【EntityFramework Core】实体实例化注入

由于逻辑需要，我希望能在EF Core实例化实体时，拿到实体并执行相关代码，所以我就研究了一番EF Core，得到以下方法。

1.创建实体初始化类，继承EntityMaterializerSource

    public class ComBoostEntityMaterializerSource : EntityMaterializerSource
    {
        public ComBoostEntityMaterializerSource(IMemberMapper memberMapper) : base(memberMapper)
        {}
    }

这个类是，我们需要注入到EF Core当中去的。

2.创建EF Core扩展

    public class ComBoostOptionExtension : IDbContextOptionsExtension
    {
        public void ApplyServices(IServiceCollection services)
        {
            services.AddScoped<IEntityMaterializerSource, ComBoostEntityMaterializerSource>();
        }
    }

创建扩展后，在ApplyServices里注入我们创建的初始化类。

3.用于实例化DbContext时的DbContextOptions参数，需要添加我们创建的扩展

new DbContextOptionsBuilder<DataContext>().UseSqlServer(Configuration.GetConnectionString("DataContext")).Options.WithExtension(new ComBoostOptionExtension()))

4.重写我们的实体初始化类

    public class ComBoostEntityMaterializerSource : EntityMaterializerSource
    {
        public ComBoostEntityMaterializerSource(IMemberMapper memberMapper, CurrentDatabaseContext current) : base(memberMapper)
        {
            //已注入的，用于业务的获取当前数据库上下文的类
            _database = current.Context;
        }

        private IDatabaseContext _database;

        //创建实例化实体的表达式
        public override Expression CreateMaterializeExpression(IEntityType entityType, Expression valueBufferExpression, int[] indexMap = null)
        {
            //获取基类实例化表达式
            BlockExpression expression = (BlockExpression)base.CreateMaterializeExpression(entityType, valueBufferExpression, indexMap);
            //业务逻辑判断是否是我需要更改的实体
            if (typeof(IEntity).IsAssignableFrom(entityType.ClrType))
            {
                var entityContext = _database.GetDynamicContext(entityType.ClrType);
                //属性表达式
                var property = Expression.Property(expression.Variables[0], typeof(IEntity).GetProperty("EntityContext"));
                //赋值表达式
                var assign = Expression.Assign(property, Expression.Constant(entityContext));
                //把几类的实例化表达式变成列表方便插入
                var list = expression.Expressions.ToList();
                //因为最后一个表达式是返回实体实例
                //所以我们的逻辑代码要放在最后一条语句之前
                list.Insert(list.Count - 1, assign);
                //重新生成表达式
                expression = Expression.Block(expression.Variables, list);
            }
            return expression;
        }
    }