使用OrElse和AndAlso表达式方法的异常

Question

我试图以编程方式构建表达式树.

我在输入中有一个条件类列表,它们具有以下形式:

public class Filter
{
    public string field { get; set; }
    public string operator { get; set; }
    public string value { get; set; }
}

当我构建Expression对象时,我Expression按以下方式为每个条件创建一个

foreach ( Filter sf in rules ) {
    Expression ex = sf.ToExpression( query );
    if ( mainExpression == null ) {
        mainExpression = ex;
    }
    else {
        if ( logicalCondition == "AND" ) {
            mainExpression = Expression.And( mainExpression, ex );
        }
        else if ( logicalCondition == "OR" ) {
            mainExpression = Expression.Or( mainExpression, ex );
        }
    }
}

Filter.ToExpression()方法就像这样实现

public override Expression ToExpression( IQueryable query ) {
    ParameterExpression parameter = Expression.Parameter( query.ElementType, "p" );
    MemberExpression memberAccess = null;
    foreach ( var property in field.Split( '.' ) )
        memberAccess = MemberExpression.Property( memberAccess ?? ( parameter as Expression ), property );
    ConstantExpression filter = Expression.Constant( Convert.ChangeType( value, memberAccess.Type ) );
    WhereOperation condition = (WhereOperation)StringEnum.Parse( typeof( WhereOperation ), operator );
    LambdaExpression lambda = BuildLambdaExpression( memberAccess, filter, parameter, condition, value );
    return lambda;
}

一切正常时,我有一个条件,但是当我尝试使用的一个表达式组合And,Or,AndAlso,OrElse静态方法我收到的InvalidOperationException,说:

二进制运算符Or未定义类型'System.Func 2[MyObject,System.Boolean]' and 'System.Func2 [MyObject,System.Boolean]'.

我有点困惑.有人能更好地解释异常的原因并提出解决方案吗？

非常感谢!

Answer 1

你结合a => a == 3和a => a == 4成(a => a == 3) || (a => a == 4),但你应该改为试图使它a => (a == 3 || a == 4).这不是很难手动完成,但有人已经为你做了.寻找"结合表达式".

编辑:根据要求,一个如何手动执行此操作的简单示例.

编辑2:它使用ExpressionVisitor.NET 4中的新功能,但在MSDN中,您可以找到早期版本的可用实现.我假设MSDN代码不符合您的"第三方"的条件.您只需要将protected virtual Expression Visit(Expression exp)方法更改为public.而且Enumerable.Zip对你来说没有必要,它现在已经消失了.

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

namespace DemoApp
{
    <include ExpressionVisitor definition here for .NET 3.5>

    public class ExpressionParameterReplacer : ExpressionVisitor
    {
        public ExpressionParameterReplacer(IList<ParameterExpression> fromParameters, IList<ParameterExpression> toParameters)
        {
            ParameterReplacements = new Dictionary<ParameterExpression, ParameterExpression>();
            for (int i = 0; i != fromParameters.Count && i != toParameters.Count; i++)
                ParameterReplacements.Add(fromParameters[i], toParameters[i]);
        }
        private IDictionary<ParameterExpression, ParameterExpression> ParameterReplacements
        {
            get;
            set;
        }
        protected override Expression VisitParameter(ParameterExpression node)
        {
            ParameterExpression replacement;
            if (ParameterReplacements.TryGetValue(node, out replacement))
                node = replacement;
            return base.VisitParameter(node);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Expression<Func<int, bool>> exprA = a => a == 3;
            Expression<Func<int, bool>> exprB = b => b == 4;
            Expression<Func<int, bool>> exprC =
                Expression.Lambda<Func<int, bool>>(
                    Expression.OrElse(
                        exprA.Body,
                        new ExpressionParameterReplacer(exprB.Parameters, exprA.Parameters).Visit(exprB.Body)),
                    exprA.Parameters);
            Console.WriteLine(exprA.ToString());
            Console.WriteLine(exprB.ToString());
            Console.WriteLine(exprC.ToString());
            Func<int, bool> funcA = exprA.Compile();
            Func<int, bool> funcB = exprB.Compile();
            Func<int, bool> funcC = exprC.Compile();
            Debug.Assert(funcA(3) && !funcA(4) && !funcA(5));
            Debug.Assert(!funcB(3) && funcB(4) && !funcB(5));
            Debug.Assert(funcC(3) && funcC(4) && !funcC(5));
        }
    }
}