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2009-09-01T12:44:36.127
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2009-09-01T13:01:51.160
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If you use the language-builting LINQ syntax, you're implicitly declaring a lamba expression - and that lamba expression is an <code>Expression<Func <>></code> - so you don't need the where clause to return the Expression, you need it to be the expression. e.g. <pre><code>var q = from row in myTable where row.a < aMax select row.b; // which translates to var q = myTable.Where(row => row.a < aMax).Select(row => row.b); </code></pre> Now you need to "cache" <code>row => row.a < aMax</code> - not just the value of <code>row.a < aMax</code>, if you will. So, if you were to write something like this... <pre><code>Expression<Func<?,bool>> cachedExpression = row => row.a < aMax; var q = from row in myTable where cachedExpression select row.b; </code></pre> Well, you're saying "where, given a row, it is true that (given a row, it is true that row.a is less than aMax)". This is nonsense, and won't compile if cachedExpression is of type <code>Expression<Func<?,bool>></code> and myTable is a LinqToSql provided table. It would translate to something like this: <pre><code>Expression<Func<?,bool>> cachedExpression = row => row.a < aMax; //nonsense .Where: var q = myTable.Where(row => cachedExpression).Select(row => row.b); //effectively says this: var q = myTable.Where(row => (row0 => row0.a < aMax)).Select(row => row.b); </code></pre> Now, this is legal in the sense that it is possible for a linq query provider to implement a whereclause with a non-boolean value, but it's a very odd thing to do; certainly no standard linq providers (such as Linq to Sql) do such a thing. So what should you do? Something like: <pre><code>//not .Where(row=>cachedExpression)! var q = myTable.Where(cachedExpression).Select(row => row.b); //or, alternatively: var q = from filteredRow in myTable.Where(cachedExpression) select filteredRow.b; </code></pre>
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1. POOptions for code sharing in Linq2SQL expressions
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