Question

I’m trying to construct a lambda expression that will match elements of one array with a second. Below is a simplified version of this query:

class Program
{
    static void Main(string[] args)
    {
        string[] listOne = new string[] { "test1", "test2", "test3" };
        MyClass[] listTwo = new MyClass[] { new MyClass("test1") };

        string[] newVals = listOne.Where(p => listTwo.Select(e => e.Name).Equals(p)).ToArray();

        //string[] newVals2 = listOne.Intersect(listTwo.Select(t => t.Name)).ToArray();
    }

    class MyClass
    {
        public MyClass(string name)
        {
            Name = name;
        }
        public string Name {get; set;}
    }
}

I would expect newVals to return an array of 1 value, but it’s empty. I realise that uncommenting myVals2 would achieve the same result, but the lists of classes differ more fundamentally than shown.

Answer 1

You are using Equals but you should use Contains. You are checking wheter IEnumerable<> is equal to p, but you want to check if IEnumerable<> contains p, so replace:

string[] newVals = listOne.
                   Where(p => listTwo.Select(e => e.Name).Equals(p)).
                   ToArray();

with

string[] newVals = listOne.
                   Where(p => listTwo.Select(e => e.Name).Contains(p)).
                   ToArray();