[SOLVED] Why is Vector<T>.Count static?

Why is Vector<T>.Count static?

I'm trying to use System.Numerics.Vector<T> (documentation).

I wrote a simple unit test:

var v = new System.Numerics.Vector<double>(new double[] { 12, 13, 14 });
Assert.AreEqual(3, v.Count);

But it gave me a build error:

Member 'Vector.Count' cannot be accessed with an instance reference; qualify it with a type name instead

To my surprise, Vector<T>.Count is static.

So I tried:

var v = new System.Numerics.Vector<double>(new double[] { 12, 13, 14 });
Assert.AreEqual(3, Vector<double>.Count);

Now the code builds but the unit test fails:

Assert.AreEqual failed. Expected:<3>. Actual:<2>.

What's going on?

Investigating I found:

Assert.AreEqual(2, Vector<double>.Count);
Assert.AreEqual(4, Vector<float>.Count);
Assert.AreEqual(4, Vector<int>.Count);
Assert.AreEqual(2, Vector<long>.Count);

Solution

The documentation suggests that this is by design:

The count of a Vector instance is fixed, but its upper limit is CPU-register dependent.

Its purpose is to allow vectorizing operations using hardware capabilities, and thus its capacity is tied to your CPU's architecture.