C# cast Dictionary to Dictionary (Involving Reflection)

Question

Is it possible to cast a Dictionary to a consistent intermediate generic type? So I would be able to cast <

Answer 1

I had the same situation and created a WrapperDictionary class and a cast extension method. Found it helpful if you also want to make operations with it like remove a item.

Example:

        const string TestKey1 = "Key";
        const string TestValue1 = "Value";
        var dict = new Dictionary<string, string>();
        dict.Add(TestKey1, TestValue1);

        var wrapper = dict.CastDictionary<string, string, string, object>();

        wrapper.Remove(TestKey1);

Code:

public class DictionaryWrapper<TKeyTarget, TValueTarget, TKeySource, TValueSource> : IDictionary<TKeyTarget, TValueTarget>
{
    #region types

    private class EnumeratorWrapper : IEnumerator<KeyValuePair<TKeyTarget, TValueTarget>>
    {
        private readonly IEnumerator<KeyValuePair<TKeySource, TValueSource>> _enumerator;
        private readonly DictionaryWrapper<TKeyTarget, TValueTarget, TKeySource, TValueSource> _dictionaryWrapper;

        public EnumeratorWrapper(IEnumerator<KeyValuePair<TKeySource, TValueSource>> enumerator, DictionaryWrapper<TKeyTarget, TValueTarget, TKeySource, TValueSource> dictionaryWrapper)
        {
            _enumerator = enumerator;
            _dictionaryWrapper = dictionaryWrapper;
        }

        public void Dispose()
        {
            _enumerator.Dispose();
        }

        public bool MoveNext()
        {
            return _enumerator.MoveNext();
        }

        public void Reset()
        {
            _enumerator.Reset();
        }

        public KeyValuePair<TKeyTarget, TValueTarget> Current => _dictionaryWrapper._kvpSourceToTargetFunc(_enumerator.Current);

        object IEnumerator.Current => Current;
    }

    #endregion

    #region fields

    private readonly IDictionary<TKeySource, TValueSource> _dictionary;
    private readonly Func<TKeySource, TKeyTarget> _keySourceToTargetFunc;
    private readonly Func<TKeyTarget, TKeySource> _keyTargetToSourceFunc;
    private readonly Func<TValueSource, TValueTarget> _valueSourceToTargetFunc;
    private readonly Func<TValueTarget, TValueSource> _valueTargetToSourceFunc;
    private readonly Func<KeyValuePair<TKeySource, TValueSource>, KeyValuePair<TKeyTarget, TValueTarget>> _kvpSourceToTargetFunc;
    private readonly Func<KeyValuePair<TKeyTarget, TValueTarget>, KeyValuePair<TKeySource, TValueSource>> _kvpTargetToSourceFunc;

    #endregion

    #region Construction

    public DictionaryWrapper(
        IDictionary<TKeySource, TValueSource> dict, 
        Func<TKeySource, TKeyTarget> keySourceToTargetFunc = null,
        Func<TKeyTarget, TKeySource> keyTargetToSourceFunc = null, 
        Func<TValueSource, TValueTarget> valueSourceToTargetFunc = null, 
        Func<TValueTarget, TValueSource> valueTargetToSourceFunc = null)
    {
        _dictionary = dict;
        _keySourceToTargetFunc = keySourceToTargetFunc ?? (i => (TKeyTarget) (object) i);
        _keyTargetToSourceFunc = keyTargetToSourceFunc ?? (i => (TKeySource) (object) i);
        _valueSourceToTargetFunc = valueSourceToTargetFunc ?? (i => (TValueTarget) (object) i);
        _valueTargetToSourceFunc = valueTargetToSourceFunc ?? (i => (TValueSource) (object) i);
        _kvpSourceToTargetFunc = 
            kvp => new KeyValuePair<TKeyTarget, TValueTarget>(_keySourceToTargetFunc(kvp.Key), _valueSourceToTargetFunc(kvp.Value));
        _kvpTargetToSourceFunc = 
            kvp => new KeyValuePair<TKeySource, TValueSource>(_keyTargetToSourceFunc(kvp.Key), _valueTargetToSourceFunc(kvp.Value));
    }

