StackOverflow2013

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Id
2231450
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0
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0
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6
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CreationDate
2010-02-09T18:26:40.287
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0
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2010-02-09T18:26:40.287
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0
OwnerUserId
88656
ParentId
2231302
PostTypeId
2
Score
30
ViewCount
0
LastEditorDisplayName
text
Body
You're attempting to build an expression tree that represents this: <pre><code>c => true && c.ClientFName == searchForClientFName </code></pre> You are actually building an expression tree that represents this: <pre><code>c => c=> true && c => c.ClientFName == searchForClientFName </code></pre> which makes no sense at all. Now, you might naively think that this will work: <pre><code>public static Expression<TDelegate> AndAlso<TDelegate>(this Expression<TDelegate> left, Expression<TDelegate> right) { // NOTICE: Combining BODIES: return Expression.Lambda<TDelegate>(Expression.AndAlso(left.Body, right.Body), left.Parameters); } </code></pre> That would produce in your case something representing <pre><code>c => true && c.ClientFName == searchForClientFName </code></pre> Which looks right. But in fact this is fragile. Suppose you had <pre><code>... d => d.City == "London" ... ... c => c.ClientName == "Fred Smith" ... </code></pre> and you used this method to combine them. You'd get an object representing <pre><code>c => d.City == "London" && c.ClientName == "Fred Smith" </code></pre> What the heck is d doing in there? Furthermore, parameters are matched by object identity, not by parameter name. If you do this <pre><code>... c => c.City == "London" ... ... c => c.ClientName == "Fred Smith" ... </code></pre> and combine them into <pre><code>c => c.City == "London" && c.ClientName == "Fred Smith" </code></pre> you're in the same boat; the "c" in "c.City" is a different c than the other two. What you actually need to do is make a third parameter object, substitute it in the bodies of both lambdas for every occurence of their parameters, and then build up a new lambda expression tree from the resulting substituted bodies. You can build a substitution engine by writing a visitor that passes over the expression tree body, rewriting it as it goes.
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1. POAppend to an expression
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1. USEric Lippert
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