Unity 2D, C# - Make object move diagonally by a certain angle?

Question

I have a sprite object which I want to move in 8 directions. Left, Right, Up, Down, Left top, Right top, Left down, Right down. The view is 2D with a isometric projection. The difficulties I am having is when the object needs to move diagonally. I need to work out how I can move the sprite in fixed degrees diagonally so its parallel to the isometric world.

I have a diagram to demonstrate.

Here the green arrows are the diagonal movement that I need. The code I have for up, down, left and right movement is already in,

void FixedUpdate ()
    {

            speed = 10f;
            //pos = Camera.main.ViewportToWorldPoint (new Vector3 (0.5F, 0.5F, Camera.main.nearClipPlane));
            //transform.position = new Vector3 (pos.x, pos.y, -1F);

            if (Input.GetKey (KeyCode.D)) {
                    ///rigidbody2D.AddForce (Vector2.right * speed);
                    rigidbody2D.velocity = Vector2.right * speed;

            } else if (Input.GetKey (KeyCode.A)) {
                    //rigidbody2D.AddForce (-Vector2.right* speed);
                    rigidbody2D.velocity = -Vector2.right * speed;
            } else if (Input.GetKey (KeyCode.S)) {
                    //rigidbody2D.AddForce (-Vector2.up * speed);
                    rigidbody2D.velocity = -Vector2.up * speed;
            } else if (Input.GetKey (KeyCode.W)) {
                    //rigidbody2D.AddForce (Vector2.up * speed);
                    rigidbody2D.velocity = Vector2.up * speed;
            } else {
                    rigidbody2D.velocity = Vector2.zero;
            }
    }

I had a look at the Transform class and the various variables and functions but I don't know what to use or start with. This is because I am very new to Unity and maths such as rotations, axis, etc. So a helping direction would be massively appreciated. Thanks.

Answer 1

//temp helper variable
var moveAmount = Point.Zero;

            if (InputHandler.KeyPressed(Keys.A) || InputHandler.KeyPressed(Keys.Left))
                moveAmount.X -= 1;
            if (InputHandler.KeyPressed(Keys.D) || InputHandler.KeyPressed(Keys.Right))
                moveAmount.X += 1;
            if (InputHandler.KeyPressed(Keys.W) || InputHandler.KeyPressed(Keys.Up))
                moveAmount.Y -= 1;
            if (InputHandler.KeyPressed(Keys.S) || InputHandler.KeyPressed(Keys.Down))
                moveAmount.Y += 1;
// do stuff
rigidbody2D.velocity = moveAmount * speed;