    #endregion

    #region Interface Members

    public IEnumerator<KeyValuePair<TKeyTarget, TValueTarget>> GetEnumerator()
    {
        return new EnumeratorWrapper(_dictionary.GetEnumerator(), this);
    }

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

    public void Add(KeyValuePair<TKeyTarget, TValueTarget> item)
    {
        _dictionary.Add(_kvpTargetToSourceFunc(item));
    }

    public void Clear()
    {
        _dictionary.Clear();
    }

    public bool Contains(KeyValuePair<TKeyTarget, TValueTarget> item)
    {
        return _dictionary.Contains(_kvpTargetToSourceFunc(item));
    }

    public void CopyTo(KeyValuePair<TKeyTarget, TValueTarget>[] array, int arrayIndex)
    {
        throw new NotImplementedException();
    }

    public bool Remove(KeyValuePair<TKeyTarget, TValueTarget> item)
    {
        return _dictionary.Remove(_kvpTargetToSourceFunc(item));
    }

    public int Count => _dictionary.Count;
    public bool IsReadOnly => _dictionary.IsReadOnly;
    public bool ContainsKey(TKeyTarget key)
    {
        return _dictionary.ContainsKey(_keyTargetToSourceFunc(key));
    }

    public void Add(TKeyTarget key, TValueTarget value)
    {
        _dictionary.Add(_keyTargetToSourceFunc(key), _valueTargetToSourceFunc(value));
    }

    public bool Remove(TKeyTarget key)
    {
        return _dictionary.Remove(_keyTargetToSourceFunc(key));
    }

    public bool TryGetValue(TKeyTarget key, out TValueTarget value)
    {
        var success = _dictionary.TryGetValue(_keyTargetToSourceFunc(key), out TValueSource result);
        value = success ? _valueSourceToTargetFunc(result) : default;
        return success;
    }

    public TValueTarget this[TKeyTarget key]
    {
        get => _valueSourceToTargetFunc(_dictionary[_keyTargetToSourceFunc(key)]);
        set => _dictionary[_keyTargetToSourceFunc(key)] = _valueTargetToSourceFunc(value);
    }

    public ICollection<TKeyTarget> Keys => _dictionary.Keys.Select(k => _keySourceToTargetFunc(k)).ToList();
    public ICollection<TValueTarget> Values => _dictionary.Values.Select(v => _valueSourceToTargetFunc(v)).ToList();

    #endregion
}

public static class DictionaryWrapperExtensions
{
    public static IDictionary<TKeyCasted, TValueCasted> CastDictionary<TKey, TValue, TKeyCasted, TValueCasted>(this IDictionary<TKey, TValue> dictionary)
    {
        return new DictionaryWrapper<TKeyCasted,TValueCasted,TKey,TValue>(dictionary);
    }
}

Answer 2

Explanation of What's Wrong: Variance

As stated in this answer:

it's not true that a Dictionary<string, bool> is a Dictionary<string,object>

Let me explain why. It all has to do with covariance and contravariance in C#. In summary, neither the key type K nor the value type V in Dictionary<K, V> is either purely an input parameter or an output parameter across the entire dictionary. So neither generic type can be cast to a weaker or a stronger type.

If you cast to a weaker type, then you break inputs. For example the Add function which expects types K, V or stronger cannot accept supertypes of either of K, V. If you cast to a stronger type, you break outputs. For example, the indexer property returns a type V. How can we cast this safely to a sub-type of V, knowing only that the original type is V? We can't.

A Type-Safe Partial Solution Using Variant Generics

There is a way to allow strongly-typed casting, but only on partial fragments of the original dictionary interface which are consistent in terms of covariance/contravariance of generic parameters. This solution uses a wrapper type which implements a bunch of partial interfaces. Each partial interface has a specific type of combination of co/contra-variance of the key/value type. Then we combine all these partial interfaces into a master interface and implement that through a backing object which is a regular dictionary. Here goes. First, the interfaces:

public interface IDictionaryBase
{
    int Count { get; }
    bool IsReadOnly { get; }
    void Clear();
}

public interface IInKeyInValueSemiDictionary<in K, in V> : IDictionaryBase, IInKeySemiDictionary<K>
{
    V this[K key] { set; }
    void Add(K key, V value);
}

