C# Linq, Searching for same items in two lists

Question

we have the following setup: We have a array of objects with a string in it (xml-ish but not normalized) and we have a list/array of strings with id.

We need to find out if a string from that list with id's is also pressent in one of the objects.

Here we have a setup that we have tried:

public class Wrapper
{
    public string MyProperty { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        List<Wrapper> wrappers = new List<Wrapper>() 
        {
            new Wrapper{ MyProperty = "<flkds,dlsklkdlsqkdkqslkdlqk><id>3</id><sqjldkjlfdskjlkfjsdklfj>"}, 
            new Wrapper{ MyProperty = "<flkds,dlsklkdlsqkdkqslkdlqk><id>2</id><sqjldkjlfdskjlkfjsdklfj>"}
        };
        string[] ids = { "<id>0</id>", "<id>1</id>", "<id>2</id>" };

        var props = wrappers.Select(w => w.MyProperty);
        var intersect = props.Intersect(ids, new MyEquilityTester());
        Debugger.Break();
    }
}

class MyEquilityTester: IEqualityComparer<string>
{
    public bool Equals(string x, string y)
    {
        return x.Contains(y);
    }

     public int GetHashCode(string obj)
     {
         return obj.GetHashCode();
     }
 }

Edit:

What we expect is when we do a .Any() on intersect that is says true because wrappers has a object with a prop that contains <id>2</id>, intersect is null.

If we are using the wrong method please say. It should work as fast as posible. A simple true when found will do!

Answer 1

For your case, you could write your IEqualitycomparer like this:

class MyEquilityTester: IEqualityComparer<string>
{
    public bool Equals(string x, string y)
    {
        return x.Contains(y) || y.Contains(x);
    }

    public int GetHashCode(string obj)
    {
        return 0;
    }
}

and it will find

<flkds,dlsklkdlsqkdkqslkdlqk><id>2</id><sqjldkjlfdskjlkfjsdklfj>

This works because GetHashCode always return 0, and the x.Contains(y) || y.Contains(x) check.

---

Another not-so-hacky solution is to use a Where in combination with Any

IEnumerable<String> intersect = props.Where(p => ids.Any (i => p.Contains(i)));

or replace the Where with another Any if you don't care about the actual items and you only want a true or false.

bool intersect = props.Any(p => ids.Any (i => p.Contains(i)));