LINQ's Distinct() on a particular property

Question

I am playing with LINQ to learn about it, but I can\'t figure out how to use Distinct when I do not have a simple list (a simple list of integers is pretty easy

Answer 1

When we faced such a task in our project we defined a small API to compose comparators.

So, the use case was like this:

var wordComparer = KeyEqualityComparer.Null().
    ThenBy(item => item.Text).
    ThenBy(item => item.LangID);
...
source.Select(...).Distinct(wordComparer);

And API itself looks like this:

using System;
using System.Collections;
using System.Collections.Generic;

public static class KeyEqualityComparer
{
    public static IEqualityComparer Null()
    {
        return null;
    }

    public static IEqualityComparer EqualityComparerBy(
        this IEnumerable source,
        Func keyFunc)
    {
        return new KeyEqualityComparer(keyFunc);
    }

    public static KeyEqualityComparer ThenBy(
        this IEqualityComparer equalityComparer,
        Func keyFunc)
    {
        return new KeyEqualityComparer(keyFunc, equalityComparer);
    }
}

public struct KeyEqualityComparer: IEqualityComparer
{
    public KeyEqualityComparer(
        Func keyFunc,
        IEqualityComparer equalityComparer = null)
    {
        KeyFunc = keyFunc;
        EqualityComparer = equalityComparer;
    }

    public bool Equals(T x, T y)
    {
        return ((EqualityComparer == null) || EqualityComparer.Equals(x, y)) &&
                EqualityComparer.Default.Equals(KeyFunc(x), KeyFunc(y));
    }

    public int GetHashCode(T obj)
    {
        var hash = EqualityComparer.Default.GetHashCode(KeyFunc(obj));

        if (EqualityComparer != null)
        {
            var hash2 = EqualityComparer.GetHashCode(obj);

            hash ^= (hash2 << 5) + hash2;
        }

        return hash;
    }

    public readonly Func KeyFunc;
    public readonly IEqualityComparer EqualityComparer;
}

More details is on our site: IEqualityComparer in LINQ.