.NET Reference source is a gold mine. In an internal System.Windows.Forms.NativeMethods+Util class you will find these helpers, that talk the same as WM_MOVE documentation (high-order word = HIWORD, low-order word = LOWORD, etc.)

public static int MAKELONG(int low, int high) {
    return (high << 16) | (low & 0xffff);
}

public static IntPtr MAKELPARAM(int low, int high) {
    return (IntPtr) ((high << 16) | (low & 0xffff));
}

public static int HIWORD(int n) {
    return (n >> 16) & 0xffff;
}

public static int HIWORD(IntPtr n) {
    return HIWORD( unchecked((int)(long)n) );
}

public static int LOWORD(int n) {
    return n & 0xffff;
}

public static int LOWORD(IntPtr n) {
    return LOWORD( unchecked((int)(long)n) );
}

public static int SignedHIWORD(IntPtr n) {
    return SignedHIWORD( unchecked((int)(long)n) );
}

public static int SignedLOWORD(IntPtr n) {
    return SignedLOWORD( unchecked((int)(long)n) );
}

public static int SignedHIWORD(int n) {
    int i = (int)(short)((n >> 16) & 0xffff);
    return i;
}

public static int SignedLOWORD(int n) {
    int i = (int)(short)(n & 0xFFFF);
    return i;
}