为什么我不能在vb.net中设置任何变量？

Question

我的vb.net存在问题。

我的代码

class business{
    buiding as string
}

我从mongodb加载数据

Dim collection1 = db1.GetCollection(Of business)("tablebusiness")
Dim list = collection1.Find(query1)
For Each abiz In list
     Dim biztemp = abiz
     biztemp.buiding = nothing '//// (but I get biztemp.building = "") why??
     'biztemp.building = "" here
Next