获得LIST唯一性的通用方法<T>

Question

I am trying to compare (values of the properties) a instance of type in a List and eliminate duplicates. According to MSDN GetHashCode() is one of the way to compare two objects.

A hash code is intended for efficient insertion and lookup in collections that are based on a hash table. A hash code is not a permanent value

Considering that, I started writing my extension method as bellow

 public static class Linq 
    {
  public static IEnumerable<T> DistinctObjects<T>(this IEnumerable<T> source)
        {
            List<T> newList = new List<T>();
            foreach (var item in source)
            {
                if(newList.All(x => x.GetHashCode() != item.GetHashCode()))
                    newList.Add(item);
            }
            return newList;
        }
}

This condition always gives me false though the data of the object is same.

newList.All(x => x.GetHashCode() != item.GetHashCode())

Finally I would like to use it like

MyDuplicateList.DistinctObjects().ToList();

If comparing all fields of the object is too much, I am okay to use it like,

 MyDuplicateList.DistinctObjects(x=>x.Id, x.Name).ToList();

Here I am telling compare only these two fields of those objects.

Answer 1

After reading your comments I would propose this solution:

   public static IEnumerable<TSource> DistinctBy<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector)
    {
        HashSet<TResult> set = new HashSet<TResult>();

        foreach(var item in source)
        {
            var selectedValue = selector(item);

            if (set.Add(selectedValue))
                yield return item;
        }
    }

Then you can use it like this:

var distinctedList = myList.DistinctBy(x => x.A);

or for multiple properties like that:

var distinctedList = myList.DistinctBy(x => new {x.A,x.B});

The advantage of this solution is you can exactly specify what properties should be used in distinction and you don't have to override Equals and GetHashCode for every object. You need to make sure that your properties can be compared.

Answer 2

You shouldn't need to create your own custom, generic method for this. Instead, provide a custom EqualityComparar for your data type:

var myDuplicates = myList.Distinct(new MyComparer());

Where you define a custom Comparer like this:

public class MyComparer : IEqualityComparer<Mine>
{
    public bool Equals(Mine x, Mine y)
    {
        if (x == null && y == null) return true;            
        if (x == null || y == null) return false;
        return x.Name == y.Name && x.Id == y.Id;
    }

    public int GetHashCode(Mine obj)
    {
        return obj.Name.GetHashCode() ^ obj.Id.GetHashCode();
    }
}

Edit: I initially had incorrect code here, this should do what you want without you having to override an Equals operator