Expression动态生成方法例子

实现和上个用DynamicMethod的方式生成的复制实体类对应一样功能

public  static class  ExpMapper<TTarget,TSource>
    {
        private static MapMethod<TTarget, TSource> mapMethod;
          public static MapMethod<TTarget, TSource> GetMapMethod()
        {
            if (mapMethod == null)
            {
                mapMethod = CreateMapMethod(typeof(TTarget), typeof(TSource));
            }
            return mapMethod;
        }

        public static TTarget Map(TSource source)
        {
            if (mapMethod == null)
            {
                mapMethod = CreateMapMethod(typeof(TTarget), typeof(TSource));
            }
            return mapMethod(source);     
        }

        private static MapMethod<TTarget, TSource> CreateMapMethod(Type targetType, Type sourceType)
        {
            var source = Expression.Parameter(sourceType, "source");
   
            var binds = (from sp in sourceType.GetProperties()
                              from tp in targetType.GetProperties()
                              where sp.Name == tp.Name
                              select Expression.Bind(tp, Expression.Property(source, sp))).ToArray();

            Expression body = Expression.MemberInit(Expression.New(typeof(TTarget)), binds);

            return Expression.Lambda<MapMethod<TTarget, TSource>>(body,source).Compile();
                             
        }

    }

 .net 4中添加了不少新的Expression。比如Expression.Assign用来赋值，Expression.Variable定义变量，等等用来生成代码会更加的直观。effective c# 第二版上有个例子

代码如下

            var source = Expression.Parameter(typeof(TSource),"source");
            var dest = Expression.Variable(typeof(TDest), "dest");
            var assignments = from srcProp in typeof(TSource).GetProperties(BindingFlags.Public |BindingFlags.Instance)
                              where srcProp.CanRead
                              let destProp = typeof(TDest).GetProperty(srcProp.Name,BindingFlags.Public |BindingFlags.Instance)
                              where (destProp != null) &&
                              (destProp.CanWrite)
                              select Expression.Assign(Expression.Property(dest,destProp),Expression.Property(source,srcProp));
            // put together the body:
            var body = new List<Expression>();
            body.Add(Expression.Assign(dest,Expression.New(typeof(TDest))));
            body.AddRange(assignments);
            body.Add(dest);
            var expr =
            Expression.Lambda<Func<TSource, TDest>>(
            Expression.Block(
            new[] { dest }, // expression parameters
            body.ToArray() // body
            ),
            source // lambda expression
            );
            var func = expr.Compile();