public void IncrementCount(Dictionary<int, int> someDictionary, int id)
     {
         int currentCount;
         if (someDictionary.TryGetValue(id, out currentCount))
         {
             someDictionary[id] = currentCount + 1;
         }
         else
         {
             someDictionary[id] = 1;
         }
     }

someDictionary.AddOrUpdate(id, 1, (id, count) => count + 1);

public void IncrementCount(Dictionary<int, int> someDictionary, int id)
{
    if (!someDictionary.ContainsKey(id))
        someDictionary[id] = 0;

    someDictionary[id]++;
}

int currentCount;

// currentCount will be zero if the key id doesn't exist..
someDictionary.TryGetValue(id, out currentCount);

someDictionary[id] = currentCount + 1;

public static void Increment<T>(this Dictionary<T, int> dictionary, T key)
    {
        int count;
        dictionary.TryGetValue(key, out count);
        dictionary[key] = count + 1;
    }

var dictionary = new Dictionary<string, int>();
dictionary.Increment("hello");
dictionary.Increment("hello");
dictionary.Increment("world");

Assert.AreEqual(2, dictionary["hello"]);
Assert.AreEqual(1, dictionary["world"]);