. Here is the implementation of this class:

class CMyCategorizedTreeGrid : public CExtPropertyGridWndCategorized
{
public:
    INT m_nIconIndex;
    CMyCategorizedTreeGrid()
        : m_nIconIndex( -1 )
    {
    }
    virtual HTREEITEM PropertyItemInsert(
        CExtPropertyItem * pPropertyItem,
        LONG nIndex = -1,
        HTREEITEM htiParent = TVI_ROOT,
        bool bRedraw = true
        )
    {
        ASSERT_VALID( this );
        HTREEITEM hTreeItem =
            CExtPropertyGridWnd::PropertyItemInsert(
                pPropertyItem, nIndex, htiParent, false );
        if( hTreeItem != NULL && m_nIconIndex >= 0
                && pPropertyItem->IsKindOf( RUNTIME_CLASS( CExtPropertyCategory ) )
                // && ANALYZE WHETHER THIS CATEGORY SHOULD HAVE AN ICON
            )
        {
            CExtGridCell * pCell = ItemGetCell( 0, hTreeItem );
            ASSERT_VALID( pCell );
            pCell->IconIndexSet( m_nIconIndex );
        }
        if( bRedraw && hTreeItem != NULL )
        {
            LONG nRowNo = pGrid->ItemGetVisibleIndexOf( hTreeItem );
            if( nRowNo >= 0 )
            {
                CRect rcInvalidate;
                if( GridCellRectsGet( 0, nRowNo, 0, 0, NULL, &rcInvalidate ) )
                {
                    CRect rcClient;
                    pGrid->GetClientRect( &rcClient );
                    rcInvalidate.left = rcClient.left;
                    rcInvalidate.right = rcClient.right;
                    InvalidateRect( &rcInvalidate );
                }
            }
        }
        return hTreeItem;
    }
};