How do I get StreamReader.ReadBlock to start at a specific line

[firespinguy]

Hello everyone. I want streamreader.Readblock to start reading a file from a specific line, but I need some help with this. So far I have have the following example code:

        Dim lineposition As Integer = 20
        Dim buffer As Char() = ""
        Dim count As Integer = 256
        Dim dataReader As New StreamReader("C:\lines.txt")
        For i = lineposition To 100

            dataReader.ReadBlock(buffer, i, count)
            'show data 
            MsgBox("Success " & buffer.ToString)

        Next i
        dataReader.Close()

but I recieve the error: "Offset and length were out of bounds for the array or count is greater than the number of elements from index to the end of the source collection."

[.paul.]

you need to initialize your buffer array, + you also need to be sure that StartPosition + 256 isn't greater than the length of the file:

Dim buffer(255) As Char
Dim dataReader As New IO.StreamReader("C:\lines.txt")
dataReader.ReadBlock(buffer, StartPosition, 256)

[jmcilhinney]

ReadBlock is going to start reading at a specific character offset from the current position. You can't tell it to start reading at a specific line unless you know the character index at which that line starts. If you want start reading a file at the 20th line and you don't know exactly how long those lines are, you have no choice but to simply read all lines and discard the ones you don't need.

[firespinguy]

If you want start reading a file at the 20th line and you don't know exactly how long those lines are, you have no choice but to simply read all lines and discard the ones you don't need.

I guess I can't use readblock then, thanks.

[youthbytes]

Heres a function that I wrote a while back that will allow you to read text from a file with parameters for maxLines / startLine. Hope this helps.

    Public Function ReadFile(ByVal Path As String, _
                             Optional ByVal MaxLines As Integer = 0, _
                             Optional ByVal StartLine As Integer = 0) As String
        Try
            If Not System.IO.File.Exists(Path) Then Return String.Empty ' Make sure the file exists
            ' Create our counter (how many lines have been read) and currentline (self explanitory) variables
            Dim Count As Integer = 0 : Dim CurrentLine As Integer = 0
            Dim sb As New System.Text.StringBuilder ' Create a new stringbuilder
            Dim sr As New System.IO.StreamReader(New System.IO.FileStream(Path, System.IO.FileMode.Open, System.IO.FileAccess.Read)) ' Read file into stream
            While Not sr.EndOfStream ' Loop while we aren't at the end of the stream
                ' If maxlines is not unlimited (0) and number of lines exceeds the maxlines exit the loop
                If Not MaxLines = 0 AndAlso Count >= MaxLines Then Exit While
                Dim Line As String = sr.ReadLine ' Read current line of stream
                If Not String.IsNullOrEmpty(Line) Then ' Make sure that the line is not an empty string
                    If Not StartLine = 0 Then ' Check if we need to monitor for the line to start reading at
                        If CurrentLine >= StartLine Then ' The current line is equal to or greater than the startline
                            sb.AppendLine(Line) ' Add line to string builder
                            Count += 1 ' Update added lines counter
                        End If
                    Else
                        sb.AppendLine(Line) ' Add line to string builder
                        Count += 1 ' Update added lines counter
                    End If
                End If
                CurrentLine += 1 ' Update current line counter
            End While
            sr.Close() ' Close stream reader
            ' Trim the contents of the string builder (so there is no blank trailing line) and return
            Return sb.ToString.Trim
        Catch ex As Exception
            Debug.Print(ex.ToString) ' Debug the exception so we can see what went wrong
            Return String.Empty ' Return empty string due to exception
        End Try
    End Function