PredicateBuilder 不适用于 Where 子句

Question

我只是复制了以下源代码

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
/// <summary>
/// Enables the efficient, dynamic composition of query predicates.
/// </summary>
public static class PredicateBuilder
{
    /// <summary>
    /// Creates a predicate that evaluates to true.
    /// </summary>
    public static Expression<Func<T, bool>> True<T>() { return param => true; }

    /// <summary>
    /// Creates a predicate that evaluates to false.
    /// </summary>
    public static Expression<Func<T, bool>> False<T>() { return param => false; }

    /// <summary>
    /// Creates a predicate expression from the specified lambda expression.
    /// </summary>
    public static Expression<Func<T, bool>> Create<T>(Expression<Func<T, bool>> predicate) { return predicate; }
    public static Expression<Func<T, bool>> Create<T>(bool value) { return param => value; }

    /// <summary>
    /// Combines the first predicate with the second using the logical "and".
    /// </summary>
    public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
    {
        return first.Compose(second, Expression.AndAlso);
    }

    /// <summary>
    /// Combines the first predicate with the second using the logical "or".
    /// </summary>
    public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
    {
        return first.Compose(second, Expression.OrElse);
    }

    /// <summary>
    /// Negates the predicate.
    /// </summary>
    public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expression)
    {
        var negated = Expression.Not(expression.Body);
        return Expression.Lambda<Func<T, bool>>(negated, expression.Parameters);
    }

    public static Func<T, bool> ToFunc<T>(this Expression<Func<T, bool>> expression)
    {
        return expression.Compile();
    }

    /// <summary>
    /// Combines the first expression with the second using the specified merge function.
    /// </summary>
    static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
    {
        // zip parameters (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 the parameters in the first
        var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

        // create a merged lambda expression with parameters from the first expression
        return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
    }

    class ParameterRebinder : ExpressionVisitor
    {
        readonly Dictionary<ParameterExpression, ParameterExpression> map;

        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);
        }
    }
}

然后，我使用它如下

public class Person
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public DateTimeOffset BirthDate { get; set; }

}
    
using Bogus;
using System.Collections.Generic;

public static class PeopleDataGenerator
{
    public static IEnumerable<Person> GetPeople(int count = 200)
    {
        var people = new Faker<Person>()
            .StrictMode(true)
            .RuleFor(p => p.Id, f => f.Random.Int(0,1000))
            .RuleFor(p => p.FirstName, f => f.Name.FirstName())
            .RuleFor(p => p.LastName, f => f.Name.LastName())
            .RuleFor(p => p.BirthDate, f => f.Person.DateOfBirth)
            ;
        return people.Generate(count);
    }

}

和

IEnumerable<Person> people = PeopleDataGenerator.GetPeople(500);

var pb = PredicateBuilder.True<Person>();

pb.And(x => x.Id > 500);
pb.And(x => x.FirstName.StartsWith("A", StringComparison.OrdinalIgnoreCase));

IEnumerable<Person> result = people.Where(pb.ToFunc()).ToList();

但是， people和result都是一样的！ 最终结果没有过滤。 我错过了什么？

Answer 1

代替：

pb.And(x => x.Id > 500);

写

pb = pb.And(x => x.Id > 500);

并更改下一个电话。 或者你可以简单地堆叠方法：

var pb = PredicateBuilder.True<Person>()
   .And(x => x.Id > 500)
   .And(x => x.FirstName.StartsWith("A", StringComparison.OrdinalIgnoreCase));