Making a list distinct in C#

Question

In C#, I have an object type 'A' that contains a list of key value pairs.

The key value pairs is a category string and a value string.

To instantiate object type A, I would have to do the following:

List<KeyValuePair> keyValuePairs = new List<KeyValuePair>();
keyValuePairs.Add(new KeyValuePair<"Country", "U.S.A">());
keyValuePairs.Add(new KeyValuePair<"Name", "Mo">());
keyValuePairs.Add(new KeyValuePair<"Age", "33">());

A a = new A(keyValuePairs);

Eventually, I will have a List of A object types and I want to manipulate the list so that i only get unique values and I base it only on the country name. Therefore, I want the list to be reduced to only have ONE "Country", "U.S.A", even if it appears more than once.

I was looking into the linq Distinct, but it does not do what I want because it I can't define any parameters and because it doesn't seem to be able to catch two equivalent objects of type A. I know that I can override the "Equals" method, but it still doesn't solve the my problem, which is to render the list distinct based on ONE of the key value pairs.

Answer 1

To expand upon Karl Anderson's suggestion of using morelinq, if you're unable to (or don't want to) link to another dll for your project, I implemented this myself awhile ago:

public static IEnumerable<T> DistinctBy<T, U>(this IEnumerable<T> source, Func<T, U>selector)
{
    var contained = new Dictionary<U, bool>();
    foreach (var elem in source)
    {
        U selected = selector(elem);
        bool has;
        if (!contained.TryGetValue(selected, out has))
        {
            contained[selected] = true;
            yield return elem;
        }
    }
}

Used as follows:

collection.DistinctBy(elem => elem.Property);

In versions of .NET that support it, you can use a HashSet<T> instead of a Dictionary<T, Bool>, since we don't really care what the value is so much as that it has already been hashed.

Answer 2

Check out the DistinctBy syntax in the morelinq project.

A a = new A(keyValuePairs);

a = a.DistinctBy(k => new { k.Key, k.Value }).ToList();