Expression builder function for String.Contains and string[]

.net c# expression-trees

Question

I'm having a hard time creating an expression builder function to pass to a query for NHibernate repository.

Basically what I want to be able to do is build a query from various object properties (strings) and compare their value to values in an array. Also be able to add other query sentences.

Something like

public static Expression<Func<TElement, bool>> BuildOrExpression<TElement, TValue>
    (Dictionary<Expression<Func<TElement, TValue>>, IEnumerable<TValue>> expressionSet)
{
    List<Expression> newExpressions = new List<Expression>();

    foreach (var item in expressionSet)
    {
        if (null == item.Key) throw new ArgumentNullException("valueSelector");
        if (null == item.Value) throw new ArgumentNullException("values");

        ParameterExpression p = item.Key.Parameters.Single();

        if (!item.Value.Any()) return e => false;

        IEnumerable<Expression> expressions = item.Value
           .Select(value => (Expression)Expression.Equal(item.Key.Body, Expression.Constant(value, typeof(TValue))));
        Expression compExpression = expressions
            .Aggregate<Expression>((accumulate, equal) => Expression.OrElse(accumulate, equal));
        newExpressions.Add(Expression.Lambda<Func<TElement, bool>>(compExpression, p));
    }

    Expression accExpression = newExpressions
        .Aggregate<Expression>((accumulate, equal) => Expression.OrElse(accumulate, equal));

    return Expression.Lambda<Func<TElement, bool>>(accExpression);
}

But instead of doing the Expression.Equal somehow doing a String.Contains.

Any help welcomed. By the way the code above is not tested.

Accepted Answer

I was able to overcome the expression combining problem but Im not sure this is the best solution. Any who I used the solution provided by Vladimir and PredicateBuilder by Joe Albahari (C# 4 book writer). This is what I came up with.

    public static Expression<Func<TElement, bool>> BuildOrExpression<TElement, TValue>(Expression<Func<TElement, TValue>> valueSelector, IEnumerable<TValue> values)
    {
        if (null == valueSelector) throw new ArgumentNullException("valueSelector");
        if (null == values) throw new ArgumentNullException("values");

        ParameterExpression p = valueSelector.Parameters.Single();

        if (!values.Any()) return e => false;

        Expression<Func<string, string, bool>> expFunc = (name, value) => name.Contains(value);
        IEnumerable<Expression> equals = values.Select(value => (Expression)Expression.Call(valueSelector.Body, typeof(String).GetMethod("Contains"), Expression.Constant(value, typeof(TValue))));
        Expression body = equals.Aggregate<Expression>((accumulate, equal) => Expression.OrElse(accumulate, equal));

        return Expression.Lambda<Func<TElement, bool>>(body, p);
    }
    public static Expression<Func<T, bool>> CombineOr<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
    {
        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
        return Expression.Lambda<Func<T, bool>>(Expression.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
    }
    public static Expression<Func<T, bool>> CombineAnd<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
    {
        var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
        return Expression.Lambda<Func<T, bool>>(Expression.AndAlso(expr1.Body, invokedExpr), expr1.Parameters);
    }

    [TestMethod()]
    public void OfferComplexSearchTest()
    {
        using (var lifetime = container.BeginLifetimeScope())
        {
            IOfferRepository offerRepository = lifetime.Resolve<IOfferRepository>();
            List<Offer> offers = new List<Offer>();
            List<Expression<Func<Offer, bool>>> expressions = new List<Expression<Func<Offer, bool>>>();

            Expression<Func<Offer, bool>> finalExp = null;

            string[] orQuery = new string[2] { "director".ToUpper(), "jefe".ToUpper() };

            expressions.Add(LinqTools.BuildOrExpression<Offer, string>(c => c.Description.ToUpper(), orQuery));
            expressions.Add(LinqTools.BuildOrExpression<Offer, string>(c => c.Title.ToUpper(), orQuery));
            expressions.Add(LinqTools.BuildOrExpression<Offer, string>(c => c.Keywords.ToUpper(), orQuery));

            finalExp = expressions.Aggregate<Expression<Func<Offer, bool>>>((accomulate, equal) => LinqTools.CombineOr<Offer>(accomulate, equal));

            offers.AddRange(offerRepository.GetMany(LinqTools.CombineAnd<Offer>(finalExp, (x) => x.Publish)));

            Assert.IsTrue(offers.Count > 0, "Error: No Offers found.");
        }
    }

Popular Answer

Instead of Expression.Equal you need to use Expression.Call and pass Contains MethodInfo instance. Something like this

IEnumerable<Expression> expressions = item.Value.Select(value => 
   (Expression)Expression.Call(item.Key.Body, 
   typeof(String).GetMethod("Contains"), 
   Expression.Constant(value, typeof(TValue))));


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Licensed under: CC-BY-SA with attribution
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Is this KB legal? Yes, learn why