public enum GridDetailExpandButtonDisplayMode

@inject NwindDataService NwindDataService

<DxGrid @ref="Grid" 
        Data="MasterGridData" 
        DetailExpandButtonDisplayMode="GridDetailExpandButtonDisplayMode.Never">
    <Columns>
        <DxGridDataColumn FieldName="ContactName" SortIndex="0" />
        <DxGridDataColumn FieldName="CompanyName" />
        <DxGridDataColumn FieldName="Country" />
        <DxGridDataColumn FieldName="City" />
    </Columns>
    <DetailRowTemplate>
        <NestedGrid_DetailContent Customer="(Customer)context.DataItem" />
    </DetailRowTemplate>
</DxGrid>

@code {
    IGrid Grid { get; set; }
    object MasterGridData { get; set; }

    protected override async Task OnInitializedAsync() {
        MasterGridData = await NwindDataService.GetCustomersAsync();
    }
    protected override void OnAfterRender(bool firstRender) {
        if(firstRender) {
            Grid.ExpandDetailRow(0);
        }
    }
}

@inject NwindDataService NwindDataService
<div class="mb-2">
    Contact Phone: @Customer.Phone
</div>
<DxGrid Data="DetailGridData"
        PageSize="5"
        AutoExpandAllGroupRows="true">
    <Columns>
        <DxGridDataColumn FieldName="OrderId" DisplayFormat="d" GroupIndex="0" />
        <DxGridDataColumn FieldName="ProductName" Width="40%" />
        <DxGridDataColumn FieldName="UnitPrice" DisplayFormat="c" />
        <DxGridDataColumn FieldName="Quantity" />
        <DxGridDataColumn FieldName="Discount" DisplayFormat="p0" />
        <DxGridDataColumn FieldName="ExtendedPrice" DisplayFormat="c" />
    </Columns>
    <GroupSummary>
        <DxGridSummaryItem SummaryType="GridSummaryItemType.Sum"
                           FieldName="ExtendedPrice"
                           FooterColumnName="ExtendedPrice" />
    </GroupSummary>
</DxGrid>

@code {
    [Parameter]
    public Customer Customer { get; set; }
    object DetailGridData { get; set; }
    protected override async Task OnInitializedAsync() {
        var invoices = await NwindDataService.GetInvoicesAsync();
        DetailGridData = invoices
            .Where(i => i.CustomerId == Customer.CustomerId)
            .ToArray();
    }
}

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace BlazorDemo.Data.Northwind {
    public partial class Customer {
        public Customer() {
            Orders = new HashSet<Order>();
        }

        public string CustomerId { get; set; }
        public string CompanyName { get; set; }
        public string ContactName { get; set; }
        public string ContactTitle { get; set; }
        public string Address { get; set; }
        public string City { get; set; }
        public string Region { get; set; }
        public string PostalCode { get; set; }
        public string Country { get; set; }
        public string Phone { get; set; }
        public string Fax { get; set; }

        public virtual ICollection<Order> Orders { get; set; }
    }
}

using System;
using System.Collections.Generic;
using System.Runtime.InteropServices;

namespace BlazorDemo.Data.Northwind {
    public class Invoice {
        public string ShipName { get; set; }
        public string ShipAddress { get; set; }
        public string ShipCity { get; set; }
        public string ShipRegion { get; set; }
        public string ShipPostalCode { get; set; }
        public string ShipCountry { get; set; }
        public string CustomerId { get; set; }
        public string CustomerName { get; set; }
        public string Address { get; set; }
        public string City { get; set; }
        public string Region { get; set; }
        public string PostalCode { get; set; }
        public string Country { get; set; }
        public string Salesperson { get; set; }
        public int OrderId { get; set; }
        public DateTime? OrderDate { get; set; }
        public DateTime? RequiredDate { get; set; }
        public DateTime? ShippedDate { get; set; }
        public string ShipperName { get; set; }
        public int ProductId { get; set; }
        public string ProductName { get; set; }
        public decimal UnitPrice { get; set; }
        public short Quantity { get; set; }
        public float Discount { get; set; }
        public decimal? ExtendedPrice { get; set; }
        public decimal? Freight { get; set; }
    }
}

// ...
builder.Services.AddScoped<NwindDataService>();