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copied!<p>The clean way is to state the fields you actually want in expression, put them in your mid-tier objects and then use any auxiliary functions to modify them latter.</p> <p>I'm not sure if you realize that a class representing SQL table for LINQ is a DTO class - it defines the grammar used by LINQ-SQL translator. Injecting a property into a DTO which is not mapped to the SQL table is not even supported - meaning that translator can fire at will. Attributes define the grammar and anything not defined by them doesn't exist for expression translator.</p> <p>Entities named in the from clause are not objects - they are just symbols used to help spelling out actual table fields that will be fetched. A field not named explicitly in select is a field not fetched - at least that's the goal of the translator, it may have to let a few slip through. For example if that ent.FormattedName is not declared, that's a slip and can explode latter.</p> <p>So, that FormattedNumber property injected into DTO class doesn't even exist in the grammar. It's not a "calculated field" - that term is strictly for SQL table definitions and if you had one it would be in DTO's grammar. Notice that error said very precisely "local expression" - very limited scope. </p> <p>You could try to cheat it by a nested lambda expression invoking a static function on the whole "tel" which might triger fetching of the whole record - or throwing another exception.</p> <p>Other LINQ-s, which are not translators, can have relaxed rules. LINQ-SQL has to be either very strict or very slow and it's already slow enough :-)</p>

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