Add ISource to handle IReadOnlyDictionary<TKey, TValue>

[dombrovsky]

Is your feature request related to a problem? Please describe.
If arguments for formatting are coming as IReadOnlyDictionary (not upcastable to IDictionary), there is no built in Source that can handle it.
,
Describe the solution you'd like
Should be a straightforward piece to add IReadOnlyDictionary support to DictionarySource, same way as it handles IDictionary.

Describe alternatives you've considered
Workaround is to convert IReadOnlyDictionary to IDictionary, but that would cause additional allocations.
There is always a possibility to implement custom Source on consumer side.

[axunonb]

You're saying that this tests fails for SmartFormat v3.2.1?

[Test]
public void ReadOnlyDictionaryTest()
{
    var dict = new Dictionary<string, object> { { "One", 1 }, { "Two", 2 }, { "Three", 3 }, };
    var roDict = new ReadOnlyDictionary<string, object>(dict);

    var formatter = Smart.CreateDefaultSmartFormat();
    var result = formatter.Format("{One}{Two}{Three}", roDict);

    Assert.That(result, Is.EqualTo("123"));
}

[dombrovsky]

No. I am saying it does not work if instance would not be possible to cast to IDictionary<,>. ReadOnlyDictionary<,> type implements both IReadOnlyDictionary<,> and IDictionary<,>.

So consider class like that:

internal sealed class MyReadOnlyDictionary<TKey, TValue> : IReadOnlyDictionary<TKey, TValue>
{
...
}

note it implements IReadOnlyDictionary and nothing else.

Changing your test so it has var roDict = new MyReadOnlyDictionary<string, object>(dict); would not work.

[axunonb]

public class CustomReadOnlyDictionary<TKey, TValue> : IReadOnlyDictionary<TKey, TValue>
{
    private readonly IDictionary<TKey, TValue> _dictionary;

    public CustomReadOnlyDictionary(IDictionary<TKey, TValue> dictionary)
    {
        _dictionary = dictionary;
    }

    public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
    {
        return _dictionary.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    public int Count => _dictionary.Count;

    public bool ContainsKey(TKey key)
    {
        return _dictionary.ContainsKey(key);
    }

    public bool TryGetValue(TKey key, out TValue value)
    {
        return _dictionary.TryGetValue(key, out value);
    }

    public TValue this[TKey key] => _dictionary[key];

    public IEnumerable<TKey> Keys => _dictionary.Keys;

    public IEnumerable<TValue> Values => _dictionary.Values;
}

var roDict = new CustomReadOnlyDictionary<string, object>(new Dictionary<string, object> { { "One", 1 }, { "Two", 2 }, { "Three", 3 }, });
var formatter = Smart.CreateDefaultSmartFormat();
var result = formatter.Format("{One}{Two}{Three}", roDict);

Assert.That(result, Is.EqualTo("123"));

This should succeed, but does not in any Smart.Format version because no ISource can handle IReadOnlyDictionary<TKey, TValue>