Mouse picking with ray casting in DirectX - c++

Question

I'm trying to implement raycasting with DirectX in C++ but have run into problems. I've tried two approaches, using XMVector3Unproject and following the instructions provided here on stackoverflow a while ago.
Unproject Approach:
//p is mouse location vector
//Don't know if I need to normalize mouse input
//p.x = (2.0f * p.x) / g_nScreenWidth - 1.0f;
//p.y = 1.0f - (2.0f * p.y) / g_nScreenHeight;
Vector3 orig = XMVector3Unproject(Vector3(p.x, p.y, 0),
0,
0,
g_nScreenWidth,
g_nScreenHeight,
0,
1,
GameRenderer.m_matProj,
GameRenderer.m_matView,
GameRenderer.m_matWorld);
Vector3 dest = XMVector3Unproject(Vector3(p.x, p.y, 1),
0,
0,
g_nScreenWidth,
g_nScreenHeight,
0,
1,
GameRenderer.m_matProj,
GameRenderer.m_matView,
GameRenderer.m_matWorld);
Vector3 direction = dest - orig;
direction.Normalize();
//shoot a bullet to visualize the ray for testing
g_cObjectManager.createObject(BUL_OBJ, "bullet", orig, direction*100);
Matrix Inversion Approach:
//Normalized device coordinates
p.x = (2.0f * p.x) / g_nScreenWidth - 1.0f;
p.y = 1.0f - (2.0f * p.y) / g_nScreenHeight;
XMVECTOR det; //Determinant, needed for matrix inverse function call
Vector3 origin = Vector3(p.x, p.y, 0);
Vector3 faraway = Vector3(p.x, p.y, 1);
XMMATRIX invViewProj = XMMatrixInverse(&det, GameRenderer.m_matView * GameRenderer.m_matProj);
Vector3 rayorigin = XMVector3Transform(origin, invViewProj);
Vector3 rayend = XMVector3Transform(faraway, invViewProj);
Vector3 raydirection = rayend - rayorigin;
raydirection.Normalize();
g_cObjectManager.createObject(BUL_OBJ, "bullet", rayorigin, raydirection * 5);
I assume at least the matrix inversion approach from stackoverflow should work, but for some reason my attempt doesn't. Do I need the world matrix as well, or is there some step I'm missing?
This is my first stackoverflow post, so if anything is unclear or more information is needed please let me know.

Answer 1

In the end I realized my problem originated with switching from perspective to orthographic view to draw the HUD and never resetting the view matrix. I was able to use the XMVector3Unproject() function with the identity matrix in place of the world matrix and everything works flawlessly now.
Thanks Nico Schertler for the information and reassurance!