switch case NaN specification

[RamType0]

In this article,
case double.NaN appears in relational pattern, and logical pattern.

But how it is specified?

Because double == double.NaN is always false, I think there are some intrinsic support for double (and float), but it is not mentioned in this article.

Further more, could we use custom NaN like value in switch?(e.g. quadruple-precision float NaN)

[tannergooding]

You can use sharplab to see what the patterns are currently translated into (https://sharplab.io/#v2:EYLgtghglgdgPgAQEwEYCwAoBBmABM3AYVwG9NcL88EAWXAWQAoBKU8yjhFATkcIBsIAZyFQAZgE9GKbM2YBuCgHoluAPYBXAC4AHbSFwAVNWtwALKAHMz7DhS68Bw0ZMYATTcH4BTAHQA5CH85RVwVdW09LQMAVRgAaxg1AHcYWzsHPkERcSkkXxoQ5VVNXX1cAEEAYyrvHS0IL29MdM4UADZ8FAAGImyXKQ8NJtwwb2ENACdvMZgtVgBeAD5R8aEpme853CFkqC0qmww7SjJjk7sAHlwAWhpfXuXcACJjU34U54AaVouVnoeuCerxM5isZm+vxOQyaASCQJWzziiRSMEh5wulAA+giXtVavVGj50ZiAL7ydKkzCkoA)

In this case, Classify is effectively the same as:

        if (!(measurement < -4.0))
        {
            if (!(measurement > 10.0))
            {
                if (double.IsNaN(measurement))
                {
                    return "Unknown";
                }
                return "Acceptable";
            }
            return "Too high";
        }
        return "Too low";

-- Noting that sharplab only shows the current. What the compiler emits here can change over time or based on changes to the pattern in your own code.

[RamType0]

-- Noting that sharplab only shows the current. What the compiler emits here can change over time or based on changes to the pattern in your own code.

Yeah, we could test what will happen by decompiling it.
But it is just actually work, but not specified that it always work (as its specification).

[tannergooding]

This, unfortunately, doesn't have a formal speclet covering NaN that I can find.

I would expect, however, that this isn't an implementation defined behavior and the intent is that NaN is properly checked for equality. I would expect float.IsNaN(x), double.IsNaN(x), and x != x would all be valid ways to check for this (!x.Equals(x) would not be valid, since Equals treats NaN as equal to any other NaN as required by the object.Equals/object.GetHashCode and IEquatable<T> contract)