Point4D.Multiply inconsistent behaviour with NaN values

Question

I've noticed some peculiar behaviour with System.Windows.Media.Media3D.Point4d.Mutiply when used with NaN values.
The following assumes that I expect that 0 * NaN should result in 0. There is an argument against that logic, but that is not the point here.
I have a 4x4 matrix defined, in the case that it is an Identity matrix and there is a NaN component in the vector, I would like to get some defined vector output where it is possible to calculate the component.
E.g. (1,2,NaN,4) * (Identity Matrix) = (1,2,NaN,4).
I was caught out for a while:
By stepping through using the debugger gives the output I want (1,2,NaN,4).
But allowing the debug code to run through uninterrupted gives the result (NaN, NaN, Nan, Nan).
There is a slightly surprising workaround: - using Matrix.IsIdentity seems to trigger the result I want
If I use the default Matrix constructor instead of a manually created one (normally created elsewhere in code), there is no problem.
Can anyone explain this inconsistency?
Simple C# console demo:
using System;
using System.Windows.Media.Media3D;
namespace TestNan
class Program
static void Main(string[] args)
Point4D testPoint = new Point4D(1.0, 2.0, Double.NaN, 4.0);
Matrix3D testMatrix = new Matrix3D(1.0, 0.0, 0.0, 0.0,
0.0, 1.0, 0.0, 0.0,
0.0, 0.0, 1.0, 0.0,
0.0, 0.0, 0.0, 1.0);
var testResult1 = Point4D.Multiply(testPoint, testMatrix);
Console.WriteLine(testResult1.ToString());// NaN,NaN,NaN,NaN
var b = testMatrix.IsIdentity;
var testResult2 = Point4D.Multiply(testPoint, testMatrix);
Console.WriteLine(testResult2.ToString());// 1, 2, NaN, 4
Console.ReadKey();

Answer 1

>>Can anyone explain this inconsistency?
The Matrix3D class has an internal boolean flag that it uses to determine if the matrix is actually an identity matrix and this flag is not set when you create a Matrix3D object using the constructor that takes the 16 double values despite the fact that
you are actually creating an identity matrix (the flag is then eventually set when you access the IsIdentity property).
If you use the Matrix3D.Identity static property, the flag will get set to the correct value immediately and the results will be the expected:
static void Main(string[] args)
Point4D testPoint = new Point4D(1.0, 2.0, Double.NaN, 4.0);
Matrix3D testMatrix = Matrix3D.Identity;
var testResult1 = Point4D.Multiply(testPoint, testMatrix);
Console.WriteLine(testResult1.ToString());
var b = testMatrix.IsIdentity;
var testResult2 = Point4D.Multiply(testPoint, testMatrix);
Console.WriteLine(testResult2.ToString());// 1, 2, NaN, 4
Console.ReadKey();
Hope that helps.
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