.NET Dictionary: get or create new

Question

I often find myself creating a Dictionary with a non-trivial value class (e.g. List), and then always writing the same code pattern when filling in data.

For example

Answer 1

Ok, different approach:

public static bool TryAddValue<TKey,TValue>(this System.Collections.Generic.IDictionary<TKey,List<TValue>> dictionary, TKey key, TValue value)
    {
        // Null check (useful or not, depending on your null checking approach)
        if (value == null)
            return false;

        List<TValue> tempValue = default(List<TValue>);

        try
        {
            if (!dictionary.TryGetValue(key, out tempValue))
            {
                dictionary.Add(key, tempValue = new List<TValue>());
            }
            else
            {
                // Double null check (useful or not, depending on your null checking approach)
                if (tempValue == null)
                {
                    dictionary[key] = (tempValue = new List<TValue>());
                }
            }

            tempValue.Add(value);
            return true;
        }
        catch
        {
            return false;
        }
    }

In this way you have to "try to add" your value to a generic List of (obviously generalizable to a generic collection), null checking and trying to get existing key/values in your Dictionary. Usage and example:

var x = new Dictionary<string,List<string>>();
x.TryAddValue("test", null); // return false due to null value. Doesn't add the key
x.TryAddValue("test", "ok"); // it works adding the key/value
x.TryAddValue("test", "ok again"); // it works adding the value to the existing list

Hope it helps.

Answer 2

I am missing the GetOrAdd for Dictionary, that does exist for ConcurrentDictionary.

ConcurrentDictionary<int,Guid> Conversion = new ConcurrentDictionary<int, Guid>();
List<int> before = new List<int> { 1, 2, 1, 3 };
List<Guid> after = before.Select(x => Conversion.GetOrAdd(x, Guid.NewGuid())).ToList();

This code will generate Guids for each number. Ending up converting both 1's in before to the same Guid.

In your case:

ConcurrentDictionary<int, List<String>> dict;
keyValues = dict.GetOrAdd(key, new List<String>());
keyValues.Add(aValueForKey);