Data Classes, Builder with Assignment Validation

LoginResource.cs

public class LoginResource {
    public struct Builder {
        private byte _assigned;
        
        private string _username;
        private string _password;
        
        public string Username {
            get => _username;
            set {
                _assigned = (byte)(_assigned | 1);
                _username = value;
            }
        }
        
        public string Password {
            get => _password;
            set {
                _assigned = (byte)(_assigned | 2);
                _password = value;
            }
        }
        
        public bool RememberMe { get; set; }
        internal byte Assigned => _assigned;
    }
    
    public static Builder Init() {
        var builder = new Builder();
        builder.RememberMe = true;
        return builder;
    }
    
    private readonly Builder _builder;
    
    public LoginResource(Builder builder) {
        byte assigned = builder.Assigned;
        if ((assigned & 1) == 0) {
            throw new InvalidOperationException($"{nameof(Username)} was not assigned.");
        }
        if ((assigned & 2) == 0) {
            throw new InvalidOperationException($"{nameof(Password)} was not assigned.");
        }
        _builder = builder;
    }
    
    public string Username => _builder.Username;
    public string Password => _builder.Password;
    public bool RememberMe => _builder.RememberMe;
    
    // other members elided for brevity
}

Pretty, pretty, pretty! me like.