Exception using OrElse and AndAlso expression meth

问题:

I am trying to build an expression tree programmatically.

I have in my input, a list of condition classes which have the following form:

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

When I build the Expression object I create an Expression for every condition in the following way

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 );
        }
    }
}

The Filter.ToExpression() method is implemented like this

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;
}

Everything works when I have a single condition but when I try to combine expressions using one of the And, Or, AndAlso, OrElse static methods I receive an InvalidOperationException that says:

The binary operator Or is not defined for the types 'System.Func2[MyObject,System.Boolean]' and 'System.Func2[MyObject,System.Boolean]'.

I am getting a little bit confused. Can somebody better explain the reasons of the exception and suggest a solution?

Thanks very much!

回答1:

You're combining a => a == 3 and a => a == 4 into (a => a == 3) || (a => a == 4), but you should instead be trying to make it a => (a == 3 || a == 4). This is not too hard to do manually, but someone has done it for you already. Look for "Combining Expressions".

Edit: as requested, a simple example of how to do this manually.

Edit 2: it uses ExpressionVisitor which is new to .NET 4, but at MSDN you can find a usable implementation for earlier versions. I'm assuming MSDN code doesn't qualify as "third party" for your purposes. You only need to change the protected virtual Expression Visit(Expression exp) method to public. And as Enumerable.Zip is unavailable for you and it isn't necessary, it is gone now.

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));
        }
    }
}