public static class Mapper<TSource, TTarget> where TSource : class where TTarget : class
 {
  public readonly static Func<TSource, TTarget> Map;

  static Mapper()
  {
   if (Map == null)
    Map = GetMap();
  }

  private static Func<TSource, TTarget> GetMap()
  {
   var sourceType = typeof(TSource);
   var targetType = typeof(TTarget);

   var parameterExpression = Expression.Parameter(sourceType, "p");
   var memberInitExpression = GetExpression(parameterExpression, sourceType, targetType);

   var lambda = Expression.Lambda<Func<TSource, TTarget(memberInitExpression, parameterExpression);
   return lambda.Compile();
  }

  /// <summary>
  /// 根据转换源和目标获取表达式树
  /// </summary>
  /// <param name="parameterExpression">表达式参数p</param>
  /// <param name="sourceType">转换源类型</param>
  /// <param name="targetType">转换目标类型</param>
  /// <returns></returns>
  private static MemberInitExpression GetExpression(Expression parameterExpression, Type sourceType, Type targetType)
  {
   var memberBindings = new List<MemberBinding>();
   foreach (var targetItem in targetType.GetProperties().Where(x => x.PropertyType.IsPublic && x.CanWrite))
   {
    var sourceItem = sourceType.GetProperty(targetItem.Name);

    //判断实体的读写权限
    if (sourceItem == null || !sourceItem.CanRead || sourceItem.PropertyType.IsNotPublic)
     continue;

    //标注NotMapped特性的属性忽略转换
    if (sourceItem.GetCustomAttribute<NotMappedAttribute>() != null)
     continue;

    var propertyExpression = Expression.Property(parameterExpression, sourceItem);

    //判断都是class 且类型不相同时
    if (targetItem.PropertyType.IsClass && sourceItem.PropertyType.IsClass && targetItem.PropertyType != sourceItem.PropertyType)
    {
     if (targetItem.PropertyType != targetType)//防止出现自己引用自己无限递归
     {
      var memberInit = GetExpression(propertyExpression, sourceItem.PropertyType, targetItem.PropertyType);
      memberBindings.Add(Expression.Bind(targetItem, memberInit));
      continue;
     }
    }

    if (targetItem.PropertyType != sourceItem.PropertyType)
     continue;

    memberBindings.Add(Expression.Bind(targetItem, propertyExpression));
   }
   return Expression.MemberInit(Expression.New(targetType), memberBindings);
  }
 }

public class A
{
 public int Id { get; set; }
 public string Name { get; set; }
 public C User { get; set; }
 
 /// <summary>
 /// 标注为notmapped特性时，不转换赋值
 /// </summary>
 [System.ComponentModel.DataAnnotations.Schema.NotMapped]
 public D UserA { get; set; }
 
}
 
public class B
{
 public int Id { get; set; }
 public string Name { get; set; }
 public D User { get; set; }<br data-filtered="filtered">
 public D UserA { get; set; }
}
 
public class C
{
 public int Id { get; set; }
 public string Name { get; set; }
}
 
public class D
{
 public int Id { get; set; }
 public string Name { get; set; }
}

var a = new A
{
 Id = 1,
 Name = "张三",
 User = new C
 {
  Id = 1,
  Name = "李四"
 }
};<br>
B b = Mapper<A, B>.Map(a);//得到转换结果

var length = 10000000;
   var listA = new List<A>();
   for (int i = 0; i < length; i++)
   {
    listA.Add(new A
    {
     Id = i,
     Name = "张三",
     User = new C
     {
      Id = i,
      Name = "李四"
     }
    });
   }

   var sw = Stopwatch.StartNew();
   for (int i = 0; i < length; i++)
   {
    var item = listA[i];
    var b = new B
    {
     Id = item.Id,
     Name = item.Name,
     User = new D
     {
      Id = i,
      Name = "李四",
     }
    };
   }
   sw.Stop();
   Console.WriteLine($"原生的时间:{sw.ElapsedMilliseconds}ms");

   //表达式
   Mapper<A, B>.Map(listA[0]);//预先编译缓存
   sw.Restart();
   for (int i = 0; i < length; i++)
   {
    Mapper<A, B>.Map(listA[i]);
   }
   sw.Stop();
   Console.WriteLine($"表达式的时间:{sw.ElapsedMilliseconds}ms");

   //AutoMapper
   AutoMapper.Mapper.Initialize(cfg => cfg.CreateMap<A, B>());
   sw.Restart();
   for (int i = 0; i < length; i++)
   {
    var b = AutoMapper.Mapper.Map<B>(listA[i]);
   }
   sw.Stop();
   Console.WriteLine($"AutoMapper时间:{sw.ElapsedMilliseconds}ms");

   //TinyMapper
   TinyMapper.Bind<A, B>();
   sw.Restart();
   for (int i = 0; i < length; i++)
   {
    var b = TinyMapper.Map<B>(listA[i]);
   }
   sw.Stop();
   Console.WriteLine($"TinyMapper时间:{sw.ElapsedMilliseconds}ms");

   Console.ReadLine();