[Flags]

public enum ErrorTypes : int {

    None = 0,

    MissingPassword = 1,

    MissingUsername = 2,

    PasswordIncorrect = 4

}

public static class EnumExtensions {

    public T Append<T>(this System.Enum type, T value) {

        return (T)(object)(((int)(object)type | (int)(object)value));

    }

    public static T Remove<T>(this System.Enum type, T value) {

        return (T)(object)(((int)(object)type & ~(int)(object)value));

    }

    public static bool Has<T>(this System.Enum type, T value) {

        return (((int)(object)type & (int)(object)value) == (int)(object)value);

    }

}

...

//used like the following...

ErrorTypes error = ErrorTypes.None;

error = error.Append(ErrorTypes.MissingUsername);

error = error.Append(ErrorTypes.MissingPassword);

error = error.Remove(ErrorTypes.MissingUsername);

//then you can check using other methods

if (error.Has(ErrorTypes.MissingUsername)) {

    ...

}