How can I delete virtual disks in virtualbox?

位置: C:\Users\user\VirtualBox VMs\vhost\vhost.vdi

Cannot close medium ‘C:\Users\user\VirtualBox VMs\vhost\vhost.vdi’ because it has 1 child media.
返回 代码:
VBOX_E_OBJECT_IN_USE (0X80BB000C)
组件:
MediumWrap
界面:
IMedium {ad47ad09-787b-44ab-b343-a082a3f2dfb1}

关闭虚拟盘文件C:\Users\user\VirtualBox VMs\vhost\vhost.vdi 失败.
Cannot close medium ‘C:\Users\user\VirtualBox VMs\vhost\vhost.vdi’ because it has 1 child media.
return:
VBOX_E_OBJECT_IN_USE (0X80BB000C)your text
组件:your text
MediumWrap
界面:
IMedium {ad47ad09-787b-44ab-b343-a082a3f2dfb1}

I have deleted all my virtual machines，but this error still exist.