error of connection open

System.invalidOperationException: There is already an open Reader that must be closed first

 
            Dim temp As Integer = 0
            str = "select CaseId  from New_Case_Id_Master where CaseId >" & temp & ""
            cmd = New SqlCommand(str, cn.getConnection())
            dr = cmd.ExecuteReader()
            If dr.Read = False Then
                Dim s_temp As String
                s_temp = temp_no + "000001"
                s_no = Val(s_temp)
 
            Else
 

 
                str = "select max(CaseId) from New_Case_Id_Master"
                cmd = New SqlCommand(str, cn.getConnection())
                dr = cmd.ExecuteReader()'showing this line number in error
                If dr.Read = True Then
                    Dim s_no_temp, s_no_temp1 As String
                    Dim no_temp As Integer
                    no_temp = dr.Item(0)
                    s_no_temp = no_temp.ToString
 
                    s_no_temp1 = s_no_temp.Substring(0, 4)
                    s_no = Val(s_no_temp1)
 
                    If temp_no > s_no Then
                        s_no = Val(temp_no.ToString + "000001")
                    Else
                        s_no = no_temp + 1
                    End If
                End If
 
            End If