Express 表达式目录树的应用

public class People
   {
       public int Age { get; set; }
       public string Name { get; set; }
       public int Id;
   }
   public class PeopleCopy
   {
 
       public int Age { get; set; }
       public string Name { get; set; }
       public int Id;
   }

　两个实体类之间的转换做法

PeopleCopy peopleCopy0 = new PeopleCopy()
               {
                   Id = people.Id,
                   Name = people.Name,
                   Age = people.Age
               };性能好，不灵活；不能共用；如果换成其他的类型就不能用了；

PeopleCopy peopleCopy1 = ReflectionMapper.Trans<People, PeopleCopy>(people);
    public class ReflectionMapper
    {
        /// <summary>
        /// 反射
        /// </summary>
        /// <typeparam name="TIn"></typeparam>
        /// <typeparam name="TOut"></typeparam>
        /// <param name="tIn"></param>
        /// <returns></returns>
        public static TOut Trans<TIn, TOut>(TIn tIn)
        {
            TOut tOut = Activator.CreateInstance<TOut>();
            foreach (var itemOut in tOut.GetType().GetProperties())
            {
                var propIn = tIn.GetType().GetProperty(itemOut.Name);
                itemOut.SetValue(tOut, propIn.GetValue(tIn));
            }
 
            foreach (var itemOut in tOut.GetType().GetFields())
            {
                var fieldIn = tIn.GetType().GetField(itemOut.Name);
                itemOut.SetValue(tOut, fieldIn.GetValue(tIn));
            }
            return tOut;
        }
    }//反射  灵活，但是性能不好！

PeopleCopy peopleCopy2 = SerializeMapper.Trans<People, PeopleCopy>(people);
                     public class SerializeMapper
                      {
                          /// <summary>
                          /// 序列化反序列化方式
                          /// </summary>
                          /// <typeparam name="TIn"></typeparam>
                          /// <typeparam name="TOut"></typeparam>
                          public static TOut Trans<TIn, TOut>(TIn tIn)
                          {
                              string strJson = JsonConvert.SerializeObject(tIn);
                              return JsonConvert.DeserializeObject<TOut>(strJson);
                          }
                      }//序列化，jason 、xml  灵活，但是性能不好！

Func<People, PeopleCopy> func = p => new PeopleCopy
            {
                Id = p.Id,
                Name = p.Name,
                Age = p.Age
            };
        PeopleCopy peopleCopynew = func.Invoke(people);灵活，但是性能不好！

如果能够把这个委托给缓存起来；根据我们的诉求，缓存一个委托；委托哪儿来？委托其实可以通过表达式目录树Compile一下，就可以得到一个委托；如果拼装一个表达式目录树，再Compile一下，然后缓存起来；就相当于拼装了这一组的转换逻辑；
拼装一下后，缓存下来，如果后面再需要使用，就可以直接使用这个委托了；

动态拼装转换过程；动态拼装了硬编码；

PeopleCopy peopleCopy = ExpressionGenericMapper<People, PeopleCopy>.Trans(people);
public class ExpressionGenericMapper<TIn, TOut>//Mapper`2
    {
        private static Func<TIn, TOut> _FUNC = null;   //静态缓存
        static ExpressionGenericMapper()
        {
            ParameterExpression parameterExpression = Expression.Parameter(typeof(TIn), "p");
            List<MemberBinding> memberBindingList = new List<MemberBinding>();
            foreach (var item in typeof(TOut).GetProperties())    //反射获取属性
            {
                MemberExpression property = Expression.Property(parameterExpression, typeof(TIn).GetProperty(item.Name));
                MemberBinding memberBinding = Expression.Bind(item, property);
                memberBindingList.Add(memberBinding);
            }
            foreach (var item in typeof(TOut).GetFields())     //反射获取字段
            {
                MemberExpression property = Expression.Field(parameterExpression, typeof(TIn).GetField(item.Name));
                MemberBinding memberBinding = Expression.Bind(item, property);
                memberBindingList.Add(memberBinding);
            }
            MemberInitExpression memberInitExpression = Expression.MemberInit(Expression.New(typeof(TOut)), memberBindingList.ToArray()); 　　　　　　  //创建System.Linq.Expressions.MemberInitExpression
            Expression<Func<TIn, TOut>> lambda = Expression.Lambda<Func<TIn, TOut>>(memberInitExpression, new ParameterExpression[]
            {
                    parameterExpression
            });
            _FUNC = lambda.Compile();//拼装是一次性的
        }
        public static TOut Trans(TIn t)
        {
            return _FUNC(t);
        }

　也可以用字典缓存，性能不如上面的，会慢几倍

public class ExpressionMapper
   {
       /// <summary>
       /// 字典缓存--hash分布  保存的是委托---委托内部是转换的动作；
       /// </summary>
       private static Dictionary<string, object> _Dic = new Dictionary<string, object>();
 
       /// <summary>
       /// 字典缓存表达式树
       /// </summary>
       /// <typeparam name="TIn"></typeparam>
       /// <typeparam name="TOut"></typeparam>
       /// <param name="tIn"></param>
       /// <returns></returns>
       public static TOut Trans<TIn, TOut>(TIn tIn)
       {
           string key = $"funckey_{typeof(TIn).FullName}_{typeof(TOut).FullName}";
           if (!_Dic.ContainsKey(key))
           {
               ParameterExpression parameterExpression = Expression.Parameter(typeof(TIn), "p");
               List<MemberBinding> memberBindingList = new List<MemberBinding>();
               foreach (var item in typeof(TOut).GetProperties())
               {
                   MemberExpression property = Expression.Property(parameterExpression, typeof(TIn).GetProperty(item.Name));
                   MemberBinding memberBinding = Expression.Bind(item, property);
                   memberBindingList.Add(memberBinding);
               }
               foreach (var item in typeof(TOut).GetFields())
               {
                   MemberExpression property = Expression.Field(parameterExpression, typeof(TIn).GetField(item.Name));
                   MemberBinding memberBinding = Expression.Bind(item, property);
                   memberBindingList.Add(memberBinding);
               }
               MemberInitExpression memberInitExpression = Expression.MemberInit(Expression.New(typeof(TOut)), memberBindingList.ToArray());
               Expression<Func<TIn, TOut>> lambda = Expression.Lambda<Func<TIn, TOut>>(memberInitExpression, new ParameterExpression[]
               {
                   parameterExpression
               });
               Func<TIn, TOut> func = lambda.Compile();//拼装是一次性的
               _Dic[key] = func;
           }
           return ((Func<TIn, TOut>)_Dic[key]).Invoke(tIn);
       }
   }
PeopleCopy peopleCopy = ExpressionMapper.Trans<People, PeopleCopy>(people);