[Parameter]
public Action<TreeListCustomizeContextMenuEventArgs> CustomizeContextMenu { get; set; }

@inject EmployeeTaskService EmployeeTaskService

<DxTreeList Data="TreeListData" 
            KeyFieldName="Id" 
            ParentKeyFieldName="ParentId"
            ContextMenus="@(TreeListContextMenus.Header | TreeListContextMenus.DataRow)"
            CustomizeContextMenu="CustomizeContextMenu">
    <Columns>
        <DxTreeListSelectionColumn Width="80px"/>
        <DxTreeListDataColumn FieldName="Name" Caption="Task" />
        <DxTreeListDataColumn FieldName="EmployeeName" />
        <DxTreeListDataColumn FieldName="StartDate" />
        <DxTreeListDataColumn FieldName="DueDate" />
    </Columns>
</DxTreeList>

@code {
    List<EmployeeTask> TreeListData { get; set; }

    protected override void OnInitialized() {
        TreeListData = EmployeeTaskService.GenerateData();
    }
    void CustomizeContextMenu(TreeListCustomizeContextMenuEventArgs args) {
        // Customizes context menu commands for the selection column header
        if (args.Context is TreeListHeaderCommandContext headerContext) {    
            if (headerContext.Column is ITreeListSelectionColumn selectionColumn) {
                var isFixed = selectionColumn.FixedPosition != TreeListColumnFixedPosition.None;
                string itemText = isFixed ? "Unfix Column" : "Fix Column to the Left";
                var newValue = isFixed ? TreeListColumnFixedPosition.None : TreeListColumnFixedPosition.Left;
                args.Items.AddCustomItem(itemText, () => {
                    headerContext.TreeList.BeginUpdate();
                    headerContext.Column.FixedPosition = newValue;
                    headerContext.TreeList.EndUpdate();
                });
            }
        }
        // Adds context menu commands for data rows
        if (args.Context is TreeListDataRowCommandContext) {
            args.Items.Add(TreeListContextMenuDefaultItemNames.ExpandAll);
            args.Items.Add(TreeListContextMenuDefaultItemNames.CollapseAll);
        }
    }
}

public class EmployeeTask {
    public int Id { get; set; }
    public int ParentId { get; set; }
    public string Name { get; set; }
    public string EmployeeName { get; set; }
    public DateTime StartDate { get; set; }
    public DateTime DueDate { get; set; }
    public int Progress { get; set; }
    public EmployeeTask() { }
    public EmployeeTask(
        int id,
        int parentId,
        string name,
        string employeeName,
        DateTime startDate,
        DateTime dueDate,
        int progress
        )
    {
        Id = id;
        ParentId = parentId;
        Name = name;
        EmployeeName = employeeName;
        StartDate = startDate;
        DueDate = dueDate;
        Progress = progress;
    }
}

