grouping with order?

Question

In WPF, I can group, but the default is group with ascending. One of my need to to be able to control the sorting of the group (Ascending or descending). For example:

group 1

• item 1.1
• item 1.2
• item 1.3

group 2

• item 2.1
• item 2.2

and also be able to switch to:

group 2

• item 2.1
• item 2.2

group 1

• item 1.1
• item 1.2

• item 1.3

  //Here is the function to setup a group for a particular column:
  private void SetupGrouping(DataGrid parentGrid, DataGridColumn col)
  {
      if (parentGrid == null || col == null || string.IsNullOrEmpty(col.SortMemberPath))
          return;
  
      ICollectionView vw = GetDefaultView();
      if (vw != null && vw.CanGroup)
      {
          if (vw.GroupDescriptions.Count != 0)
          {
              vw.GroupDescriptions.Clear();
          }
  
          PropertyGroupDescription gd = new PropertyGroupDescription(col.SortMemberPath);
  
          // Check to see if the column is Priority, if it is
          // then do the grouping with high priority (3) on top.
          // The order should be High(3), Normal (2), Low(1)
          DataGridColumn priCol = GetColumnByID(ColumnFlags.Priority);
          if(col == priCol)
          {
              // Attempted to change the direction of the sort added by adding group.
              // However, it has error complaining SortDescription is sealed 
              // and can't be changed.
              //if (vw.SortDescriptions != null && vw.SortDescriptions.Count > 0)
              //{
              //    SortDescription sd = vw.SortDescriptions[0];
              //    if (sd.PropertyName == col.SortMemberPath)
              //    {
              //        sd.Direction = ListSortDirection.Descending;
              //    }
              //}
          }
  
          // Info: when we add a new GroupDescription to GroupDescriptions list, 
          // guest what? a new SortDescription is also added to the 
          // SortDescriptions list.
          vw.GroupDescriptions.Add(gd);
      }
  
      // Save off the column for later use
      GroupedColumn = col;
  
      // Set the DataGrid's Tag so that the GroupSyle can get the column name
      parentGrid.Tag = DispatchAttachedProperties.GetColumnHeader(col);
  }
  

Answer 1

You had the right idea. Add a SortDescription to the ICollectionView based on the same property you are grouping on. If you want to change sort directions, you have to clear the existing one and add a new for the opposite direction. You can't change it once its created as you discovered.