Use the Lock procedure in situations where multiple programs or more than one instance of your program may need read and write access to the same data file. Rules at a Glance Use the Lock procedure with only the filenumber argument to lock the whole file. record is interpreted as a record number in the case of random files and a byte number in the case of binary files. Records and bytes in a file are always numbered sequentially from 1 onward. To lock a particular record, specify its record number as record, and only that record will be locked. The Lock procedure locks an entire file opened in Input or Output (sequential) mode, regardless of the record argument. If you omit the start argument, Lock will lock all records from the start of the file to record or byte number end. Attempting to access a locked file or portion of a file returns runtime error 70, “Permission denied.” Programming Tips and Gotchas You must take care to remove all file locks with the Unlock procedure before either closing a file or ending the application; otherwise, you can leave the file in an unstable state. This of course means that, where appropriate, your error-handling routines must be made aware of any locks you currently have in place so that they may be removed if necessary. You use the Lock and Unlock procedures in pairs, and the argument lists of both statements must match exactly. The Lock procedure does not guarantee under all circumstances that the locked file will be protected from access by other processes. There are two major circumstances under which an apparent access violation can occur: o The file has already been opened but has not been locked by a process when the current process locks it. However, the first process will not be able to perform operations on the file once the second file successfully locks it. o The block of code responsible for opening the file and then locking it is interrupted by the scheduling policy of the operating system before the file can be locked. If a second process then opens and locks the file, it and not the first process will have sole use of the file. Because of this, the Lock procedure should immediately follow the FileOpen procedure in code. This reduces, but does not eliminate, the problems that result from the fact that opening and locking a file is not an atomic operation. VB .NET/VB 6 Differences In the VB 6 Lock statement, you can separate the fromrecord and torecord arguments with the To keyword. In the VB .NET Lock procedure, this syntax is not supported. See Also Unlock Procedure LOF Function

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FileOpen Procedure, LOF Function Lock Procedure Class Microsoft.VisualBasic.FileSystem Syntax Lock(filenumber[, record] or: Lock(filenumber[, fromrecord,torecord] filenumber Use: Required Data Type: Integer Any valid file number record Use: Optional Data Type: Long The record or byte number at which to commence the lock fromrecord Use: Optional Data Type: Long The first record or byte number to lock torecord Use: Optional Data Type: Long The last record or byte number to lock Description The Lock procedure prevents another process from accessing a record, section, or whole file until it is unlocked by the Unlock function.

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Loc Function Class Microsoft.VisualBasic.FileSystem Syntax Loc(filenumber) filenumber Use: Required Data Type: Integer Any valid file number Return Value A Long indicating the current position of the read/write pointer in a file Description Determines the current position of the file read/write pointer Rules at a Glance If you have opened the file in Random mode, Loc returns the record number of the last record read or written. If you have opened the file in Input or Output modes (sequential), Loc returns the current byte position in the file divided by 128. If you have opened the file in Binary mode, Loc returns the position of the last byte read or written. Example Dim fr As Integer = FreeFile( ) Dim sChar As Char FileOpen(fr, “c:data.txt”, OpenMode.Binary, OpenAccess.Read) Do While Loc(fr) < LOF(fr) FileGet(fr, sChar) Debug.Write(Loc(fr) & ": ") Debug.WriteLine(sChar) Loop Programming Tips and Gotchas For sequential files, the return value of Loc is not required and should not be used. Note that you cannot set the position of the file pointer using Loc. See Also

