How to reuse LINQ Select Expression with arguments

Question

I write a LINQ query and for Select clause I created an Expression to reuse it.

My query looks like this

 DataContext.Single.Select(SearchSelector).ToList();

Where as Search Selector defined as

 private Expression<Func<Singles, SearchSingles>> SearchSelector = s =>
    new SearchSingles
    {
    };

The above works fine, but what if I want to use two input parameters? How would I invoke it?

 private Expression<Func<Singles,string, SearchSingles>> SearchSelector = (s,y) =>
    new SearchSingles
    {
    };

Where is the other parameter suppose to come from? Have you tried Select(s => SearchSelector(s,"Your String Here"))? — juharr
– juharr, Commented Nov 14, 2014 at 19:34
Parameter suppose to come come from method which needed to be use inside expression. The above show syntax error — Ammar Khan
– Ammar Khan, Commented Nov 14, 2014 at 19:38
Maybe private Func<string,Expression<Func<Singles,SearchSingles>>> SearchSelector = y => s => new SearchSingles{}; and Select(SearchSelector("Your String Here")) — juharr
– juharr, Commented Nov 14, 2014 at 19:49
No luck, seems like I can't invoke like a normal function, it says Delegate, method or event expected — Ammar Khan
– Ammar Khan, Commented Nov 14, 2014 at 19:53
Well I don't work with EF, and I'm out of ideas, I've up voted you so maybe someone else can help you out. — juharr
– juharr, Commented Nov 14, 2014 at 19:58

Servy · Accepted Answer · 2014-11-14 21:17:10Z

5

Rather than having a field that stores the expression, have a method that creates the expression that you need given a particular string:

private static Expression<Func<Singles, SearchSingles>> CreateSearchSelector(
    string foo)
{
    return s =>
        new SearchSingles
        {
            Foo = foo,
        };
}

You can then use this method like so:

DataContext.Single.Select(CreateSearchSelector("Foo")).ToList();

answered Nov 14, 2014 at 21:17

Servy

204k27 gold badges347 silver badges470 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Ammar Khan Over a year ago

However, is it not possible with the way I was trying to do?

Peter Duniho Over a year ago

What are you trying to do?, specifically? Servy's answer seems to address your question, but maybe your question isn't clear enough. If you want to do it without a named method, you can apply his approach but without the named method. E.g.

DataContext.Single.Select(((Func<string, Expression<Func<Singles, SearchSingles>>>)(y => s => new SearchSingles { Foo = y }))("Foo")).ToList();

It's just like Servy's answer, but with an anonymous method instead, so I'm not really sure if that's the difference you're trying to solve.

Servy Over a year ago

@AmmarKhan You can create an expression that actually takes to parameters, sure. You created one in your question just fine. If you're passing it to something that expects an expression with that signature, you don't need to do anything.

thewyzard Over a year ago

the 'trick' is that the signature that Select is expecting is fixed. Whatever you do must resolve to a delegate of Func<Singles, SearchSingles>. So however you go about that...

thewyzard · Accepted Answer · 2014-11-14 21:46:34Z

0

what about leaving the signature alone and passing additional parameters as captured values? It might have limited use as an initialized member variable, like this, but if you assign from within some worker function, rather than initialize it during class construction you'd have more power.

private Func<Singles, SearchSingles> SearchSelector = s =>
    new SearchSingles
    {
         someVal = capturedVariable,
         someOther = s.nonCapturedVar
    };

that would work if capturedVariable were a static member

or

private Func<Singles, SearchSingles> SearchSelector = null;
private void WorkerFunction(string capturedVariable, bool capAgain, bool yetAgain)
{
  SearchSelector = s => {
    bool sample = capAgain;
    if (capturedTestVar)
        sample = yetAgain;
    return new SearchSingles
    {
         someVal = capturedVariable,
         someOther = s.nonCapturedVar,
         availability = sample
    };
  };
};

you could have all sorts of fun

*EDIT* references to Expression removed and clarifications

edited Nov 14, 2014 at 21:46

answered Nov 14, 2014 at 21:03

thewyzard

3121 silver badge6 bronze badges

4 Comments

Servy Over a year ago

statement lambdas cannot be used with expressions. As for your first option, what variable to you expect to close over from that context?

thewyzard Over a year ago

yea I glazed over that 'Expression' as I copy/pasted and was just looking into whether it's necessary. In the first, capturedVariable could be a parameter to a worker member function like

private void DoIt(string capturedVariable)      { SearchSelector = s=> new SearchSingles         {          someVal = capturedVariable,          someOther = s.nonCapturedVar         };     }

Servy Over a year ago

It could be that, but that's not what you showed in your answer. Had you actually shown that, then you'd have a correct answer. That said you would almost certainly want to have such a method return the expression using that string, not have it assign it to a field.

thewyzard Over a year ago

I didn't want to limit imagination. I'll get an edit done after I test the need of the "Expression"

Collectives™ on Stack Overflow

How to reuse LINQ Select Expression with arguments

2 Answers 2

4 Comments

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related