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POLinq PredicateBuilder with conditional AND, OR and NOT filters
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  1. POLinq PredicateBuilder with conditional AND, OR and NOT filters
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    copied!<p>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 <em>and</em> or an <em>or</em> search.</p> <p>I though about redoing the LINQ queries using <a href="http://www.albahari.com/nutshell/predicatebuilder.aspx" rel="noreferrer">PredicateBuilder</a> and have got this working pretty well I think. I effectively have a class containing my predicates, e.g.:</p> <pre><code>internal static Expression&lt;Func&lt;Job, bool&gt;&gt; Description(string term) { return p =&gt; p.Description.Contains(term); } </code></pre> <p>To perform the search i'm doing this (some code omitted for brevity):</p> <pre><code>public Expression&lt;Func&lt;Job, bool&gt;&gt; ToLinqExpression() { var predicates = new List&lt;Expression&lt;Func&lt;Job, bool&gt;&gt;&gt;(); // build up predicates here if (SearchType == SearchType.And) { query = PredicateBuilder.True&lt;Job&gt;(); } else { query = PredicateBuilder.False&lt;Job&gt;(); } foreach (var predicate in predicates) { if (SearchType == SearchType.And) { query = query.And(predicate); } else { query = query.Or(predicate); } } return query; } </code></pre> <p>While i'm reasonably happy with this, I have two concerns:</p> <ol> <li>The if/else blocks that evaluate a SearchType property feel like they could be a potential code smell.</li> <li>The client is now insisting on being able to perform 'and not' / 'or not' searches.</li> </ol> <p>To address point 2, I think I could do this by simply rewriting my expressions, e.g.:</p> <pre><code>internal static Expression&lt;Func&lt;Job, bool&gt;&gt; Description(string term, bool invert) { if (invert) { return p =&gt; !p.Description.Contains(term); } else { return p =&gt; p.Description.Contains(term); } } </code></pre> <p>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.</p>
 

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