How to parameterize a selector with a function in EF query?

Question

I have a projection function that I pass to IQueryable<>.Select() method:

private static Expression<Func<VendorPrice, PriceItem>> GetPriceSelector(){
    return e => new PriceItem {
        Id = e.Id,
        Price = Math.Round(e.Price, 4)
    };
}

Everything works just fine but I want to parameterize it like that:

private static Expression<Func<VendorPrice, PriceItem>> GetPriceSelector(Func<VendorPrice, decimal> formula){
    return e => new PriceItem {
        Id = e.Id,
        Price = formula(e)
    };
}

so that I can call it like

prices.Select(GetPriceSelector(e => Math.Round(e.Price, 4)))

Unfortunately, EF complains about it:

The LINQ expression node type 'Invoke' is not supported in LINQ to Entities

How to rewrite the code to make EF happy?

Answer 1

First, the GetPriceSelector method needs to take in an expression, not a function. The difference is that an expression is code as data so it can be translated to SQL, while a function is compiled code so it cannot be translated to SQL.

Next, you need a way to merge the two expressions. Doing this manually is hard. Fortunately, there is a library called LINQKit that can do that. Here is how you can solve your problem with LINQKit:

private static Expression<Func<VendorPrice, PriceItem>> GetPriceSelector(
    Expression<Func<VendorPrice, decimal>> formula)
{
    Expression<Func<VendorPrice, PriceItem>> expression = e => new PriceItem
    {
        Id = e.Id,
        Price = formula.Invoke(e) //use the forumla expression here
    };

    return expression.Expand(); //This causes formula.Invoke(e) to be converted 
                                //to something like Math.Round(e.Price, 4)
}