EF Core WinForms开发遇CS1660错误：Lambda无法转为INavigationBase

跟随微软官方文档学习WinForms结合Entity Framework Core开发时，出现编译错误，报错代码如下：

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();
        }
    }
}

错误信息：
CS1660 Cannot convert lambda expression to type 'INavigationBase' because it is not a delegate type
错误定位在a => a.Products处。

补充的DB Context类代码：

internal class ContextDB : DbContext
{
    public DbSet<Product> Products { get; set; }
    public DbSet<ProductCategory> categories { get; set; }

    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" }
        );
    }
}

解决方法 #

1. 修正类型推断问题：category变量被默认推断为object类型，contextDB.Entry(object)返回的EntityEntry没有接受lambda表达式的Collection重载，必须明确转换为实体类型。
   修改变量声明：

   var category = (ProductCategory)this.dgvCategories.CurrentRow.DataBoundItem;
   

2. 补充导航属性定义：确保ProductCategory类中存在正确的一对多导航属性，EF Core才能识别a => a.Products：

   public class ProductCategory
   {
       public int CategoryId { get; set; }
       public string Name { get; set; }
       // 定义一对多导航属性
       public ICollection<Product> Products { get; set; } = new List<Product>();
   }
   

3. 完善Product类关联：同时确认Product类包含对应的外键和多对一导航属性：

   public class Product
   {
       public int ProductId { get; set; }
       public int CategoryId { get; set; }
       public string Name { get; set; }
       public decimal? Price { get; set; }
       // 定义多对一导航属性
       public ProductCategory Category { get; set; }
   }
   

内容的提问来源于stack exchange，提问作者Muhammad Faizan Khan