Define “custom” integer-based type?

Question

I have a program that is interfacing with an external library that, among other things, has an unsigned 12-bit value packed in a larger struct.

This used to be 8 bit

Answer 1

You should create a struct that overrides the implicit conversion operator:

struct PackedValue {
    private PackedValue(ushort val) {
         if(val >= (1<<12)) throw new ArgumentOutOfRangeException("val");
         this._value = val;
    }
    private ushort _value;
    public static explicit operator PackedValue(ushort value) {
        return new PackedValue(value);
    }

    public static implicit operator ushort(PackedValue me) {
        return me._value;
    }
}