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  • C#基础 Lambda

    原文:https://blog.csdn.net/zhifeiya/article/details/68489089

    扩展类

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Linq.Expressions;
    using System.Text;
     
    namespace Mvc.Util
    {
        public class ParameterRebinder : ExpressionVisitor
        {
            private readonly Dictionary<ParameterExpression, ParameterExpression> map;
            public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
            {
                this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
            }
            public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
            {
                return new ParameterRebinder(map).Visit(exp);
            }
            protected override Expression VisitParameter(ParameterExpression p)
            {
                ParameterExpression replacement;
                if (map.TryGetValue(p, out replacement))
                {
                    p = replacement;
                }
                return base.VisitParameter(p);
            }
        }
        public static class LambdaHelper
        {
            public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
            {
                var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
                var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
                return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
            }
            public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
            {
                return first.Compose(second, Expression.And);
            }
            public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
            {
                return first.Compose(second, Expression.Or);
            }
        }
    }
    

    使用示例

    TyMall.Entity.DbEntities db = new DbEntities();
    Expression<Func<tb_advert, bool>> expression = t => true;
    expression = expression.And(t => t.id == 1);
    expression = expression.And(t => t.sort > 1);
    var rs=db.tb_advert.Where(expression);
    
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  • 原文地址:https://www.cnblogs.com/guxingy/p/13207203.html
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