public static async Task <IDictionary <TKey, bool> > ExistsFilesByKey <TKey>(this IFileSystemOperator @operator, IDistinctValuedDictionary <TKey, string> filePathsByKey)
{
var keysByFilePath = filePathsByKey.Invert();
var existsByFilePath = await @operator.ExistsFiles(keysByFilePath.Keys.AsDistinct());
var output = keysByFilePath.Join(existsByFilePath,
keys => keys.Key,
exists => exists.Key,
(keys, exists) => (Key: keys.Value, Exists: exists.Value))
.ToDictionary(
x => x.Key,
x => x.Exists);
return(output);
}