asp.net - 即使在使用 IsDBNull 检查后也出现 NullReferenceException

Question

我有以下代码：

Dim getProspect = (From p In dbContext.IRF_Prospects _
                   Where p.url = prospect_url _
                   Select p).FirstOrDefault
' If they have a record...
If Not IsDBNull(getProspect) Then
    If IsDBNull(getProspect.user_id) Then
        ' Prepopulate the form with their information.
        txtFirst.Text = getProspect.first_name
Else
        ' Redirect them to login.
        Response.Redirect("login.aspx")
End If

当我执行它时，它会抛出一个未设置为对象错误实例的对象引用getProspect.user_id。为什么要这样做？我IsDBNull首先验证它是否存在的事实不应该防止这种情况发生吗？

Answer 1

DBNullNothing和你所拥有的不一样Nothing。FirstOrDefault，顾名思义，返回第一项或默认值，Nothing用于引用类型 - never DBNull。

Dim getProspect = (From p In dbContext.IRF_Prospects _
                   Where p.url = prospect_url _
                   Select p).FirstOrDefault
' If they have a record...
If getProspect IsNot Nothing Then
    If IsDBNull(getProspect.user_id) Then
        ' Prepopulate the form with their information.
        txtFirst.Text = getProspect.first_name
    Else
        ' Redirect them to login.
        Response.Redirect("login.aspx")
    End If