public class EmployeeTaskService {
    public List<EmployeeTask> GenerateData() {
        return new List<EmployeeTask>() {
            new EmployeeTask(1, 0, "Simplify & Clarify Product Messaging", "John Heart", new DateTime(2018, 4, 3), new DateTime(2018, 4, 14), 14),
            new EmployeeTask(2, 1, "Prepare Financial Reports", "Samantha Bright", new DateTime(2018, 4, 3), new DateTime(2018, 4, 7), 17),
            new EmployeeTask(3, 1, "Prepare Marketing Plan", "Arthur Miller", new DateTime(2018, 4, 7), new DateTime(2018, 4, 14), 11),
            new EmployeeTask(4, 0, "Create Action Plan to Improve Customer Engagement", "Robert Reagan", new DateTime(2017, 8, 8), new DateTime(2018, 4, 8), 23),
            new EmployeeTask(5, 4, "Update Personnel Files", "Greta Sims", new DateTime(2017, 8, 8), new DateTime(2017, 10, 18), 100),
            new EmployeeTask(6, 4, "Review Health Insurance Options", "Brett Wade", new DateTime(2017, 9, 27), new DateTime(2017, 11, 10), 37),
            new EmployeeTask(7, 4, "Choose Between PPO and HMO Health Plan", "Sandra Johnson", new DateTime(2017, 12, 13), new DateTime(2018, 3, 23), 17),
            new EmployeeTask(8, 4, "Update Google Adwords Strategy", "Ed Holmes", new DateTime(2017, 8, 23), new DateTime(2017, 12, 23), 45),
            new EmployeeTask(9, 4, "Create New Brochure Design", "Barb Banks", new DateTime(2018, 1, 3), new DateTime(2018, 3, 14), 17),
            new EmployeeTask(10, 4, "Obtain Price Quote for New Brochure", "Kevin Carter", new DateTime(2018, 2, 1), new DateTime(2018, 3, 15), 18),
            new EmployeeTask(11, 4, "Brochure Design Review", "Cindy Stanwick", new DateTime(2017, 8, 22), new DateTime(2017, 10, 28), 4),
            new EmployeeTask(12, 4, "Review Website Redesign Strategy", "Sammy Hill", new DateTime(2017, 9, 16), new DateTime(2018, 3, 6), 73),
            new EmployeeTask(13, 4, "Roll Out New Website", "Davey Jones", new DateTime(2017, 11, 7), new DateTime(2018, 2, 6), 9),
            new EmployeeTask(14, 4, "Update Sales/Marketing Strategy", "Victor Norris", new DateTime(2017, 12, 13), new DateTime(2018, 4, 2), 5),
            new EmployeeTask(15, 4, "Update Sales/Revenue Report", "Mary Stern", new DateTime(2017, 12, 25), new DateTime(2018, 4, 2), 4),
            new EmployeeTask(16, 4, "Direct vs Online Sales Comparison Report", "Robin Cosworth", new DateTime(2018, 1, 2), new DateTime(2018, 3, 20), 14),
            new EmployeeTask(17, 4, "Review Sales Report and Approve Modifications", "Kelly Rodriguez", new DateTime(2017, 9, 4), new DateTime(2017, 10, 30), 8),
            new EmployeeTask(18, 4, "Update R&D Strategy", "James Anderson", new DateTime(2017, 11, 13), new DateTime(2017, 12, 4), 12),
            new EmployeeTask(19, 4, "Discuss Updated R&D Strategy", "Antony Remmen", new DateTime(2017, 10, 29), new DateTime(2017, 12, 31), 14),
            new EmployeeTask(20, 4, "Update QA Strategy", "Olivia Peyton", new DateTime(2017, 10, 31), new DateTime(2017, 11, 2), 18),
            new EmployeeTask(21, 4, "Schedule Training Events", "Taylor Riley", new DateTime(2017, 11, 19), new DateTime(2018, 4, 7), 21),
            new EmployeeTask(22, 4, "Approve Hiring of John Jeffers", "Amelia Harper", new DateTime(2018, 1, 7), new DateTime(2018, 4, 8), 10),
            new EmployeeTask(23, 0, "Increase Average Subscription Price", "Wally Hobbs", new DateTime(2017, 8, 9), new DateTime(2017, 9, 13), 96),
            new EmployeeTask(24, 23, "Update Non-Compete Agreements", "Brad Jameson", new DateTime(2017, 8, 9), new DateTime(2017, 9, 3), 96),
            new EmployeeTask(25, 23, "Update Employee Records with New NDA", "Karen Goodson", new DateTime(2017, 8, 23), new DateTime(2018, 9, 10), 100)
        };
    }
}

