Why "vSpeed(j).Z = -vSpeed(j).Z" ---> Error: Expression is a value and therefore cannot be the target of an assignment.

I don't understand why this is not allowed :S

This is the general situation:

Dim vSpeed As New List (Of Vector3d)

For statement...

vSpeed(j).Z = -vSpeed(j).Z

Next...

Error: Expression is a value and therefore cannot be the target of an assignment.

I've solved it with this:

Dim vZ As Vector3d

Dim vSpeed As New List (Of Vector3d)

For statement...

dZ = vSpeed(j)
dZ.Z = -dZ.Z
vSpeed(j) = dZ

Next

Anyone knows why this happens?

Replies to This Discussion

Permalink Reply by David Rutten on August 17, 2012 at 4:10am

It happens because Vector3d is a Value Type ('Structure' in VB).

When you get a structure out of a list using vSpeed(i) you get a copy of the actual vector stored inside the list. When you then access the Z of this vector, you are only modifying the copy. Since this copy is not stored anywhere the compiler sees that you are performing a meaningless operation and thus warns you about it.

Your solution of getting the vector, modifying it, then putting it back is the correct approach.

David Rutten

david@mcneel.com

Poprad, Slovakia

Permalink Reply by Ángel Linares on August 17, 2012 at 5:53am