Lambda expression not behaving as expected

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.

• user541686: listTwo.Select(e => e.Name).Equals(p)? I don't think that's doing what you intended...
• TYY: I believe you are trying to do a join. So instead of using the where clause, use a the regualr linq join (it is clearer as to what you are trying to achieve).

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();

• DaveShaw: You want to be careful using this. listTwo will be iterated for each entry in listOne.

Answer 2

Try this:

string[] listOne = new string[] { "test1", "test2", "test3" };
 MyClass[] listTwo = new MyClass[] { new MyClass("test1") };
 string[] newVals = listOne
 .Where(p => listTwo.Select(e => e.Name).Contains(p))
 .ToArray();

listTwo.Select(e => e.Name) is a IEnumerable<string>

Answer 3

You probably want to perform a Join on the 2 collections.

var q = 
 listOne
 .Join(
 listTwo,
 l2 => l2,
 l1 => l1.Name,
 (l2, l1) => new { l2, l1, });

You can change the selector (the last parameter) to suit your needs, if it's just values from listOne for example then have (l2, l1) => l1.

The other solutions will work, but maybe not as you would expect.

Using Linq-Objects Contains within a where clause will cause the entire of listTwo to be iterated for each entry in listOne.

Answer 4

How about something like this:

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

or to be more strict use == instead of Contains.

But if you want to obtain the items that are common between the 2 why not just call .Intersect()??

• Servy: Intersect won't work because the sequences are not of the same type. You need to use Join, which is why this is the best answer.

Answer 5

You are trying to perform a join, technically you are better off simplifying your linq statement to use a join. An example is included below.

 static void Main(string[] args)
 {
 string[] listOne = new [] { "test1", "test2", "test3" };
 MyClass[] listTwo = new [] { new MyClass("test1") };
 string[] newVals = (from str1 in listOne
 join str2 in listTwo.Select(e => e.Name) on str1 equals str2
 select str1).ToArray();
 foreach (var newVal in newVals)
 {
 Console.WriteLine(newVal);
 }
 //string[] newVals2 = listOne.Intersect(listTwo.Select(t => t.Name)).ToArray();
 }
 class MyClass
 {
 public MyClass(string name)
 {
 Name = name;
 }
 public string Name { get; set; }
 }