解析Expression<Func<T, object>>表达式

前言

　　时光荏苒，2020年不觉中已成过去。2021刚刚开头，新的开始本应意气风发、本应有会当凌绝顶豪情壮志，再不济也该“鸿图霸业谈笑间”，且不管一入江湖是不是岁月摧的洒脱。很不幸，有些事情就是会突入其来，搞得你触不及防，什么心情都没有。但是我居然还是写下了这篇博客。。。。。。嗯，总算是有地方表达以我糟糕心情了。

　　本来是想解决一下合并两个Expression<Func<T, object>>类型的表达式的问题，比如：

　　Expression<Func<T, object>> exp1 = t => t.updateTime;　Expression<Func<T, object>> exp2 = t => new {t.age, t.name};

　　合并为一个表达式：Expression<Func<T, object>> exp1 = t => new {t.age, t.name， t.updateTime};

　　之前尝试过一些办法（比如下图方法1和方法2，但是都失败了）也请教过一些大佬，都没有解决这个问题。于是我只能绕过这个问题用其他方法去实现了业务需求（在这里抛出这个问题，也希望能有人帮我解惑）

　　

退而求其次

　　我通过解析出来表达式中指定的属性也能满足我的需求，下面是我的实现方式：

        private static List<MemberInfo> SelectMembers(Expression properties)
        {
            if( properties.NodeType == ExpressionType.Convert)
            {
                List<MemberInfo> newArrMembers = new List<MemberInfo>
                {
                    ((MemberExpression)((UnaryExpression)properties).Operand).Member
                };
                return newArrMembers;
            }
            else
            {
                var newExp = (NewExpression)properties;
                if (newExp != null)
                {
                    return newExp.Members.ToList();
                }
                else
                {
                    var newArr = (NewArrayExpression)properties;
                    if (newArr != null)
                    {
                        List<MemberInfo> newArrMembers = new List<MemberInfo>();
                        foreach (var newArrExp in newArr.Expressions)
                            newArrMembers.AddRange(SelectMembers(newArrExp));
                        return newArrMembers.Distinct().ToList();
                    }
                    return null;
                }
            }
        }

如何使用

        public override Task<bool> UpdateAsync(T model, Expression<Func<T, object>> properties)
        {
            var mem = properties.SelectMembers();
            if (mem != null)
            {
                var sc = mem.Select(t => t.Name).ToArray();
                if (!sc.Contains("Updatedtime")) throw new Exception("表达必须指定updatedtime字段");
                if (sc.Contains("Createdtime")) throw new Exception("不允许更新的字段");
            }
            model.UpdatedTime = DateTime.Now;
            return base.UpdateAsync(model, properties);
        }

下面附上一些实用的扩展

using System;
using System.Collections.Generic;
using System.Dynamic;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;

namespace Syspetro.Core.Extensions
{
    /// <summary>
    /// 合并表达式 And Or  Not扩展
    /// </summary>
    public static class ExpressionHelper
    {
        /// <summary>
        /// 合并表达式 expr1 AND expr2
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="expr1"></param>
        /// <param name="expr2"></param>
        /// <returns></returns>
        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
        {
            if (expr1 == null)
                return expr2;
            else if (expr2 == null)
                return expr1;
            ParameterExpression newParameter = Expression.Parameter(typeof(T), "c");
            MyExpressionVisitor visitor = new MyExpressionVisitor(newParameter);
            var left = visitor.Visit(expr1.Body);
            var right = visitor.Visit(expr2.Body);

            return Expression.Lambda<Func<T, bool>>(Expression.And(left, right), newParameter);
        }
        /// <summary>
        /// 合并表达式 expr1 or expr2
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="expr1"></param>
        /// <param name="expr2"></param>
        /// <returns></returns>
        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
        {
            if (expr1 == null)
                return expr2;
            else if (expr2 == null)
                return expr1;
            ParameterExpression newParameter = Expression.Parameter(typeof(T), "c");
            MyExpressionVisitor visitor = new MyExpressionVisitor(newParameter);

            var left = visitor.Visit(expr1.Body);
            var right = visitor.Visit(expr2.Body);
            var body = Expression.Or(left, right);
            return Expression.Lambda<Func<T, bool>>(body, newParameter);
        }
        /// <summary>
        /// 取反表达式
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="expr"></param>
        /// <returns></returns>
        public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expr)
        {
            if (expr == null) return null;
            var candidateExpr = expr.Parameters[0];
            var body = Expression.Not(expr.Body);

            return Expression.Lambda<Func<T, bool>>(body, candidateExpr);
        }
        /// <summary>
        /// 获取表达式属性列表
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="expr"></param>
        /// <returns></returns>
        public static List<MemberInfo> SelectMembers<T>(this Expression<Func<T, object>> expr)
        {
            return SelectMembers(expr.Body);
        }
        private static List<MemberInfo> SelectMembers(Expression properties)
        {
            if( properties.NodeType == ExpressionType.Convert)
            {
                List<MemberInfo> newArrMembers = new List<MemberInfo>
                {
                    ((MemberExpression)((UnaryExpression)properties).Operand).Member
                };
                return newArrMembers;
            }
            else
            {
                var newExp = (NewExpression)properties;
                if (newExp != null)
                {
                    return newExp.Members.ToList();
                }
                else
                {
                    var newArr = (NewArrayExpression)properties;
                    if (newArr != null)
                    {
                        List<MemberInfo> newArrMembers = new List<MemberInfo>();
                        foreach (var newArrExp in newArr.Expressions)
                            newArrMembers.AddRange(SelectMembers(newArrExp));
                        return newArrMembers.Distinct().ToList();
                    }
                    return null;
                }
            }
        }
    }
}

总结

　　纵然此时你的内心是千疮百孔，万般无奈，那又怎样，你还想继续生活吗？那就只能继续前向，继续拼搏，努力让自己更加强大。所以该做的事情还是要做，加油。