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Syntax LineInput(filenumber) filenumber Use: Required Data Type: Integer Any valid file number Return Value A String containing the line read from the file Description Assigns a single line from a sequential file opened in Input mode to a string variable Rules at a Glance Data is read into a buffer one character at a time until a line feed or carriage-return sequence (either Chr(13) or Chr(13)+Chr(10)) is encountered. When this happens, all the characters in the buffer are returned as a string, without the carriage-return sequence, and the buffer is cleared. After reading a line, the file pointer advances to the first character after the end of the line or to the end-of-file marker. Example The following code reads all of the lines in a text file and sends them to the Output window: Dim fr As Integer = FreeFile( ) Dim sLine As String FileOpen(fr, “c:data.txt”, OpenMode.Input, OpenAccess.Read) Do While Not EOF(fr) Debug.WriteLine(LineInput(fr)) Loop FileClose(fr) Programming Tips and Gotchas You use the LineInput function to read data from text files. To write data back to this type of file, use the PrintLine function. VB .NET/VB 6 Differences The VB .NET LineInput function corresponds directly to the VB 6 LineInput statement, with the following differences. The VB 6 LineInput statement has a second argument, varname, which is the variable to receive the line read by the function. It is not supported by the VB .NET LineInput function, since the line read is the return value of the function. The first argument of the VB 6 LineInput statement, filenumber, could be preceded by the # symbol. In VB .NET, this format is not supported.
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[!list] Any single character not in list [] A zero-length string (“”) list is used to match a group of characters in pattern to a single character in string and can contain almost all available characters, including digits. Use a hyphen (-) in list to create a range of characters to match a character in string. For example, [A-D] will match A, B, C, or D in that character position in string. Multiple ranges of characters can be included in list without the use of a delimiter. For example, [A-D J-L]. Ranges of characters should appear in sort order. For example, [c-k]. Use the hyphen at the start or end of list to match to itself. For example, [-A-G] matches a hyphen or any character from A to G. The exclamation point in pattern matching is like the negation operator in C. Use an exclamation point before a character or range of characters in list to match all but that character. For example, [!A-G] matches all characters apart from the characters from A to G. The exclamation point outside of the bracket matches itself. To use any special character as a matching character, you should enclose the special character in brackets. For example, to match to a question mark, use [?]. Example The following example will display OK if the text entered into Text1 starts with either V or A, followed by any characters, and ends with “in a Nutshell.” Therefore, “Paul in a Nutshell” returns Wrong, whereas either “ASP in a Nutshell” or “VB .NET Language in a Nutshell” returns OK. Private Sub Button1_Click(ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles Button1.Click Dim sTitle As String = “in a Nutshell” Dim sPattern As String = “[V A]* ” & sTitle If TextBox1.Text Like sPattern Then MsgBox(“OK”) Else MsgBox(“Wrong”) End If End Sub Programming Tips and Gotchas Different languages place different priority on particular characters with relation to sort order. Therefore, the same program using the same data may yield different results when run on machines in different parts of the world, depending upon the locale settings of the systems. Regular expressions provide an even more powerful method for searching and comparing strings. You can use regular expressions through the .NET Framework’s System.Text.RegularExpressions.RegEx class. LineInput Function Class Microsoft.VisualBasic.FileSystem
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the Length method can be used only on strings, whereas Len can be used on all data types other than strongly typed objects. Like Operator Syntax result = string Like pattern string Use: Required Data Type: String The string to be tested against pattern pattern Use: Required Data Type: String A series of characters used by the Like operator to determine if string and pattern match Return Type Boolean Description If string matches pattern, result is True; otherwise, result is False. Rules at a Glance If either string or patternis Nothing, then result will be Nothing. The default comparison method for the Like operator is Binary. This can be overridden using the OptionCompare statement. Binary comparison is based on comparing the internal binary number representing each character; this produces a case-sensitive comparison. Text comparison, the alternative to binary comparison, is case insensitive; therefore, A = a. The sort order is based on the code page currently being used, as determined by the Windows regional settings. The following table describes the special characters to use when creating a pattern; all other characters match themselves. Character Meaning ? Any single character * Zero or more characters # Any single digit (0-9) [list] Any single character in list
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Len Function Class Microsoft.VisualBasic.Strings Syntax Len(expression) expression Use: Required Data Type: Any Any valid variable name or expression Return Value Integer Description Counts the number of characters within a string or the size of a given variable Rules at a Glance If expression contains Nothing, Len returns 0. For a string or String variable, Len returns the number of characters in the string. For a nonobject and nonstructure variable, Len returns the number of bytes required to store the variable in memory. For a variable of type Object, Len returns the length of its data subtype. If the object is uninitialized, its length is 0. However, if the object contains a strongly typed class instance, an InvalidCastException exception is thrown. For a structure, Len returns the number of bytes required to store the structure as a file. (But see the comment in Programming Tips and Gotchas.) For a strongly typed object variable, such as one defined by the Class… EndClass construct, Len generates an InvalidCastException exception. If varname is an array, you must also specify a valid subscript. In other words, Len cannot be used to determine the total number of elements in or the total size of an array. Programming Tips and Gotchas Len cannot accurately report the number of bytes required to store structures that contain variable-length strings. If you need to know how many bytes of storage space will be required by a structure that includes string members, you can fix the length of the strings by using the attribute in the Structure statement. For details, see the “Structure Statement” entry. Len is functionally similar to the BCL’s System.String.Length public instance method. One significant difference is that Len retuns a 0 in the case of an uninitialized String variable, whereas the Length method raises a NullReferenceException exception. In addition, of course,

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Microsoft.VisualBasic.Strings Syntax Left(str, length) str Use: Required Data Type: String The string to be processed length Use: Required Data Type: Long The number of characters to return from the left of the string Return Value String Description Returns a string containing the leftmost length characters of str Rules at a Glance If length is 0, a zero-length string (“”) is returned. If length is greater than the length of str, str is returned. If string is Nothing, Left returns Nothing. Programming Tips and Gotchas Use the Len function to determine the overall length of str. The Left function corresponds to the BCL System.String class’ Substring method. For example, the following two assignments to the sCity variable are functionally identical: Dim sCity As String Dim sLocation As String = “New York, New York” sCity = Left(sLocation, sCity = sLocation.Substring(0, Note that the Substring method uses a zero-based index to determine the starting position of the substring. See Also Mid Function, Right Function
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Since VB 6 offers a number of ways to set the lower bound of all arrays or a specific array, the LBound function is particularly useful when iterating the elements of an array. In VB .NET, its use is a matter of choice. See Also UBound Function LCase Function Class Microsoft.VisualBasic.Strings Syntax LCase(value) value Use: Required Data Type: String or Char A valid string expression or a character Return Value String or Char Description Converts a string to lowercase Rules at a Glance LCase only affects uppercase letters; all other characters in value are unaffected. LCase returns Nothing if value contains a Nothing. LCase returns the same data type as value. See Also UCase Function Left Function Class

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Microsoft.VisualBasic.Information Syntax LBound(array[, rank]) array Use: Required Data Type: Any array An array whose lower bound is to be determined rank Use: Optional Data Type: Integer The dimension whose lower bound is desired Return Value An Integer whose value is 0 Description Determines the lower boundary of a specified dimension of an array. The lower boundary is the smallest subscript you can access within the specified array. Rules at a Glance Unless it is passed an invalid argument, the LBound function always returns 0. If array is uninitialized, it generates an ArgumentNullException error when passed to the LBound function. You can prevent this by comparing array to Nothing, as in the following code fragment: If Not oArray Is Nothing Then To determine the lower limit of the first dimension of an array, set rank to 1, set it to 2 for the second, and so on. If rank isn’t specified, 1 is assumed. Programming Tips and Gotchas Since VB .NET does not allow you to change the lower bound of an array, the LBound function would appear to be superfluous except for reasons of backward compatibility. Its continued use may be a good idea, though, in the event that a future version of VB .NET allows you to set the lower boundary of an array. VB .NET/VB 6 Differences
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