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    <p>The first thing to do is to put on logging, to see what TSQL was generated; for example:</p> <pre><code>ctx.Log = Console.Out; </code></pre> <p>LINQ-to-SQL seems to treat nulls a little inconsistently (depending on literal vs value):</p> <pre><code>using(var ctx = new DataClasses2DataContext()) { ctx.Log = Console.Out; int? mgr = (int?)null; // redundant int? for comparison... // 23 rows: var bosses1 = ctx.Employees.Where(x =&gt; x.ReportsTo == (int?)null).ToList(); // 0 rows: var bosses2 = ctx.Employees.Where(x =&gt; x.ReportsTo == mgr).ToList(); } </code></pre> <p>So all I can suggest is use the top form with nulls!</p> <p>i.e.</p> <pre><code>Expression&lt;Func&lt;Category,bool&gt;&gt; predicate; if(categoryId == null) { predicate = c=&gt;c.ParentId == null; } else { predicate = c=&gt;c.ParentId == categoryId; } var subCategories = this.Repository.Categories .Where(predicate).ToList().Cast&lt;ICategory&gt;(); </code></pre> <hr> <p>Update - I got it working "properly" using a custom <code>Expression</code>:</p> <pre><code> static void Main() { ShowEmps(29); // 4 rows ShowEmps(null); // 23 rows } static void ShowEmps(int? manager) { using (var ctx = new DataClasses2DataContext()) { ctx.Log = Console.Out; var emps = ctx.Employees.Where(x =&gt; x.ReportsTo, manager).ToList(); Console.WriteLine(emps.Count); } } static IQueryable&lt;T&gt; Where&lt;T, TValue&gt;( this IQueryable&lt;T&gt; source, Expression&lt;Func&lt;T, TValue?&gt;&gt; selector, TValue? value) where TValue : struct { var param = Expression.Parameter(typeof (T), "x"); var member = Expression.Invoke(selector, param); var body = Expression.Equal( member, Expression.Constant(value, typeof (TValue?))); var lambda = Expression.Lambda&lt;Func&lt;T,bool&gt;&gt;(body, param); return source.Where(lambda); } </code></pre>
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    1. POCompare nullable types in Linq to Sql
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    1. USMarc Gravell
    1. USMarc Gravell
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    1. COIt seems there is no better way to handle this. Thanks!
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      1. USAli Ersöz
    2. COI ran into this exact same problem, made the same workaround and was about to ask if there was a better way to do it. Looks like there isn't :( This behaviour is really counter intuitive.
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      1. USAndrew Barrett
    3. COI would say the inconsistency between literals and variables is worse that counter intuitive. Thanks for confirming my suspicion. +1
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