Prevent construction of invalid messages

[rkm]

It should not be possible to construct a message class that is not valid; however, all message types currently have public setters. Some classes have a "validate" method, but this is too easy to forget about.

[rkm]

Using records might also help here as they can generate a lot of the boilerplate code.

[rkm]

Having looked at this in some more detail, I think using records is a good idea but we'll still likely need to add some sort of Validate method as part of the interface that is checked before messages are sent/received. I had hoped that we would be able to implement validation using the type system e.g.,

// https://www.youtube.com/watch?v=KFx9XHpoYV4
public readonly record struct NonEmptyTrimmedString(string Value)
{
    public required string Value { get; init; } =
        !string.IsNullOrWhiteSpace(Value) ? Value.Trim() :
        throw new ArgumentException("Value cannot be null or whitespace", nameof(Value));

    public NonEmptyTrimmedString() : this(string.Empty) { } // Throws

    public static implicit operator string(NonEmptyTrimmedString value) => value.Value;
}

However, it's always possible to zero-initialize a struct so the above doesn't actually provide a guarantee that the validation code is executed. Maybe this would work as a class instead of a struct, but then you lose the value-type semantics.

In any case, these changes should wait until we've implemented the CTP C# wrapper so that we are free to tweak the message classes and not have to worry about mirroring it in Java.