UserService.cs

public class UserService : IUserService
{
    private readonly IDictionary<string, User> _users;

    public UserService(IDictionary<string, User> users) => _users = users;

    public Task<(bool, User)> ValidateUserCredentialsAsync(string username, string password)
    {
        var isValid = _users.ContainsKey(username) && 
                      string.Equals(_users[username].Password, password, StringComparison.Ordinal);
        var result = (isValid, isValid ? _users[username] : null);
        return Task.FromResult(result);
    }
}