public static class DictionaryExtensions
{
    public static T GetKey<T, K>(this IDictionary<K, T> dict, K key, T defValue = default) where K : struct
    {
        return dict.TryGetValue(key, out T value) ? value : defValue;
    }

    public static T GetKey<T, K>(this IDictionary<K?, T> dict, K? key, T defValue = default) where K : struct
    {
        return key.HasValue && dict.TryGetValue(key, out T value) ? value : defValue;
    }

    public static T GetKey<T, K>(this IDictionary<K, T> dict, K? key, T defValue = default) where K : struct
    {
        return key.HasValue && dict.TryGetValue(key.Value, out T value) ? value : defValue;
    }

    public static T GetKeyC<T, K>(this IDictionary<K, T> dict, K key, T defValue = default) where K : class
    {
        return key != null && dict.TryGetValue(key, out T value) ? value : defValue;
    }
}