Getting Proper Type Inference for a more complicated type in a Generic Method

Answer 1

You could define the Out dictionary as an out parameter. Type inference does not look the type of the variable you're assigning to, only the types of the parameters. That's the reason this doesn't compile.

Answer 2

You need to tell it what TValue and TKey are. Unless they are defined up in the signature of the method calling this code, they don't have any specific types. You need to give it something like:

Dictionary<string, int> MirroredDictionary = MirrorDictionary(StartDictionary);

Answer 3

Here's a 3.5 solution (you can also use it in 2.0 with VS2008 and LinqBridge)

IDictionary<TValue, TKey> MirrorDictionary<TKey, TValue>(IDictionary<TKey, TValue> dict)
{
    return dict.ToDictionary(kvp => kvp.Value, kvp => kvp.Key);
}

And a pure 2.0 solution

IDictionary<TValue, TKey> MirrorDictionary<TKey, TValue>(IDictionary<TKey, TValue> dict)
{
    Dictionary<TValue, TKey> newDict = new Dictionary<TValue, TKey>();
    foreach(KeyValuePair<TKey, TValue> kvp in dict)
    {
        newDict.Add(kvp.Value, kvp.Key);
    }
    return newDict;
}

Type inference should work fine with both solutions (as they have the same signature)

Answer 4

You can make life much easier for the compiler by telling it the key and value types thus:

public static Dictionary<TValue, TKey> MirrorDictionary<TKey, TValue>
    (Dictionary<TKey, TValue> source)
{
    Dictionary<TValue, TKey> destination = new Dictionary<TValue, TKey>();

    foreach (KeyValuePair<TKey, TValue> kvp in source)
    {
        destination.Add(kvp.Value, kvp.Key);
    }

    return destination;
}

I don't think you need reflection here at all.

Sample usage:

static void Main(string[] args)
{
    Dictionary<int, string> source = new Dictionary<int, string>();
    source.Add(3, "foo");
    source.Add(4, "bar");

    DumpDic(source);

    DumpDic(MirrorDictionary(source));

    Console.ReadLine();

}

where DumpDic is:

public static void DumpDic<TK, TV>(Dictionary<TK, TV> dic)
{
    foreach (KeyValuePair<TK, TV> keyValuePair in dic)
    {
        Console.WriteLine("{0} => {1}", keyValuePair.Key, keyValuePair.Value);
    }
}