Linq PredicateBuilder with conditional AND, OR and

We have a project using LINQ to SQL, for which I need to rewrite a couple of search pages to allow the client to select whether they wish to perform an and or an or search.

I though about redoing the LINQ queries using PredicateBuilder and have got this working pretty well I think. I effectively have a class containing my predicates, e.g.:

internal static Expression<Func<Job, bool>> Description(string term)
{
    return p => p.Description.Contains(term);
}

To perform the search i'm doing this (some code omitted for brevity):

public Expression<Func<Job, bool>> ToLinqExpression()
{
    var predicates = new List<Expression<Func<Job, bool>>>();
    // build up predicates here

    if (SearchType == SearchType.And)
    {
        query = PredicateBuilder.True<Job>();
    }
    else
    {
        query = PredicateBuilder.False<Job>();
    }

    foreach (var predicate in predicates)
    {
        if (SearchType == SearchType.And)
        {
            query = query.And(predicate);
        }
        else
        {
            query = query.Or(predicate);
        }
    }
    return query;
}

While i'm reasonably happy with this, I have two concerns:

The if/else blocks that evaluate a SearchType property feel like they could be a potential code smell.
The client is now insisting on being able to perform 'and not' / 'or not' searches.

To address point 2, I think I could do this by simply rewriting my expressions, e.g.:

internal static Expression<Func<Job, bool>> Description(string term, bool invert)
{
    if (invert)
    {
        return p => !p.Description.Contains(term);
    }
    else
    {
        return p => p.Description.Contains(term);
    }
}

However this feels like a bit of a kludge, which usually means there's a better solution out there. Can anyone recommend how this could be improved? I'm aware of dynamic LINQ, but I don't really want to lose LINQ's strong typing.

If you are looking for less lines you can replace the if/else with ternary operator:

query = SearchType == SearchType.And ? PredicateBuilder.True<Job>() : PredicateBuilder.False<Job>();

   foreach (var predicate in predicates)
   {
        query = SearchType == SearchType.And ? query.And(predicate) : query.Or(predicate);
   }

for the 'and not' / 'or not' part the ! operator should do the trick.

PD: Did you test the foreach part is correctly setting the predicates?, as far as i remember you are building the expression that will be executed at later point in time, so you may have a literal reference just to the last set predicate in the final iteration, and that's why you must use a temp variable to save the value of each iteration.

EDIT: If you want to negate a expression programmatic, that's a tricky one, you can try something like:

internal static Expression<Func<Job, bool>> Description(string term, bool invert)
        {
           return NegateExp<Func<Job, bool>>(p => p.Description.Contains(term), invert);
        }

And the NegateExp method will be something like:

public static Expression<TDelegate> NegateExp<TDelegate>(Expression<TDelegate> expression, bool inverse)
        {
            if (inverse)
            {
                return Expression.Lambda<TDelegate>(Expression.Not(expression.Body), expression.Parameters);
            }
            return expression;
        }

You can take a look at this question for more examples Is there any way to negate a Predicate?