I am attempting to set the field width programmatically and it's not working. I am also updating the Field position (the ActualPosition) and the SortedFields and that's working without issue.

Here is the code I'm using:

                string[] fieldList = ViewModel.ColumnNameList.Split(',');
                string[] sizeList = ViewModel.ColumnSizeList.Split(',');
                List<SortColumn> sortColumns = ViewModel.GridSortColumns;
                List<SortColumn> groupByColumns = ViewModel.GridGroupbyColumns;

                FieldLayout currentFields = _xamDataGrid.FieldLayouts[0];
                currentFields.SortedFields.Clear();

                for (int i = 0; i < fieldList.Length; ++i)
                {
                    if (currentFields.Fields.IndexOf(fieldList[i]) > -1)
                    {
                        int fieldIndex = currentFields.Fields[fieldList[i]].ActualPosition.Column;

                        if (fieldIndex != i)
                        {
                            currentFields.Fields[fieldList[i]].ActualPosition = new FieldPosition(i, 0, 1, 1);
                            currentFields.EnsureUniqueFieldPositions();
                        }

                        if (i < sizeList.Length)
                        {
                            int columnSize = 0;
                            if (int.TryParse(sizeList[i], out columnSize))
                            {
                                currentFields.Fields[fieldList[i]].Width = new FieldLength(columnSize);  //  <= SETTING WIDTH HERE
                            }
                        }

                        if (groupByColumns.Exists(p => p.ColumnName == fieldList[i]))
                        {
                            SortColumn sortColumn = groupByColumns.Find(p => p.ColumnName == fieldList[i]);

                            FieldSortDescription currentSort = new FieldSortDescription();
                            currentSort.Direction = sortColumn.SortOrder ==
                                SortOrder.Ascending ? ListSortDirection.Ascending : ListSortDirection.Descending;
                            currentSort.FieldName = fieldList[i];
                            currentSort.IsGroupBy = true;
                            currentFields.SortedFields.Add(currentSort);
                        }
                        else if (sortColumns.Exists(p => p.ColumnName == fieldList[i]))
                        {
                            SortColumn sortColumn = sortColumns.Find(p => p.ColumnName == fieldList[i]);

                            FieldSortDescription currentSort = new FieldSortDescription();
                            currentSort.Direction = sortColumn.SortOrder ==
                                SortOrder.Ascending ? ListSortDirection.Ascending : ListSortDirection.Descending;
                            currentSort.FieldName = fieldList[i];
                            currentSort.IsGroupBy = false;
                            currentFields.SortedFields.Add(currentSort);
                        }
                    }
                }

While debugging I noticed that the Width property of the field was not being updated when I resized the column in the grid (interactively, by dragging with the mouse).