How to open .VHDX files in virtualbox?

I used disk2vhd by microsoft so I tried to create a new virtualbox using it and I got this error

Failed to open the hard disk C:\Users\user\Documents\Virtual Box Images\win7vir.VHDX

Could not open the medium 'C:\Users\user\Documents\Virtual Box Images\win7vir.VHDX'.

VHDX: Required flag of metadata item does not meet expectations 'C:\Users\user\Documents\Virtual Box Images\win7vir.VHDX' (VERR_VD_GEN_INVALID_HEADER).

VHDX: BAT region in image 'C:\Users\user\Documents\Virtual Box Images\win7vir.VHDX' is missing (VERR_VD_GEN_INVALID_HEADER).

VD: error VERR_VD_GEN_INVALID_HEADER opening image file 'C:\Users\user\Documents\Virtual Box Images\win7vir.VHDX' (VERR_VD_GEN_INVALID_HEADER).

Result Code: E_FAIL (0x80004005)
Component: Medium
Interface: IMedium {29989373-b111-4654-8493-2e1176cba890}
Callee: IVirtualBox {3b2f08eb-b810-4715-bee0-bb06b9880ad2}
Callee RC: VBOX_E_OBJECT_NOT_FOUND (0x80BB0001)

Short answer: use the powershell commandlet to convert the VHDX to VHD. AFAIK it requires hyperv or one of it's variants to be installed to have the convert-vhd commandlet available.

Convert-VHD –Path c:\test\MY-VM.vhdx –DestinationPath c:\test\MY-NEW-VM.vhd

Just uncheck "Make VHDX" when creating the image in Disk2vhd. Virtual Box doesn't support the .VHDX format.