检查文件流是否已打开或未使用vb.net中的if语句 [英] check file stream is open or not using if statement in vb.net

问题描述

有人能说明检查文件流当前是否打开的条件是什么？

Can anyone show what would be the condition to check if the filestream is currently open?

fs = New FileStream(sfile, FileMode.Open, FileAccess.Read)
            Dim picByte As Byte() = New Byte(fs.Length - 1) {}
            fs.Read(picByte, 0, System.Convert.ToInt32(fs.Length))
            fs.Close()

            u_qry = "update nstudent set yer='" & cmb_year.Text & "',photo = @photo where stid='" & txt_sID.Text & "'"

            'store image as Binary value using SQLParameter
            Dim imgP As New SqlParameter
            imgP.SqlDbType = SqlDbType.Binary
            imgP.ParameterName = "photo"
            imgP.Value = picByte
            ucmd = New SqlCommand(u_qry, con)
            ucmd.Parameters.Add(imgP)
            ucmd.ExecuteNonQuery()
            Dim st = "select * from nstudent"
            scmd = New SqlCommand(st, con)
            Dim dt As New DataTable
            dt.Load(scmd.ExecuteReader)
            With dgrid
                .AutoGenerateColumns = False
                .DataSource = dt
                Call dgrid_col()
            End With
            MsgBox("Details Successfully Updated with Image")

这是一个常见的程序，代码性质是更新。我有两个选择。更新照片，更新没有照片。



低于我检查的条件。

Its a common procedure and the code nature is update. I''ve 2 option. update with photo, update without photo.

below the condition i''ve checked.

if(sfile <= 0 ) then 'Here sfile = OpenFileDialog1.FileName
{
' here update qry without image field
}
else {
'Here upadate qry with image field.
}

i不知道上述情况是否正确。请任何人说正确的方法来检查和操作。

i dont know the above condition is correct.? pls anyone say the correct way to check and do operation.

推荐答案

首先，正如我所看到的，你只是在阅读你用它的名字打开的文件。在这个和其他简单的情况下，你最好使用类 System.IO.StreamReader 或 System.IO.BinaryReader ：

http://msdn.microsoft.com /en-us/library/system.io.streamreader.aspx [ ^ ]，

http://msdn.microsoft.com/en-us/library/system.io.binaryreader.aspx [ ^ ]。



现在，根据检查流状态的技术的想法是错误的。相反，您应该只保证正确的状态：在阅读或写作之前创建一个阅读器或流，在完成后关闭它。更确切地说，请注意所有这些类实现接口 System.IDisposable 。这意味着您始终必须保证在完成任何流时调用 System.IDisposable.Disposed / reader / writer对象。最好的方法是使用使用语句：

http://msdn.microsoft.com/en-us/library/htd05whh.aspx [ ^ ]。



顺便说一下，声明是在读者的例子上说明。这正是你应该如何使用的。



祝你好运，

-SA

First of all, as I can see, you are just reading the file which you open using its name. In this and other simple cases case, you should better use either the class System.IO.StreamReader or System.IO.BinaryReader:
http://msdn.microsoft.com/en-us/library/system.io.streamreader.aspx[^],
http://msdn.microsoft.com/en-us/library/system.io.binaryreader.aspx[^].

Now, the idea of the technique depending on checking the stream status is wrong. Instead, you should simply guarantee right status: create a reader or stream before reading or writing, close it when you are done. More exactly, please pay attention that all those classes implement the interface System.IDisposable. It means that you always must guarantee that the System.IDisposable.Disposed is called when you are done with any of stream/reader/writer objects. The best way to do this is using the using statement:
http://msdn.microsoft.com/en-us/library/htd05whh.aspx[^].

By the way, the statement is illustrated on the example of the reader. This is exactly how you should use it.

Good luck,

—SA

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