vb.net 如何修复此错误“未将对象引用设置到对象的实例”
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How to fix this error "Object reference not set to an instance of an object"
提问by Neqia
Server Error in '/' Application. Object reference not set to an instance of an object. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.
“/”应用程序中的服务器错误。你调用的对象是空的。说明：在执行当前 Web 请求期间发生未处理的异常。请查看堆栈跟踪以获取有关错误及其在代码中的来源的更多信息。
异常详细信息：System.NullReferenceException：未将对象引用设置为对象的实例。
Source Error:
源错误：
Line 844: Dim dt As New DataTable
Line 845:
Line 846: format_type_id = Request.QueryString("ID").ToString.Trim
Line 847:
Line 848: 'take the value from db so easy to maintain the format name
Stack Trace:
堆栈跟踪：
[NullReferenceException: Object reference not set to an instance of an object.]
Doccon.ESER_Doc_Form.get_format_type() in C:\Users\naquid9065\Documents\Q SMTT\Doccon\Doccon\ESER-Doc-Form.aspx.vb:846
Doccon.ESER_Doc_Form.Page_Load(Object sender, EventArgs e) in C:\Users\naquid9065\Documents\Q SMTT\Doccon\Doccon\ESER-Doc-Form.aspx.vb:57
System.Web.UI.Control.OnLoad(EventArgs e) +95
System.Web.UI.Control.LoadRecursive() +59
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +678
Code:
代码：
Private Sub get_format_type()
Dim strSQL As String = ""
Dim params As New Hashtable
Dim dt As New DataTable
format_type_id = Request.QueryString("ID").ToString.Trim (error from here)
strSQL = "SELECT * FROM tb_setup_format_type WHERE status ='1' and ID = @ID "
params.Add("@ID", format_type_id)
Common.OpenConn()
Common.execReader(strSQL, params, dt, Common.txn)
If dt.Rows.Count <> 0 Then
Me.txtformat.Text = dt.Rows(0)("Report_Format").ToString.Trim
End If
Common.CloseConn()
End Sub
回答by Mary
Procedures that are named Get... are usually functions. If you are getting something then it is returned.
名为 Get... 的过程通常是函数。如果你得到了一些东西，那么它就会被退回。
If you only need one piece of data don't pull down the whole record. You can use ExecuteScalar which returns the first column of the first row.
如果您只需要一条数据，请不要下拉整个记录。您可以使用 ExecuteScalar 返回第一行的第一列。
It is really easier to use ADO.net directly. Then you can keep track of your data objects local and ensure that they are closed and disposed even if there is an error. The Using...End Using blocks accomplish this.
直接使用ADO.net确实更容易。然后，您可以在本地跟踪您的数据对象，并确保即使出现错误，它们也已关闭和处理。Using...End Using 块实现了这一点。
Private Sub FillFormatTextBox()
Dim Format As Object = Nothing
Dim strSQL = "SELECT Report_Format FROM tb_setup_format_type WHERE status ='1' and ID = @ID "
Using cn As New SqlConnection("Your connection string")
Using cmd As New SqlCommand(strSQL, cn)
'Your code did not provide a value for the Id parameter
cmd.Parameters.Add("@ID", SqlDbType.Int).Value = 4
cn.Open()
Format = cmd.ExecuteScalar()
End Using
End Using
If Format Is Nothing Then
MessageBox.Show("No format Found")
Else
txtformat.Text = Format.ToString.Trim
End If
End Sub
