我收到**"不支持异常：当我尝试插入到ASP.NET Core 2.2中多对多的链接表中时，集合大小固定"**。我可以在SQL Server中手动插入值!
我的实体是：

public class Product
    {
        [key]
        public int Id {get;set;}

        public ICollection<ProductCategory> ProductCategories { get; set; } = new List<ProductCategory>();
    }

public class Category
    {
        [key]
        public int Id {get;set;}

        public string Name { get; set; }

        public ICollection<ProductCategory> ProductCategories { get; set; } = new List<ProductCategory>();

    }

public class ProductCategory
    {
        [Key]
        public int ProductId { get; set; }
        public Product Product { get; set; }

        [Key]
        public int CategoryId { get; set; }
        public Category Category { get; set; }
    }

在DbContext中：

protected override void OnModelCreating(ModelBuilder modelBuilder)
        {

            modelBuilder.Entity<ProductCategory>().HasKey(t => new { t.ProductId, t.CategoryId });

            modelBuilder.Entity<ProductCategory>()
                .HasOne(bc => bc.Product)
                .WithMany(b => b.ProductCategories)
                .HasForeignKey(bc => bc.ProductId);

            modelBuilder.Entity<ProductCategory>()
                .HasOne(bc => bc.Category)
                .WithMany(c => c.ProductCategories)
                .HasForeignKey(bc => bc.CategoryId);

            base.OnModelCreating(modelBuilder);
        }

        public DbSet<Product> Products { get; set; }
        public DbSet<Category> Categories { get; set; }
        public DbSet<ProductCategory> ProductCategories {get; set;}

我尝试将记录插入到Product and Category中，插入的记录没有问题。但当我尝试插入链接记录时，我收到**"NotSupportedException：集合具有固定大小"**错误。
插入：

var Electronics = new Category
            {
                Name = "Electronics",
            };

            var Exir = new Product
            {
                Name = "Exir",
            };

            var pc1 = new ProductCategory();

            bc1.Product = Exir ;
            bc1.Category = Electronics;
            Exir.ProductCategories.Add(pc1);

            sqlContext.Categories.Add(Electronics);
            sqlContext.Products.Add(Exir);
            sqlContext.SaveChanges();