从文件读取文本

Question

I am writing some code to search a file at the start of the file until a string is found.

Here is the code that works:

    Private Function ReadTextFileUpto(ByVal txtFile As String, ByVal searchString As String) As String
        Dim result As String = String.Empty
        Dim sb As New System.Text.StringBuilder()
        Dim previousLength As Integer = 0
        Using reader As New IO.StreamReader(txtFile)
            'Read the file 1 char at a time and append it to sb
            While reader.Peek >= 0
                sb.Append(Convert.ToChar(reader.Read))
                'store the current length of sb
                previousLength = sb.Length
                'attemp to replace the search string with an empty string. If the length changed after the replacement
                'then the following conditions must be true:
                ' 1. we have found the search string in sb.
                ' 2. the search string was at the end of sb.
                sb.Replace(searchString, String.Empty)
                If sb.Length < previousLength Then
                    'since we have found the search string,  exit the loop and return the string currently in sb.
                    result = sb.ToString()
                    Exit While
                End If
            End While
        End Using
        Return result
    End Function

I am wanting to edit thsi code such that I can choose a start position in the file to start searching.

Here is the code that I am working on:

    Public Function ReadTextFileUpto(ByVal txtFile As String, ByVal searchString As String, integerStartPosition As Integer) As String
    Dim result As String = String.Empty
    Dim sb As New System.Text.StringBuilder()
    Dim previousLength As Integer = 0
    Using reader As New IO.StreamReader(txtFile)
        'Read the file 1 char at a time and append it to sb
        While reader.Peek >= integerStartPosition
            sb.Append(Convert.ToChar(reader.Read))
            'store the current length of sb
            previousLength = sb.Length
            'attemp to replace the search string with an empty string. If the length changed after the replacement
            'then the following conditions must be true:
            ' 1. we have found the search string in sb.
            ' 2. the search string was at the end of sb.
            sb.Replace(searchString, String.Empty)
            If sb.Length < previousLength Then
                'since we have found the search string,  exit the loop and return the string currently in sb.
                result = sb.ToString()
                Exit While
            End If
        End While
    End Using
    Return result
End Function

Can I please have some help to get this working?

FINAL CODE

Public Function ReadTextFileUpto(ByVal txtFile As String, ByVal searchString As String, integerStartPosition As Integer) As String
    Dim result As String = String.Empty
    Dim sb As New System.Text.StringBuilder()
    Dim tmpString As String = ""
    Dim previousLength As Integer = -1
    Dim integerCurrentPosition As Integer 
    Using reader As New IO.StreamReader(txtFile)
        While reader.Peek >= 0

            integerCurrentPosition += 1

            tmpString = Convert.ToChar(reader.Read)

            If integerCurrentPosition > integerStartPosition Then
                sb.Append(tmpString)
                previousLength = sb.Length
                sb.Replace(searchString, String.Empty)
                If sb.Length < previousLength Then
                    result = sb.ToString()
                    Exit While
                End If
            End If

        End While
    End Using
    Return result
End Function

Answer 1

You should put position checking inside the while loop. Here's a simple example how to do it.

Private Function ReadTextFileUpto(ByVal txtFile As String, ByVal searchString As String) As String
            Dim result As String = String.Empty
            Dim sb As New System.Text.StringBuilder()
            Dim previousLength As Integer = 0
            Dim currentPosition as Integer = 0

            Using reader As New IO.StreamReader(txtFile)
                'Read the file 1 char at a time and append it to sb
                While reader.Peek >= 0
                    If currentPosition < integerStartPosition Then
                       Continue While
                    End if 

                    sb.Append(Convert.ToChar(reader.Read))
                    'store the current length of sb
                    previousLength = sb.Length
                    'attemp to replace the search string with an empty string. If the length changed after the replacement
                    'then the following conditions must be true:
                    ' 1. we have found the search string in sb.
                    ' 2. the search string was at the end of sb.
                    sb.Replace(searchString, String.Empty)
                    If sb.Length < previousLength Then
                        'since we have found the search string,  exit the loop and return the string currently in sb.
                        result = sb.ToString()
                        Exit While
                    End If

                    currentPosition = currentPosition + 1
                End While
            End Using
            Return result
        End Function

UPDATE:

Or you can try StreamReader.Read Method (Char[], Int32, Int32) method to read stream into array, specifying starting position.

Reads a maximum of count characters from the current stream into buffer, beginning at index.

For more info check tutorial

StreamReader

Good Luck!