Etwas aufwändig:

var result = new Dictionary<string, string>();
result["username"] = username;
result["mail"] = email;
return result;

Besser ist:

return new Dictionary<string, string>()
    .With("username", username)
    .With("mail", email);

Mit dieser Extension-Methode:

public static Dictionary<T, W> With<T, W>(this Dictionary<T, W> dict, T key, W value)
{
    dict[key] = value;
    return dict;
}