Question

警告1变量“dr”在赋值前被使用。可能会在运行时导致空引用异常。 请高手指教！！

代码如下：
Dim strsql As String
Dim conn As New constr.db
strsql = "select * from stuloginfo where stuid='" & Textbox1.Text & " 'and stupassword='" & TextBox2.Text & " ' "
Dim myconn As New SqlConnection(constr.db.getstr)
Dim mycommand As New SqlCommand(strsql, myconn)
Session("stuno") = Textbox1.Text
Dim dr As SqlDataReader
Try
myconn.Open()
dr = mycommand.ExecuteReader(CommandBehavior.CloseConnection)
While dr.Read()
If Trim(dr.Item(3)) = "1111" Then
Response.Redirect("stulogin.aspx")
Else
Response.Redirect("student.aspx")
End If
End While
Catch ex As SqlException
Label1.Text = ex.Message
Finally
dr.Close()--------------------------------dr画了绿线
myconn.Close()
mycommand.Dispose()
End Try

Answer 1

这个警告是因为有可能存在dr没赋值的情况在try block抛出异常进入了finally时dr是空值
可以把Dim dr As SqlDataReader
改成
Dim dr As SqlDataReader = Nothing消除这个警告。不过为了避免这种情况出现，可以在dr.Close加上判断：
If dr IsNot Nothing Then dr.Close()
myconn，mycommand也类似