Input Function

Returns a String containing characters from a file opened in Input or Binary mode.

Syntax

Input(number, [#]filenumber)

Parameters

number (Required): Long expression specifying the number of characters to return
filenumber (Required): Integer file number used in the Open statement (the # is optional)

Return Value

Returns a String containing number characters read from the file. If fewer than number characters remain in the file, returns all remaining characters.

Remarks

The Input function reads data from files: - Used with files opened in Input or Binary mode - Returns exactly the number of characters requested (or fewer if end of file reached) - Does not skip or ignore any characters (unlike Input # statement) - Reads all characters including commas, quotes, line feeds, carriage returns, etc. - The file pointer advances by the number of characters read - Use EOF function to check for end of file before reading - For Binary mode files, reads raw bytes - For Input mode files, reads text characters - Cannot be used with files opened in Output or Append mode - The # symbol before filenumber is optional but commonly used

Typical Uses

Binary File Reading: Read fixed-size chunks from binary files
Text File Reading: Read specific number of characters from text files
Fixed-Width Records: Read fixed-width record data
File Parsing: Read file content for custom parsing
Header Reading: Read file headers of known size
Buffer Reading: Read file content into memory buffers

Basic Usage Examples

' Example 1: Read 10 characters from a file
Dim fileNum As Integer
Dim content As String
fileNum = FreeFile
Open "data.txt" For Input As #fileNum
content = Input(10, #fileNum)
Close #fileNum
' Example 2: Read entire file
Dim fileNum As Integer
Dim fileContent As String
Dim fileSize As Long
fileNum = FreeFile
Open "document.txt" For Input As #fileNum
fileSize = LOF(fileNum)
fileContent = Input(fileSize, #fileNum)
Close #fileNum
' Example 3: Read file in chunks
Dim fileNum As Integer
Dim chunk As String
fileNum = FreeFile
Open "data.bin" For Binary As #fileNum
Do While Not EOF(fileNum)
    chunk = Input(1024, #fileNum)
    ProcessChunk chunk
Loop
Close #fileNum
' Example 4: Read fixed-width record
Dim fileNum As Integer
Dim record As String
fileNum = FreeFile
Open "records.dat" For Binary As #fileNum
record = Input(80, #fileNum)  ' Read 80-character record
Close #fileNum

