Csharp CS1660无法将lambda表达式转换为类型INavigationBase，因为它不是委托类型

我目前正在使用Windows窗体探索实体框架，同时遵循我遇到的official documentation个代码 这会产生编译时错误:

private void dgvCategories_SelectionChanged(object sender, EventArgs e)
        {
            if(this.contextDB != null && this.dgvCategories.CurrentRow !=null)
            {
                var category = this.dgvCategories.CurrentRow.DataBoundItem;
                if(category != null)
                {
                  //  this.contextDB.Entry(category).Collection(a => a.Products).Load();
                    this.contextDB.Entry(category).Collection(a => a.Products).Load();

                }
            }
        }

CS1660无法将lambda表达式转换为类型‘INavigationBase’，因为它不是委托类型.

错误在a=&gt；a产品中.

编辑:这是我的数据库上下文类

 internal class ContextDB : DbContext
    {

        public DbSet<Product> Products { get; set; }
        public DbSet<ProductCategory> categories { get; set; }
        //public DbSet<PurchaseOrder> purchases { get; set; }
        // public DbSet<cate>
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlite("Data Source=products.db");
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<ProductCategory>().HasData(
                new ProductCategory { CategoryId = 1, Name = "Cheese" },
                new ProductCategory { CategoryId = 2, Name = "Meat" },
                new ProductCategory { CategoryId = 3, Name = "Fish" },
                new ProductCategory { CategoryId = 4, Name = "Bread" }
                );

            modelBuilder.Entity<Product>().HasData(
  new Product { ProductId = 1, CategoryId = 1, Name = "Cheddar", Price = 50 },
            new Product { ProductId = 10, CategoryId = 1, Name = "Parmesan" },
            new Product { ProductId = 12, CategoryId = 2, Name = "Beef" },
            new Product { ProductId = 13, CategoryId = 2, Name = "Chicken" },
            new Product { ProductId = 17, CategoryId = 2, Name = "Mutton" },
            new Product { ProductId = 21, CategoryId = 3, Name = "Salmon" },
            new Product { ProductId = 22, CategoryId = 3, Name = "Tuna" }
                );
        }
    }