Error When Calling Intersect With Expression Tree API

Question

I am learning Expression Trees so bear with me on this one.

The idea is to call intersect an to receive the num 3.

Guess I missing something. Can you tell me what am I doing wrong here please?

    static void Main(string[] args)
    {
        List<int> arr1 = new List<int> { 1, 2, 3, 4, 5 };
        List<int> arr2 = new List<int> { 6, 7, 8, 9, 3 };

        var ex =
            Expression.Lambda<Func<List<int>>>(
                Expression.Call(
                    Expression.Constant(arr1), typeof(List<int>).GetMethod("Intersect"), Expression.Constant(arr2)));
     ....

Why is this throwing that value cannot be null?

Error text is value cannot be null. I wrote it at the bottom. — dev hedgehog, Nov 08 '13 at 08:30
possible duplicate of [Why does it return null when I try to invoke generic methods?](http://stackoverflow.com/questions/8668772/why-does-it-return-null-when-i-try-to-invoke-generic-methods) — nawfal, Jan 18 '14 at 05:49

Grundy · Accepted Answer · 2013-11-08T09:24:19.007

3

Intersect is extension method, so typeof(List<int>).GetMethod("Intersect") return null
for solve try get them from Enumerable

UPDATE

for get Intersect try this

var intersectMethod = typeof(Enumerable).GetMethods().First(a => a.Name == "Intersect" && a.GetParameters().Count() == 2).MakeGenericMethod(typeof(int));

better

var intersectMethod = typeof(Enumerable).GetMember("Intersect").First().MakeGenericMethod(typeof(int));

edited Nov 08 '13 at 09:24

answered Nov 08 '13 at 08:37

Grundy

13,356
3
35
55

Good suggestion but now I get Unhandled Exception: System.Reflection.AmbiguousMatchException: Ambiguous match found. Any ideas??? – dev hedgehog Nov 08 '13 at 08:38
yes, find method that you want, `public static IEnumerable Intersect(this IEnumerable first, IEnumerable second);` or `public static IEnumerable Intersect(this IEnumerable first, IEnumerable second, IEqualityComparer comparer);` and get concrete – Grundy Nov 08 '13 at 08:40
The first one shall match to what I wrote is my expression why is it still wrong? – dev hedgehog Nov 08 '13 at 08:41
it wrong because `GetMethod` in you case get only by name, and names same – Grundy Nov 08 '13 at 08:43
I dont get it. Could you edit your question with the correct code please? – dev hedgehog Nov 08 '13 at 08:44
And why is it not working the way I wrote it down? Why do I need First() and MakeGenericMethod? – dev hedgehog Nov 08 '13 at 09:21
You need `First()` to specify method what you want, because `Enumerable` class has several methods with same name, you need `MakeGenericMethod` to specify generic type what you want use – Grundy Nov 08 '13 at 09:29
I get that but is it not working with GetMethod("Intersect", new Type[] { type })? – dev hedgehog Nov 08 '13 at 09:30
no, for more see [GetMethod for generic method](http://stackoverflow.com/questions/4035719/getmethod-for-generic-method) – Grundy Nov 08 '13 at 10:17

Error When Calling Intersect With Expression Tree API

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