public string GetGroupSummaryDisplayText(
    IGridSummaryItem item,
    int visibleIndex
)

@using Microsoft.EntityFrameworkCore
@inject IDbContextFactory<NorthwindContext> NorthwindContextFactory
@implements IDisposable

<DxGrid Data="@Data"
        ShowGroupPanel="true"
        CustomizeSummaryDisplayText="Grid_CustomizeSummaryDisplayText">
    <Columns>
        <DxGridDataColumn FieldName="Country" GroupIndex="0" Width="200px">
            <GroupRowTemplate>
                <text>@context.DataColumn.FieldName : @context.GroupValue</text>
                @{
                    var summaryItems = context.Grid.GetGroupSummaryItems();
                    if (summaryItems.Any())
                    {
                        <text> (</text>
                        foreach (var i in summaryItems)
                        {
                            <i>@context.Grid.GetGroupSummaryDisplayText(i, context.VisibleIndex)</i>
                        }
                        <text>)</text>
                    }
                }
            </GroupRowTemplate>
        </DxGridDataColumn>
        <DxGridDataColumn FieldName="City" />
        <DxGridDataColumn FieldName="OrderDate" Width="200px" />
        <DxGridDataColumn FieldName="UnitPrice" DisplayFormat="c" Width="120px" />
        <DxGridDataColumn FieldName="Quantity" Width="120px" />
    </Columns>
    <GroupSummary>
        <DxGridSummaryItem SummaryType="GridSummaryItemType.Count" FieldName="Country" />
    </GroupSummary>
</DxGrid>
@* ... *@
@code {
    object Data { get; set; }
    NorthwindContext Northwind { get; set; }
    IGrid Grid1 { get; set; }
    @* ... *@
    protected override void OnInitialized() {
        Northwind = NorthwindContextFactory.CreateDbContext();
        Data = Northwind.Invoices
            .ToList();
    }

    void Grid_CustomizeSummaryDisplayText(GridCustomizeSummaryDisplayTextEventArgs e) {
        e.DisplayText = string.Format("Count of Orders : {0}", e.Value);
    }

    public void Dispose() {
        Northwind?.Dispose();
    }
}

using System;
#nullable disable

namespace Grid.Northwind {
    public partial 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; }
    }
}

using Microsoft.EntityFrameworkCore;
#nullable disable

namespace Grid.Northwind {
    public partial class NorthwindContext : DbContext {

        public NorthwindContext(DbContextOptions<NorthwindContext> options)
            : base(options) {
        }
        // ...
        public virtual DbSet<Invoice> Invoices { get; set; }
        // ...
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) {
            if(!optionsBuilder.IsConfigured) {
                optionsBuilder.UseSqlServer("Server=.\\sqlexpress;Database=Northwind;Integrated Security=true");
            }
        }
        // ...
        protected override void OnModelCreating(ModelBuilder modelBuilder) {
        // ...
            modelBuilder.Entity<Invoice>(entity => {
                entity.HasNoKey();
                entity.ToView("Invoices");
                entity.Property(e => e.Address).HasMaxLength(60);
                entity.Property(e => e.City).HasMaxLength(15);
                entity.Property(e => e.Country).HasMaxLength(15);
                entity.Property(e => e.CustomerId)
                    .HasMaxLength(5)
                    .HasColumnName("CustomerID")
                    .IsFixedLength(true);
                entity.Property(e => e.CustomerName)
                    .IsRequired()
                    .HasMaxLength(40);
                entity.Property(e => e.ExtendedPrice).HasColumnType("money");
                entity.Property(e => e.Freight).HasColumnType("money");
                entity.Property(e => e.OrderDate).HasColumnType("datetime");
                entity.Property(e => e.OrderId).HasColumnName("OrderID");
                entity.Property(e => e.PostalCode).HasMaxLength(10);
                entity.Property(e => e.ProductId).HasColumnName("ProductID");
                entity.Property(e => e.ProductName)
                    .IsRequired()
                    .HasMaxLength(40);
                entity.Property(e => e.Region).HasMaxLength(15);
                entity.Property(e => e.RequiredDate).HasColumnType("datetime");
                entity.Property(e => e.Salesperson)
                    .IsRequired()
                    .HasMaxLength(31);
                entity.Property(e => e.ShipAddress).HasMaxLength(60);
                entity.Property(e => e.ShipCity).HasMaxLength(15);
                entity.Property(e => e.ShipCountry).HasMaxLength(15);
                entity.Property(e => e.ShipName).HasMaxLength(40);
                entity.Property(e => e.ShipPostalCode).HasMaxLength(10);
                entity.Property(e => e.ShipRegion).HasMaxLength(15);
                entity.Property(e => e.ShippedDate).HasColumnType("datetime");
                entity.Property(e => e.ShipperName)
                    .IsRequired()
                    .HasMaxLength(40);
                entity.Property(e => e.UnitPrice).HasColumnType("money");
            });
            // ...
            OnModelCreatingPartial(modelBuilder);
        }

        partial void OnModelCreatingPartial(ModelBuilder modelBuilder);
    }
}

using Microsoft.EntityFrameworkCore;
// ...
builder.Services.AddDbContextFactory<NorthwindContext>((sp, options) => {
    var env = sp.GetRequiredService<IWebHostEnvironment>();
    var dbPath = Path.Combine(env.ContentRootPath, "Northwind-5e44b51f.mdf");
    options.UseSqlServer("Server=(localdb)\\MSSQLLocalDB;Integrated Security=true;AttachDbFileName=" + dbPath);
});