public static void AssertForeignKeyExists(string input, string key, GTFSCalendarCollection collection, string collectionName)
{
if (input == null)
{
throw new PropertyNullException(key);
}
if (!collection.Contains(input))
{
if (input == "")
{
throw new PropertyNullException(key);
}
else
{
throw new PropertyForeignKeyException(key, $"The collection {collectionName} doesn't contain the key {input}.");
}
}
}