// ...
builder.Services.AddSingleton<EmployeeTaskService>();

void CustomizeContextMenu(TreeListCustomizeContextMenuEventArgs args) {    
    // Returns the first item
    IContextMenuItem firstItem = args.Items[0];
    // Returns the "Auto Fit All" item
    IContextMenuItem autoFitItem;
    if (args.Items.Contains(TreeListContextMenuDefaultItemNames.AutoFitAll))
        autoFitItem = args.Items[TreeListContextMenuDefaultItemNames.AutoFitAll];
}

void CustomizeContextMenu(TreeListCustomizeContextMenuEventArgs args) {  
      // Inserts the "Auto Fit All" item at the end of the item collection
      args.Items.Add(TreeListContextMenuDefaultItemNames.AutoFitAll);
      // Inserts the "Show Column Chooser" item at the first position in the item collection
      args.Items.Add(0, TreeListContextMenuDefaultItemNames.ShowColumnChooser);
}

void CustomizeContextMenu(TreeListCustomizeContextMenuEventArgs args) {
    // Inserts a custom item at the first position in the item collection
    var isFooterVisible = args.TreeList.FooterDisplayMode != TreeListFooterDisplayMode.Never;
    string footerItemText = isFooterVisible ? "Hide Footer" : "Show Footer";
    var newState = isFooterVisible ? TreeListFooterDisplayMode.Never : TreeListFooterDisplayMode.Always;
    args.Items.AddCustomItem(0, footerItemText, () => {
        args.TreeList.BeginUpdate();
        args.TreeList.FooterDisplayMode = newState;
        args.TreeList.EndUpdate();
    });
    // Inserts a custom item at the end of the item collection
    if (args.Context is TreeListHeaderCommandContext headerContext) {
        if (headerContext.Column is ITreeListCommandColumn commandColumn) {
            var isFixed = commandColumn.FixedPosition != TreeListColumnFixedPosition.None;
            string fixedItemText = isFixed ? "Unfix Column" : "Fix Column to the Left";
            var newValue = isFixed ? TreeListColumnFixedPosition.None : TreeListColumnFixedPosition.Left;
            args.Items.AddCustomItem(fixedItemText, () => {
                headerContext.TreeList.BeginUpdate();
                headerContext.Column.FixedPosition = newValue;
                headerContext.TreeList.EndUpdate();
            });
        }
    }
}

void CustomizeContextMenu(TreeListCustomizeContextMenuEventArgs args) {  
    if (args.Context is TreeListHeaderCommandContext headerContext) {
        // Inserts a custom item at the first position in the item collection
        var sortItem = args.Items.AddCustomItem(0, "Sort...");
        // Adds default items to the item collection of the inserted item
        sortItem.Items.Add(TreeListContextMenuDefaultItemNames.SortColumnAscending);
        sortItem.Items.Add(TreeListContextMenuDefaultItemNames.SortColumnDescending);
        sortItem.Items.Add(TreeListContextMenuDefaultItemNames.ClearColumnSorting);
    }
}

void CustomizeContextMenu(TreeListCustomizeContextMenuEventArgs args) {
    // Removes the first item
    args.Items.Remove(0);
    // Removes the "Show Column Chooser" item
    args.Items.Remove(TreeListContextMenuDefaultItemNames.ShowColumnChooser);
    // Removes all items
    args.Items.Clear();
}

void CustomizeContextMenu(TreeListCustomizeContextMenuEventArgs args) {
    // Hides icons for all built-in commands
    foreach(var item in args.Items)
        item.IconUrl = string.Empty;   
    if (args.Context is TreeListHeaderCommandContext headerContext) {
        // Removes the "Sort Column Descending" item
        args.Items.Remove(TreeListContextMenuDefaultItemNames.SortColumnDescending);
        // Changes the "Sort Column Ascending" item caption
        args.Items[TreeListContextMenuDefaultItemNames.SortColumnAscending].Text = "Sort";
        // Adds and configures a custom command for the command column header
        if (headerContext.Column is ITreeListCommandColumn commandColumn) {
            var isFixed = commandColumn.FixedPosition != TreeListColumnFixedPosition.None;
            string itemText = isFixed ? "Unfix Column" : "Fix Column to the Left";
            var newValue = isFixed ? TreeListColumnFixedPosition.None : TreeListColumnFixedPosition.Left;

            var fixColumnItem = args.Items.AddCustomItem(itemText, () => {
                headerContext.TreeList.BeginUpdate();
                headerContext.Column.FixedPosition = newValue;
                headerContext.TreeList.EndUpdate();
            });
            fixColumnItem.BeginGroup = true;
        }
    }
}