public RefreshToken GenerateRefreshToken(User user)
{
    // Create the refresh token
    RefreshToken refreshToken = new RefreshToken()
    {
        Token = GenerateRefreshToken(),
        Expiration = DateTime.UtcNow.AddMinutes(35) // Make this configurable
    }

    // Add it to the list of of refresh tokens for the user
    user.RefreshTokens.Add(refreshToken);

    // Update the user along with the new refresh token
    UserRepository.Update(user);

    return refreshToken;
}

public string GenerateRefreshToken()
{
    var randomNumber = new byte[32];
    using (var rng = RandomNumberGenerator.Create())
    {
        rng.GetBytes(randomNumber);
        return Convert.ToBase64String(randomNumber);
    }
}