@(
Html.Kendo().TreeList<DevelopmentEmployeeDirectoryRemoteModel>()
    .Name("development-tree-list-basic")
    .DataSource(dataSource => dataSource
    .Read(read => read.Url($"{Url.Content("~")}/Development?handler=KendoTreeListDataSourceRead").Data("forgeryToken"))
    .Model(m =>
    {
        m.Id(f => f.EmployeeId);
        m.ParentId(f => f.ReportsTo).Nullable(true);
        m.Field(f => f.FirstName);
        m.Field(f => f.LastName);
        m.Field(f => f.ReportsTo);

    })
    .ServerOperation(false))
    .Columns(columns =>
        {
            columns.Add().Field(f => f.FirstName).Width(250);
            columns.Add().Field(e => e.LastName);
            columns.Add().Field(e => e.Position);
            columns.Add().Field(e => e.Extension).Title("Ext").Format("{0:#}");
        })
    .Toolbar(tools => {                                            
        tools.Pdf();
        tools.Excel();
        tools.Spacer();
        tools.Search();
    })    
    .Excel(excel => excel.FileName("Kendo UI TreeList Export.xlsx").ProxyURL(Url.Action("Excel_Export_Save", "Development")))
    .Pdf(pdf => pdf.FileName("Kendo UI TreeList Export.pdf")
               .AllPages()               
               .PaperSize("A4")
               .Margin("2cm", "1cm", "1cm", "1cm")
               .Landscape(true))
    //.ColumnMenu()
    .Filterable()    
    .Selectable(true)
    .Scrollable(false)
    .Sortable()
    .Resizable(true)
    .Reorderable(true)
    .Deferred(true)
)

AppSettings.json (Rest Project)

{
    "Logging": {
        "LogLevel": {
            "Default": "Information",
            "Microsoft.AspNetCore": "Warning"
        }
    },
    "AllowedHosts": "*",
    "ConnectionStrings": {
        "Telerik.Reporting.Examples.CSharp.Properties.Settings.TelerikConnectionString": "Data Source =.\\SQLEXPRESS; Initial Catalog=AdventureWorks; Intgrated Security = true"
    }
    
}

namespace TopLevelStatements.Controllers
{
    using System.Net;
    using System.Net.Mail;
    using Microsoft.AspNetCore.Mvc;
    using Telerik.Reporting.Services;
    using Telerik.Reporting.Services.AspNetCore;

    [Route("api/[controller]")]
    [ApiController]
    public class ReportsController : ReportsControllerBase
    {
        public ReportsController(IReportServiceConfiguration reportServiceConfiguration)
        : base(reportServiceConfiguration)
        {
        }

        protected override HttpStatusCode SendMailMessage(MailMessage mailMessage)
        {
            throw new System.NotImplementedException("This method should be implemented in order to send mail messages");

            // using (var smtpClient = new SmtpClient("smtp01.mycompany.com", 25))
            // {
            //     smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
            //     smtpClient.EnableSsl = false;
            //     smtpClient.Send(mailMessage);
            // }
            // return HttpStatusCode.OK;
        }
    }
}

using Microsoft.Extensions.DependencyInjection.Extensions;
using Telerik.Reporting.Services;
using Telerik.Reporting.Cache.File;


var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
builder.Services.AddRazorPages();
builder.Services.AddControllers().AddNewtonsoftJson();

// Configure dependencies for ReportsController.
builder.Services.TryAddSingleton<IReportServiceConfiguration>(sp =>
    new ReportServiceConfiguration
    {
        // The default ReportingEngineConfiguration will be initialized from appsettings.json or appsettings.{EnvironmentName}.json:
        ReportingEngineConfiguration = sp.GetService<IConfiguration>(),
        // In case the ReportingEngineConfiguration needs to be loaded from a specific configuration file, use the approach below:
        //ReportingEngineConfiguration = ResolveSpecificReportingConfiguration(sp.GetService<IWebHostEnvironment>()),
        HostAppId = "ReportingNet6",
        Storage = new FileStorage(),
        ReportSourceResolver = new UriReportSourceResolver(System.IO.Path.Combine(sp.GetService<IWebHostEnvironment>().ContentRootPath, "Reports"))
    });

builder.Services.AddCors(corsOption => corsOption.AddPolicy(
    "ReportingRestPolicy",
    corsBuilder =>
    {
        corsBuilder.AllowAnyOrigin()
            .AllowAnyMethod()
            .AllowAnyHeader();
    }
));


var app = builder.Build();