public interface IInKeyOutValueSemiDictionary<in K, out V> : IDictionaryBase, IInKeySemiDictionary<K>, IOutValueSemiDictionary<V>
{
    V this[K key] { get; }
    ISuccessTuple<V> TryGetValue(K key);
}

public interface ISuccessTuple<out V>
{
    bool WasSuccessful { get; }
    V Value { get; }
}

public class SuccessTupleImpl<V> : ISuccessTuple<V>
{
    public bool WasSuccessful { get; }
    public V Value {get;}

    public SuccessTupleImpl(bool wasSuccessful, V value)
    {
        WasSuccessful = wasSuccessful;
        Value = value;
    }
}

public interface IInKeySemiDictionary<in K> : IDictionaryBase
{
    bool ContainsKey(K key);
    bool Remove(K key);
}

public interface IOutKeySemiDictionary<out K> : IDictionaryBase
{
    IEnumerable<K> Keys { get; }
}

public interface IOutValueSemiDictionary<out V> : IDictionaryBase
{
    IEnumerable<V> Values { get; }
}

Note: we don't have to cover all combinations of in/out here for the generic parameters, and also note that some of the interfaces only need a single generic parameter.The reason is some combinations of covariance/contravariance don't have any associated methods, so there's not need for corresponding types. And note that I'm leaving out the KeyValuePair<K, V> type- you'd need to do a similar trick on this if you wanted to include it, and it doesn't seem worth it at this stage.

So, next, the union of all those interfaces:

public interface IVariantDictionary<K, V> : IInKeyInValueSemiDictionary<K, V>, IInKeyOutValueSemiDictionary<K, V>,
    IOutKeySemiDictionary<K>, IDictionary<K, V>
{ }

And then, the wrapper class:

class VariantDictionaryImpl<K, V> : IVariantDictionary<K, V>
{
    private readonly IDictionary<K, V> _backingDictionary;

    public VariantDictionaryImpl(IDictionary<K, V> backingDictionary)
    {
        _backingDictionary = backingDictionary ?? throw new ArgumentNullException(nameof(backingDictionary));
    }

    public V this[K key] { set => _backingDictionary[key] = value; }

    V IInKeyOutValueSemiDictionary<K, V>.this[K key] => _backingDictionary[key];

    V IDictionary<K, V>.this[K key] { get => _backingDictionary[key]; set => _backingDictionary[key] = value; }

    public int Count => _backingDictionary.Count;

    public bool IsReadOnly => _backingDictionary.IsReadOnly;

    public IEnumerable<K> Keys => _backingDictionary.Keys;

    public ICollection<V> Values => _backingDictionary.Values;

    ICollection<K> IDictionary<K, V>.Keys => _backingDictionary.Keys;

    IEnumerable<V> IOutValueSemiDictionary<V>.Values => Values;

    public void Add(K key, V value)
    {
        _backingDictionary.Add(key, value);
    }

    public void Add(KeyValuePair<K, V> item)
    {
        _backingDictionary.Add(item);
    }

    public void Clear()
    {
        _backingDictionary.Clear();
    }

    public bool Contains(KeyValuePair<K, V> item)
    {
        return _backingDictionary.Contains(item);
    }

    public bool ContainsKey(K key)
    {
        return _backingDictionary.ContainsKey(key);
    }

    public void CopyTo(KeyValuePair<K, V>[] array, int arrayIndex)
    {
        _backingDictionary.CopyTo(array, arrayIndex);
    }

    public IEnumerator<KeyValuePair<K, V>> GetEnumerator()
    {
        return _backingDictionary.GetEnumerator();
    }

    public bool Remove(K key)
    {
        return _backingDictionary.Remove(key);
    }

    public bool Remove(KeyValuePair<K, V> item)
    {
        return _backingDictionary.Remove(item);
    }

    public ISuccessTuple<V> TryGetValue(K key)
    {
        bool wasSuccessful = _backingDictionary.TryGetValue(key, out V v);
        return new SuccessTupleImpl<V>(wasSuccessful, v);
    }

    public bool TryGetValue(K key, out V value)
    {
        return _backingDictionary.TryGetValue(key, out value);
    }

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

Once you wrap a dictionary in this class, you can then use it either as a regular dictionary, or as any of the covariant/contravariant fragment interface types defined.