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copied!<p><a href="http://msdn.microsoft.com/en-us/library/bb291976.aspx" rel="nofollow">First()</a> will throw an exception if the collection is empty. Using <a href="http://msdn.microsoft.com/en-us/library/bb337697.aspx" rel="nofollow">Any()</a> in your <code>Where()</code> call should indeed solve the problem, but you would have to write something like:</p> <pre><code>data.Where(d => d.ObjectsA != null && d.ObjectsA.Any() && d.ObjectsA.First().ObjectsB != null && d.ObjectsA.First().ObjectsB.Any() && d.ObjectsA.First().ObjectsB.First().Nr == 1); </code></pre> <p>That's not very nice, because <code>First()</code> ends up being called several times on the same data. I'd suggest adding a body to your lambda expression and using intermediate variables with <a href="http://msdn.microsoft.com/en-us/library/bb340482.aspx" rel="nofollow">FirstOrDefault()</a>:</p> <pre><code>data.Where(d => { if (d.ObjectsA != null) { var firstA = d.ObjectsA.FirstOrDefault(); if (firstA != null && firstA.ObjectsB != null) { var firstB = firstA.ObjectsB.FirstOrDefault(); if (firstB != null) { return (firstB.Nr == 1); } } } return false; }); </code></pre> <p><strong>EDIT:</strong> The second code fragment above apparently does not work with LINQ to NHibernate. If you can use query syntax and don't have to check if <code>ObjectsA</code> and <code>ObjectsB</code> are <code>null</code>, you can write:</p> <pre><code>from d in data let firstA = d.ObjectsA.FirstOrDefault() let firstB = (firstA != null ? firstA.ObjectsB.FirstOrDefault() : null) where (firstB != null && firstB.Nr == 1) select d; </code></pre>

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