Linq help with IEnumerable<KeyValuePair<string, string>> to a Dictionary<string, List<string>>

Question

Seems to be a common issue, my use case is hampered by the fact I will enumerate multiple occurrences of the same key for which possibly non unique string values may be returned.

Is there syntactically short linq that can produce this, I am accessing an api for which this pattern is rampant.

public static class Extensions
{
    public static Dictionary<string, List<string>> ToDictionary(this IEnumerable<KeyValuePair<string, string>> source)
    {
        Dictionary<string, List<string>> results = new Dictionary<string, List<string>>();

        foreach (KeyValuePair<string, string> kp in source)
        {
            if (results.ContainsKey(kp.Key))
            {
                results[kp.Key].Add(kp.Value);
            }
            else
            {
                results.Add(kp.Key, new List<string>() { kp.Value });
            }
        }

        return results;
    }
}

I know I can use extensions, but I am working towards some auto generated code and not relying hooks to insert boiler plate is a plus in this use case.

Thanks!

Answer 1

Hi Ritmo2k,

Actually I think it's already a very simple code snippet, if you insist on using linq to do the samething, it may be like this:

public static class Extensions
    {
        public static Dictionary<string, List<string>> ToDictionary(this IEnumerable<KeyValuePair<string, string>> source)
        {
            Dictionary<string, List<string>> results = new Dictionary<string, List<string>>();

            //use linq expression
            ((from kp in source
              where results.ContainsKey(kp.Key)
              select kp).ToList()).ForEach(kp => results[kp.Key].Add(kp.Value));

            ((from kp in source
              where !results.ContainsKey(kp.Key)
              select kp).ToList()).ForEach(kp => results.Add(kp.Key, new List<string>() { kp.Value }));

            //or use the extension methods
            //source.Where(kp => results.ContainsKey(kp.Key)).ToList().ForEach(kp => results[kp.Key].Add(kp.Value));
            //source.Where(kp => !results.ContainsKey(kp.Key)).ToList().ForEach(kp => results.Add(kp.Key, new List<string>() { kp.Value }));

            //foreach (KeyValuePair<string, string> kp in source)
            //{

            //    if (results.ContainsKey(kp.Key))
            //    {
            //        results[kp.Key].Add(kp.Value);
            //    }
            //    else
            //    {
            //        results.Add(kp.Key, new List<string>() { kp.Value });
            //    }
            //}

            return results;
        }
    }

As you can see, using the extension method is the easist way to do what you want. So I recommend that you go in this way.

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Answer 2

Here is my preference

            List<List<object>> input = new List<List<object>>();

            Dictionary<object, List<List<object>>> dict = input.AsEnumerable()
                .GroupBy(x => x[0], y => y)
                .ToDictionary(x => x.Key, y => y.ToList());

If you just have two column input then you can use this

            Dictionary<object, List<object>> dict = input.AsEnumerable()
                .GroupBy(x => x[0], y => y[1])
                .ToDictionary(x => x.Key, y => y.ToList());

Note : the 2nd field of the dictionary is a list when you have a key with multiple values.  You don't need to list when you have a dictionary with a unique value for each key.

Answer 3

 public static Dictionary<string, List<string>> ToDictionary(this IEnumerable<KeyValuePair<string, string>> source)
        {
            return source
                .GroupBy(
                    func => func.Key,
                    (key, list) => new { Key = key, Value = list.Select(f => f.Value).ToList() }
                    )
                .ToDictionary(
                    func => func.Key,
                    func => func.Value
                    );
        }

Answer 4

Thanks everyone, this was all very informative as I have not used Linq extensively and seeing examples helped.