表达式目录树的应用

1.使用表达式目录树实现两个不同类型的属性赋值：

首先，准备两个实体类

    /// <summary>
    /// 实体类
    /// </summary>
    public class People
    {
        public int Age { get; set; }
        public string Name { get; set; }
        public int Id;
    }
    /// <summary>
    /// 实体类Target
    /// PeopleDTO
    /// </summary>
    public class PeopleCopy
    {

        public int Age { get; set; }
        public string Name { get; set; }
        public int Id;
    }

接着写，数据处理类

  /// <summary>
    /// 生成表达式目录树  泛型缓存
    /// </summary>
    /// <typeparam name="TIn"></typeparam>
    /// <typeparam name="TOut"></typeparam>
    public class ExpressionGenericMapper<TIn, TOut>//Mapper`2
    {
        private static Func<TIn, TOut> _FUNC = null;
        static ExpressionGenericMapper()
        {
            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 = lambda.Compile();//拼装是一次性的
        }
        public static TOut Trans(TIn t)
        {
            return _FUNC(t);
        }
    }

最后，程序中调用该代码处理

                    Stopwatch watch = new Stopwatch();
                    watch.Start();
                    for (int i = 0; i < 1_000_000; i++)
                    {
                        PeopleCopy peopleCopy = ExpressionGenericMapper<People, PeopleCopy>.Trans(people);
                    }
                    watch.Stop();
                    generic = watch.ElapsedMilliseconds;

2. 使用表达式目录树，拼装sql语句

  internal static class SqlOperator
    {
        internal static string ToSqlOperator(this ExpressionType type)
        {
            switch (type)
            {
                case (ExpressionType.AndAlso):
                case (ExpressionType.And):
                    return "AND";
                case (ExpressionType.OrElse):
                case (ExpressionType.Or):
                    return "OR";
                case (ExpressionType.Not):
                    return "NOT";
                case (ExpressionType.NotEqual):
                    return "<>";
                case ExpressionType.GreaterThan:
                    return ">";
                case ExpressionType.GreaterThanOrEqual:
                    return ">=";
                case ExpressionType.LessThan:
                    return "<";
                case ExpressionType.LessThanOrEqual:
                    return "<=";
                case (ExpressionType.Equal):
                    return "=";
                default:
                    throw new Exception("不支持该方法");
            }

        }
    }

   public static class ExpressionExtension
    {
        public static void BatchDelete<T>(this IQueryable<T> entities, Expression<Func<T, bool>> expr)
        {
            ConditionBuilderVisitor visitor = new ConditionBuilderVisitor();
            visitor.Visit(expr);
            string condition = visitor.Condition();

            string sql = string.Format("DELETE FROM [{0}] WHERE {1};"
                , typeof(T).Name//有可能还得根据泛型去获取
                , condition);
            //然后执行sql
        }

    }

3， 合并表达式

 /// <summary>
    /// 合并表达式 And Or  Not扩展
    /// </summary>
    public static class ExpressionExtend
    {
        /// <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)
        {
            //return Expression.Lambda<Func<T, bool>>(Expression.AndAlso(expr1.Body, expr2.Body), expr1.Parameters);
            if (expr1 == null)
                return expr2;
            else if (expr2 == null)
                return expr1;
            ParameterExpression newParameter = Expression.Parameter(typeof(T), "c");
            NewExpressionVisitor visitor = new NewExpressionVisitor(newParameter);
            var left = visitor.Replace(expr1.Body);
            var right = visitor.Replace(expr2.Body);
            var body = Expression.And(left, right);
            return Expression.Lambda<Func<T, bool>>(body, 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");
            NewExpressionVisitor visitor = new NewExpressionVisitor(newParameter);

            var left = visitor.Replace(expr1.Body);
            var right = visitor.Replace(expr2.Body);
            var body = Expression.Or(left, right);
            return Expression.Lambda<Func<T, bool>>(body, newParameter);
        }
        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);
        }
    }

方法调用

                Expression<Func<People, bool>> lambda1 = x => x.Age > 5;
                Expression<Func<People, bool>> lambda2 = x => x.Id > 5;
                
                Expression<Func<People, bool>> lambda3 = lambda1.And(lambda2);
                Expression<Func<People, bool>> lambda4 = lambda1.Or(lambda2);
                Expression<Func<People, bool>> lambda5 = lambda1.Not();
                Do1(lambda3);
                Do1(lambda4);
                Do1(lambda5);