EntityFramework的linq扩展where

代码

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

namespace Common
{
    /// <summary>
    /// 摘自：https://blogs.msdn.microsoft.com/meek/2008/05/02/linq-to-entities-combining-predicates/
    /// </summary>
    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 ExpressionUtility
    {
        public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
        {
            // build parameter map (from parameters of second to parameters of first) 
            var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);
            // replace parameters in the second lambda expression with parameters from the first 
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
            // apply composition of lambda expression bodies to parameters from the first expression  
            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);
        }
    }
}

Expression<Func<TSource, bool>> predicate = c=>c.Id==1;

predicate = predicate.And(c=>c.Name=="jay");