Using Where() with a dynamic Func fails, but works with a hard coded Where() clause

See the two Linq (to SharePoint) code samples below.

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The only differences are the highlighted sections of code. The first statement works as expected with a hard-coded where clause, but the 2nd set of code throws the error “Value does not fall in within the expected range” when I try to do a count on the items. What am I missing?

Works

relatedListItems = dc.GetList<GeneralPage>("Pages")
    .Where(x => x.RelatedPracticesTitle.Any(y=>y=="Foo"))                           

if (relatedListItems.Count() == 0)
                    {…}

Fails - “Value does not fall within the expected range”

Func<GeneralPage, bool> f = x => x.RelatedPracticesTitle.Any(y => y == "Foo");
relatedListItems = dc.GetList<GeneralPage>("Pages")                         
    .Where(f)                                  

if (relatedListItems.Count() == 0)
                    {…}

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If it's LINQ to Sharepoint, presumably that means it should be using expression trees, not delegates. Try:

Expression<Func<GeneralPage, bool>> f = 
    x => x.RelatedPracticesTitle.Any(y => y == "Foo");
relatedListItems = dc.GetList<GeneralPage>("Pages").Where(f);

By the way, it's generally a better idea to use Any() rather than Count() if you just want to find out if there are any results - that way it can return as soon as it's found the first one. (It also expresses what you're interested in more clearly, IMO.)

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In the first case, you're using the Expression<Func<GeneralPage, bool>> overload and pass an expression which I assume LINQ to SharePoint will try to convert to CAML and execute.

In the second case, you're passing the plain Func<GeneralPage, bool> so LINQ to SharePoint can't figure out how to compose a query (it only sees the delegate, not the expression).