// Configure the HTTP request pipeline.
if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/Error");
    // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
    app.UseHsts();
}

app.UseHttpsRedirection();
app.UseStaticFiles();

app.UseRouting();

app.UseEndpoints(endpoints =>
{
    endpoints.MapRazorPages();
    endpoints.MapControllers();
    //...
});

app.UseStaticFiles();

app.UseAuthorization();

app.MapRazorPages();

app.UseCors("ReportingRestPolicy");

app.Run();

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>Telerik HTML5 Report Viewer Demo in ASP.NET Core in .NET 5</title>
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale-1, maximum-scale=1" />
    <script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>
    <link href="https://kendo.cdn.telerik.com/2020.3.1118/styles/kendo.common.min.css" rel="stylesheet" />
    <link href="https://kendo.cdn.telerik.com/2020.3.1118/styles/kendo.blueopal.min.css" rel="stylesheet" />
    <script src="https://localhost:44320/api/reports/resources/is/telerikReportViewer"></script>
    <style>
        #reportViewer1 {
            position: absolute;
            left: 5px;
            right: 5px;
            top: 50px;
            bottom: 5px;
            overflow: hidden;
            font-family: verdana, Arial;
    </style>
</head>
<body>
    <div id="reportViewer1">
        loading...
    </div>
    <script>
        $(document).ready(function () {
            $("#reportViewer1")
                .telerik_ReportViewer({
                    serviceurl: "https://localhost:44320/api/reports/", reportSource: {
                    },
                    report: "Barcodes Report.trdp", parameters: {}
viewMode: telerikReportViewer.ViewModes.INTERACTIVE, scaleMode: telerikReportViewer.ScaleModes.SPECIFIC, scale: 1.0,
                    enableAccessibility: false,
                    sendEmail: enabled: true }
});
});</script>
</body>
</html>

public class EditOrderLineDto
{
    public long Id { get; set; }
    public long OrderId { get; set; }
    public int Quantity { get; set; }

    public EditOrderLineProductDto Product { get; set; }

    public EditOrderLineDto()
    {
        Product = new EditOrderLineProductDto();
    }

    public EditOrderLineDto(long id, long orderId, string productId, string productName,
         int quantity)
    {
        Id = id;
        OrderId = orderId;
        Quantity = quantity;
        Product = new EditOrderLineProductDto(productId, productName);
    }
}

public class EditOrderLineProductDto
{
    public string ProductId { get; set; }
    public string Name { get; set; }

    public EditOrderLineProductDto()
    {
        ProductId = string.Empty;
        Name = string.Empty;
    }

    public EditOrderLineProductDto(string productId, string name)
    {
        ProductId = productId;
        Name = name;
    }
}

Html.Kendo().Grid<EditOrderLineDto>()
    .Name("orderLines")
    .ToolBar(t =>
    {
        t.Create();
        t.Save();
    })
    .Columns(c =>
    {
        c.Bound(x => x.Id).Hidden(true);
        c.Bound(x => x.OrderId).Hidden(true);
        c.Group(g =>
            g.Title("Product").Columns(p =>
            {
                p.Bound(x => x.Product.Name);
                p.Bound(x => x.Product.ProductId);
            })
        );
        c.Bound(x => x.Quantity);

        c.Command(x => x.Edit());
        c.Command(x => x.Destroy());
    })
    .Editable(x => x.Mode(GridEditMode.InLine))
    .DataSource(d =>
    {
        d.Ajax()
        .Batch(false)
        .Model(m =>
        {
            m.Id(x => x.Id);
            m.Field(x => x.Id).DefaultValue(0);
            m.Field(x => x.OrderId).DefaultValue(Model.Id);
            m.Field(x => x.Product).DefaultValue(new EditOrderLineProductDto()).Editable(true);
            m.Field(x => x.Quantity);
        })
        .Create(c => c.Action("CreateOrderLine", "Orders"))
        .Read(r => r.Action("EditOrderOrderLines", "Orders", new { orderId = Model.Id }))
        .Update(u => u.Action("EditOrderLine", "Orders"))
        .Destroy(d => d.Action("DestroyOrderLine", "Orders"))
        .Events(e => e.Error("onError"));
    })
    .Events(e => e.Save("onGridSave"))