FileGet Function
Reads data from an open disk file into a variable.
The My feature gives you greater productivity and performance in file I/O operations than FileGet. For more information, see My.Computer.FileSystem Object.
Public Overloads Sub FileGet( _
ByVal FileNumber As Integer, _
ByRef Value As Object, _
Optional RecordNumber As Integer = -1 _
)
' -or-
Public Overloads Sub FileGet( _
ByVal FileNumber As Integer, _
ByRef Value As Short, _
Optional RecordNumber As Integer = -1 _
)
' -or-
Public Overloads Sub FileGet( _
ByVal FileNumber As Integer, _
ByRef Value As Integer, _
Optional RecordNumber As Integer = -1 _
)
' -or-
Public Overloads Sub FileGet( _
ByVal FileNumber As Integer, _
ByRef Value As Single, _
Optional RecordNumber As Integer = -1 _
)
' -or-
Public Overloads Sub FileGet( _
ByVal FileNumber As Integer, _
ByRef Value As Double, _
Optional RecordNumber As Integer = -1 _
)
' -or-
Public Overloads Sub FileGet( _
ByVal FileNumber As Integer, _
ByRef Value As Decimal, _
Optional RecordNumber As Integer = -1 _
)
' -or-
Public Overloads Sub FileGet( _
ByVal FileNumber As Integer, _
ByRef Value As Byte, _
Optional RecordNumber As Integer = -1 _
)
' -or-
Public Overloads Sub FileGet( _
ByVal FileNumber As Integer, _
ByRef Value As Boolean, _
Optional RecordNumber As Integer = -1 _
)
' -or-
Public Overloads Sub FileGet( _
ByVal FileNumber As Integer, _
ByRef Value As Date, _
Optional RecordNumber As Integer = -1 _
)
' -or-
Public Overloads Sub FileGet( _
ByVal FileNumber As Integer, _
ByRef Value As System.Array, _
Optional RecordNumber As Integer = -1, _
Optional ArrayIsDynamic As Boolean = False _
)
' -or-
Public Overloads Sub FileGet(
ByVal FileNumber As Integer, _
ByRef Value As String, _
Optional RecordNumber As Integer = -1, _
Optional StringIsFixedLength As Boolean = False _
)
Parameters
- FileNumber
Required. Any valid file number.
- Value
Required. Valid variable name into which data is read.
- RecordNumber
Optional. Record number (Random mode files) or byte number (Binary mode files) at which reading begins.
- ArrayIsDynamic
Optional. Applies only when writing an array. Specifies whether the array is to be treated as dynamic and whether an array descriptor describing the size and bounds of the array is necessary.
- StringIsFixedLength
Optional. Applies only when writing a string. Specifies whether to write a two-byte descriptor for the string describing the length. The default is False.
Exceptions
| Exception type | Error number | Condition |
|---|---|---|
RecordNumber < 1 and not equal to -1. |
||
FileNumber does not exist. |
||
IOException |
File mode is invalid. |
See the "Error number" column if you are upgrading Visual Basic 6.0 applications that use unstructured error handling. (You can compare the error number against the Number Property (Err Object).) However, when possible, you should consider replacing such error control with Structured Exception Handling Overview for Visual Basic.
Remarks
FileGet is valid only in Random and Binary mode.
Data read with FileGet is usually written to a file with FilePut.
The first record or byte in a file is at position 1, the second record or byte is at position 2, and so on. If you omit RecordNumber, the next record or byte following the last FileGet or FilePut function (or pointed to by the last Seek function) is read.
.gif) Security Note |
|---|
When reading from files, do not make decisions about the contents of a file based on the file name extension. For example, a file named Form1.vb may not be a Visual Basic source file. |
Random Mode
For files opened in Random mode, the following rules apply:
If the length of the data being read is less than the length specified in the RecordLength clause of the FileOpen function, FileGet reads subsequent records on record-length boundaries. The space between the end of one record and the beginning of the next record is padded with the existing contents of the file buffer. Because the amount of padding data cannot be determined with any certainty, it is generally a good idea to have the record length match the length of the data being read.
If the variable being read into is a string, by default FileGet reads a two-byte descriptor containing the string length and then reads the data that goes into the variable. Therefore, the record length specified by the RecordLength clause of the FileOpen function must be at least two bytes greater than the actual length of the string. Visual Basic 6.0 and earlier versions support fixed-length strings; when put to a file, the length descriptor is not written. If you want to read a string without the descriptor, you should pass True to the StringIsFixedLength parameter, and the string you read into should be the correct length.
If the variable being read into is an array, you can choose whether to read a descriptor for the size and dimension of the array. To write the descriptor, set the ArrayIsDynamic parameter to True. When reading the array, you need to match the way the array was written. If it was written with the descriptor, you need to read the descriptor. If the descriptor is not used, then the size and bounds of the array passed into FileGet determine what to read.
The descriptor specifies the rank of the array, the size, and the lower bounds for each rank. Its length equals 2 plus 8 times the number of dimensions: (2 + 8 * NumberOfDimensions). The record length specified by the RecordLength parameter in the FileOpen function must be greater than or equal to the sum of all the bytes required to write the array data and the array descriptor. For example, the following array declaration requires 118 bytes when the array is written to disk.
Dim MyArray(4,9) As IntegerThe 118 bytes are distributed as follows:
18 bytes for the descriptor: (2 + 8 * 2)
100 bytes for the data: (5 * 10 * 2).
If the variable being read into is any other type of variable (not a variable-length string or an object), FileGet reads only the variable data. The record length specified by the RecordLength clause in the FileOpen function must be greater than or equal to the length of the data being read.
FileGet reads elements of structures as if each were being read individually, except that there is no padding between elements. On disk, a dynamic array in a user-defined type (written with FilePut) is prefixed by a descriptor whose length equals 2 plus 8 times the number of dimensions: (2 + 8 * NumberOfDimensions). The record length specified by the RecordLength clause in the FileOpen function must be greater than or equal to the sum of all the bytes required to read the individual elements, including any arrays and their descriptors. The VBFixedString attribute can be applied to string fields in the structures to indicate the size of a string when written to disk.
Binary Mode
For files opened in Binary mode, most of the Random mode rules apply, with a few exceptions. The following rules for files opened in Binary mode differ from the rules for Random mode:
The RecordLength clause in the FileOpen function has no effect. FileGet reads all variables from disk contiguously; that is, with no padding between records.
For any array other than an array in a structure, FileGet reads only the data. No descriptor is read.
FileGet reads variable-length strings that are not elements of structures without expecting the two-byte length descriptor. The number of bytes read equals the number of characters already in the string.
Security NoteReading from a file with the FileGet function requires Read access from the FileIOPermissionAccess enumeration.
Smart Device Developer Notes
This function is not supported.
Requirements
Namespace: Microsoft.VisualBasic
Module: FileSystem
Assembly: Visual Basic Runtime Library (in Microsoft.VisualBasic.dll)
See Also
Reference
FileOpen Function
FilePut Function
Seek Function
FileGetObject Function
Other Resources
Reading from Files in Visual Basic
Writing to Files in Visual Basic