public UserDto GetUser(string email) {
    if (!ValidationUtil.IsValidEmail(email))
        throw new ArgumentException("Must be a properly formatted email", nameof(email));
    UserDto toReturn = null;
    toReturn = Users?.FirstOrDefault(user => user?.Email == email);
    return toReturn;
}

public bool AreCredentialsValid(string email, string password) {
    bool toReturn = false;
    if (!ValidationUtil.IsValidEmail(email))
        throw new ArgumentException("Must be a properly formatted email", nameof(email));
    if (password == null)
        new ArgumentNullException(nameof(password));
    UserDto user = this.GetUser(email);
    if (user?.Email == email && user?.Password == password)
        toReturn = true;
    return toReturn;
}

public UserDto GetUserIfCredentialsAreValid(string email, string password) {
    UserDto toReturn = null;
    if (AreCredentialsValid(email, password))
        toReturn = GetUser(email);
    return toReturn;
}