谓词生成器不适用于Where子句

Question

我刚刚复制了以下源代码

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，我在下面使用它

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是相同的！。最终结果中没有筛选。我错过什么了吗？

Answer 1

代替：

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

写入

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

并更改下一个呼叫。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));