WPF无模式对话框呈现文本框不可编辑

A WPF form that I launch from a WinForms window shows up with all textboxes as uneditable when it's launched as a modeless dialog. It works well when it's a modal window. I'm still able to type spaces or paste text. But regular typing doesn't work. I'm using 3.5 with SP1. Would anybody know how to resolve this?A WPF form that I launch from a WinForms window