Common Patterns

' Pattern 1: Read entire file into string
Function ReadFileToString(fileName As String) As String
    Dim fileNum As Integer
    Dim fileSize As Long
    fileNum = FreeFile
    Open fileName For Input As #fileNum
    fileSize = LOF(fileNum)
    If fileSize > 0 Then
        ReadFileToString = Input(fileSize, #fileNum)
    Else
        ReadFileToString = ""
    End If
    Close #fileNum
End Function
' Pattern 2: Read file in fixed-size chunks
Sub ReadFileInChunks(fileName As String, chunkSize As Long)
    Dim fileNum As Integer
    Dim chunk As String
    fileNum = FreeFile
    Open fileName For Binary As #fileNum
    Do While Not EOF(fileNum)
        chunk = Input(chunkSize, #fileNum)
        Debug.Print "Read " & Len(chunk) & " bytes"
    Loop
    Close #fileNum
End Sub
' Pattern 3: Read file header
Function ReadFileHeader(fileName As String, headerSize As Long) As String
    Dim fileNum As Integer
    fileNum = FreeFile
    Open fileName For Binary As #fileNum
    If LOF(fileNum) >= headerSize Then
        ReadFileHeader = Input(headerSize, #fileNum)
    Else
        ReadFileHeader = ""
    End If
    Close #fileNum
End Function
' Pattern 4: Read until delimiter found
Function ReadUntilDelimiter(fileNum As Integer, delimiter As String) As String
    Dim result As String
    Dim char As String
    result = ""
    Do While Not EOF(fileNum)
        char = Input(1, #fileNum)
        If char = delimiter Then
            Exit Do
        End If
        result = result & char
    Loop
    ReadUntilDelimiter = result
End Function
' Pattern 5: Peek at file content without closing
Function PeekFileContent(fileNum As Integer, bytes As Long) As String
    Dim currentPos As Long
    Dim content As String
    currentPos = Seek(fileNum)
    content = Input(bytes, #fileNum)
    Seek fileNum, currentPos  ' Restore position
    PeekFileContent = content
End Function
' Pattern 6: Read line character by character
Function ReadCustomLine(fileNum As Integer) As String
    Dim result As String
    Dim char As String
    result = ""
    Do While Not EOF(fileNum)
        char = Input(1, #fileNum)
        If char = vbCr Or char = vbLf Then
            ' Skip additional line feed if CRLF
            If char = vbCr And Not EOF(fileNum) Then
                If Input(1, #fileNum) <> vbLf Then
                    Seek fileNum, Seek(fileNum) - 1
                End If
            End If
            Exit Do
        End If
        result = result & char
    Loop
    ReadCustomLine = result
End Function
' Pattern 7: Read binary structure
Function ReadBinaryStruct(fileNum As Integer, structSize As Long) As Byte()
    Dim data As String
    Dim bytes() As Byte
    Dim i As Long
    data = Input(structSize, #fileNum)
    ReDim bytes(0 To Len(data) - 1)
    For i = 0 To Len(data) - 1
        bytes(i) = Asc(Mid$(data, i + 1, 1))
    Next i
    ReadBinaryStruct = bytes
End Function
' Pattern 8: Safe read with EOF check
Function SafeRead(fileNum As Integer, numChars As Long) As String
    Dim available As Long
    Dim toRead As Long
    available = LOF(fileNum) - Seek(fileNum) + 1
    toRead = IIf(numChars < available, numChars, available)
    If toRead > 0 Then
        SafeRead = Input(toRead, #fileNum)
    Else
        SafeRead = ""
    End If
End Function
' Pattern 9: Read with progress tracking
Function ReadFileWithProgress(fileName As String) As String
    Dim fileNum As Integer
    Dim fileSize As Long
    Dim bytesRead As Long
    Dim chunk As String
    Dim result As String
    Const CHUNK_SIZE As Long = 4096
    fileNum = FreeFile
    Open fileName For Binary As #fileNum
    fileSize = LOF(fileNum)
    result = ""
    bytesRead = 0
    Do While Not EOF(fileNum)
        chunk = Input(CHUNK_SIZE, #fileNum)
        result = result & chunk
        bytesRead = bytesRead + Len(chunk)
        ' Update progress (0 to 100)
        DoEvents
        Debug.Print "Progress: " & (bytesRead * 100 / fileSize) & "%"
    Loop
    Close #fileNum
    ReadFileWithProgress = result
End Function
' Pattern 10: Read specific byte range
Function ReadByteRange(fileName As String, startPos As Long, numBytes As Long) As String
    Dim fileNum As Integer
    fileNum = FreeFile
    Open fileName For Binary As #fileNum
    Seek fileNum, startPos
    ReadByteRange = Input(numBytes, #fileNum)
    Close #fileNum
End Function

Advanced Usage Examples

' Example 1: Binary file reader class
Public Class BinaryFileReader
    Private m_fileNum As Integer
    Private m_fileName As String
    Private m_isOpen As Boolean
    Public Sub OpenFile(fileName As String)
        If m_isOpen Then CloseFile
        m_fileName = fileName
        m_fileNum = FreeFile
        Open m_fileName For Binary As #m_fileNum
        m_isOpen = True
    End Sub
    Public Function ReadBytes(numBytes As Long) As String
        If Not m_isOpen Then
            Err.Raise 5, , "File not open"
        End If
        If EOF(m_fileNum) Then
            ReadBytes = ""
        Else
            ReadBytes = Input(numBytes, #m_fileNum)
        End If
    End Function
    Public Function ReadAll() As String
        If Not m_isOpen Then
            Err.Raise 5, , "File not open"
        End If
        Dim fileSize As Long
        fileSize = LOF(m_fileNum) - Seek(m_fileNum) + 1
        If fileSize > 0 Then
            ReadAll = Input(fileSize, #m_fileNum)
        Else
            ReadAll = ""
        End If
    End Function
    Public Sub CloseFile()
        If m_isOpen Then
            Close #m_fileNum
            m_isOpen = False
        End If
    End Sub
    Private Sub Class_Terminate()
        CloseFile
    End Sub
End Class
' Example 2: Custom file parser
Function ParseFixedWidthFile(fileName As String) As Collection
    Dim fileNum As Integer
    Dim records As New Collection
    Dim recordData As String
    Const RECORD_SIZE As Long = 100
    fileNum = FreeFile
    Open fileName For Binary As #fileNum
    Do While Not EOF(fileNum)
        recordData = Input(RECORD_SIZE, #fileNum)
        If Len(recordData) = RECORD_SIZE Then
            records.Add ParseRecord(recordData)
        End If
    Loop
    Close #fileNum
    Set ParseFixedWidthFile = records
End Function
' Example 3: File comparison utility
Function CompareFiles(file1 As String, file2 As String) As Boolean
    Dim fileNum1 As Integer, fileNum2 As Integer
    Dim chunk1 As String, chunk2 As String
    Const CHUNK_SIZE As Long = 8192
    fileNum1 = FreeFile
    Open file1 For Binary As #fileNum1
    fileNum2 = FreeFile
    Open file2 For Binary As #fileNum2
    ' Check file sizes
    If LOF(fileNum1) <> LOF(fileNum2) Then
        Close #fileNum1
        Close #fileNum2
        CompareFiles = False
        Exit Function
    End If
    ' Compare content
    Do While Not EOF(fileNum1)
        chunk1 = Input(CHUNK_SIZE, #fileNum1)
        chunk2 = Input(CHUNK_SIZE, #fileNum2)
        If chunk1 <> chunk2 Then
            Close #fileNum1
            Close #fileNum2
            CompareFiles = False
            Exit Function
        End If
    Loop
    Close #fileNum1
    Close #fileNum2
    CompareFiles = True
End Function
' Example 4: Large file reader with buffering
Function ReadLargeFile(fileName As String, Optional bufferSize As Long = 32768) As String
    Dim fileNum As Integer
    Dim result As String
    Dim chunk As String
    Dim chunks As Collection
    Dim i As Long
    Set chunks = New Collection
    fileNum = FreeFile
    Open fileName For Binary As #fileNum
    ' Read in chunks
    Do While Not EOF(fileNum)
        chunk = Input(bufferSize, #fileNum)
        chunks.Add chunk
    Loop
    Close #fileNum
    ' Concatenate chunks
    result = ""
    For i = 1 To chunks.Count
        result = result & chunks(i)
    Next i
    ReadLargeFile = result
End Function

