test.cs

public interface IDictionaryValueResolver
{
  public string ResolveValue(string key);
}

public class ConcreteDictionaryValueResolver : IDictionaryValueResolver
{
  public string ResolveValue(string key) {
     // Do your thing
  }
}

public class MockDictionaryValueResolver : IDictionaryValueResolver
{
  public string ResolveValue(string key) {
     // Do your thing
  }
}

public class DictionaryValueResolverFactory {

  private static IDictionaryValueResolver _resolver;
  public static IDictionaryValueResolver Get()
  {
    if(_resolver == null) {
      _resolver = new ConcreteDictionaryValueResolver();
    }
    return _resolver;
  }
  
  internal static Set(IDictionaryValueResolver value) {
    _resolver = value;
  }
}