Question

I need to pass in a “where” lambda expression that’ll be used in a LINQ query inside a method. The problem is, I don’t know what the where value will be compared against until I get into the method.

Now to explain further and clarify some of what I said above I’ll come up with a bit of a contrived example.

Imagine I have a List<Products> and I need to narrow that list down to a single record using a productId property of the Products object. Normally I would do this:

var product = productList.Where(p=>p.productId == 123).FirstOrDefault();

Now take it a step further – I need to put the above logic into a method that isn’t limited to a List<Products> but is instead a List<T> so ideally, I’d be calling it like this (and I know the below won’t work, it’s simply here to show what I am trying to achieve):

myMethod(productList, p => p.productId == X)

With the caveat being that X isn’t known until I’m inside the method.

Finally, for what it’s worth, I need to point out that my collection of data is an OData DataServiceQuery.

So, to re-summarize my question: I need to know how to construct a lambda “where” expression that I can pass into a method and how to use it against a collection of objects in a LINQ query.

Answer 1

myMethod(productList, p => p.productId == X) – you can emulate with this trick

    static void myMethod<T>(List<T> list, Func<T,bool> predicate, ref int x)
    {
        x = 5;
        var v = list.Where(predicate);
        foreach (var i in v)
            Console.Write(i);
        Console.ReadLine();
    }

    static void Main(string[] args)
    {
        List<int> x = new List<int> { 1, 2, 3, 4, 5 };
        int z = 0;
        myMethod(x, p => p == z, ref z);
    }

but not sure if it solves your problem in whole