Error Handling

The Input function can raise several errors: - Bad file mode (Error 54): If file is not opened in Input or Binary mode - Bad file number (Error 52): If filenumber is invalid or file is not open - Input past end of file (Error 62): Only if reading past EOF (rare, usually returns partial data) - Type Mismatch (Error 13): If number parameter is not numeric

On Error GoTo ErrorHandler
Dim fileNum As Integer
Dim content As String
fileNum = FreeFile
Open "data.txt" For Input As #fileNum
content = Input(LOF(fileNum), #fileNum)
Close #fileNum
Exit Sub
ErrorHandler:
    If fileNum > 0 Then Close #fileNum
    MsgBox "Error reading file: " & Err.Description, vbCritical

Performance Considerations

Buffer Size: Reading in larger chunks (4KB-32KB) is more efficient than single bytes
String Concatenation: For large files, use collection of chunks then join
Memory Usage: Reading entire large files into memory can cause issues
File Mode: Binary mode is faster than Input mode for raw data
LOF Function: Call LOF once and store result rather than calling repeatedly

Best Practices

Check EOF: Always check EOF before reading to avoid errors
Close Files: Always close files in error handlers to prevent leaks
Use LOF: Use LOF to determine file size before reading entire file
Chunk Reading: Read large files in chunks to manage memory
Binary Mode: Use Binary mode for most file reading operations
Error Handling: Wrap file operations in proper error handling
Free Resources: Close files as soon as done reading

Comparison with Other Functions

Function/Statement	Purpose	Usage
`Input`	Read exact number of characters	`s = Input(100, #1)`
`Input #`	Read comma-delimited data	`Input #1, var1, var2`
`Line Input #`	Read entire line	`Line Input #1, s`
`Get`	Read binary data into variables	`Get #1, , myVar`
`LOF`	Get file length	`size = LOF(1)`
`EOF`	Check end of file	`If EOF(1) Then...`

Platform and Version Notes

Available in all VB6 versions
Consistent behavior across Windows platforms
Maximum string length limitations apply (approximately 2GB in VB6)
File must be opened before using Input function
The # symbol before filenumber is optional

Limitations

Cannot be used with files opened in Output or Append mode
Reading very large files into single string may cause memory issues
No built-in Unicode support (use ADODB.Stream for Unicode)
String concatenation for large files can be slow
Limited to approximately 2GB string size on 32-bit systems
No built-in compression or encoding support

Input #: Statement for reading delimited data from files
Line Input #: Statement for reading complete lines
Get: Statement for reading binary data into variables
LOF: Returns the size of an open file in bytes
EOF: Returns True if at end of file
Seek: Function/statement for getting/setting file position
Open: Statement for opening files
Close: Statement for closing files