1. 首先，在DbContext中添加一个bool类型的属性，用于指示种子数据是否已添加。在下例中，我们将其命名为_isSeeded:

public class ApplicationDbContext : DbContext
{
    private bool _isSeeded;

    public ApplicationDbContext(DbContextOptions options)
        : base(options)
    {
    }

    // Add entities here
    public DbSet Products { get; set; }
    public DbSet Categories { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        // Map entities to tables
        modelBuilder.Entity().ToTable("Products");
        modelBuilder.Entity().ToTable("Categories");

        // Add seed data here
        if (!_isSeeded)
        {
            modelBuilder.Entity().HasData(
                new Category { Id = 1, Name = "Electronics" },
                new Category { Id = 2, Name = "Clothing" },
                new Category { Id = 3, Name = "Books" }
            );

            modelBuilder.Entity().HasData(
                new Product { Id = 1, Name = "Laptop", Price = 1000m, CategoryId = 1 },
                new Product { Id = 2, Name = "Smartphone", Price = 800m, CategoryId = 1 },
                new Product { Id = 3, Name = "T-shirt", Price = 10m, CategoryId = 2 },
                new Product { Id = 4, Name = "Jeans", Price = 50m, CategoryId = 2 },
                new Product { Id = 5, Name = "The Catcher in the Rye", Price = 5m, CategoryId = 3 }
            );

            _isSeeded = true;
        }
    }
}

2. 接下来，在Program.cs文件中配置WebHost，并添加以下代码，以在应用程序启动时应用迁移并添加种子数据：

public class Program
{
    public static void Main(string[] args)
    {
        var host = CreateWebHostBuilder(args).Build();

        using (var scope = host.Services.CreateScope())
        {
            var services = scope.ServiceProvider;
            var context = services.GetRequiredService();

            context.Database.Migrate();

            // Manually apply seed data if not already seeded
            if (!context._isSeeded)
            {
                context.Database.ExecuteSqlCommand("SET IDENTITY_INSERT [Categories] ON");
                context.Categories.AddRange(
                    new Category { Id = 1, Name = "Electronics" },
                    new Category { Id = 2, Name