Reading Text from a File

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How do I... Read Text Files in VBCorLib?

Reading information from files is a common task. VBCorLib provides many facilities for reading files. One of the most common reasons to read a file is to read the text within. A text file is a series of bytes that represent text that is encoded using a specific Encoding. To read the encoded bytes of text, they must be decoded using an appropriate Encoding back into the original text. This is what the StreamReader does. A StreamReader takes a series of bytes from a Stream (such as a FileStream) and decodes them using an Encoding, returning the decoded text in one of many methods supplied by StreamReader. By default a StreamReader uses an Encoding returned by Encoding.UTF8.

VBCorLib provides several methods for opening a file to be read from. The simplest way to open a StreamReader for reading a file is directly creating the object, passing in the filename.

    Dim reader As StreamReader

    ' create a StreamReader directly, passing in the file name.
    Set reader = NewStreamReader("lines.txt")

This will open an existing file using the default encoding Encoding.UTF8. If the file does not exist, then a FileNotFoundException will be thrown. Since the file is to be read, then it makes sence that the file must exist for a StreamReader to be used.

The File class provides many methods for opening a file for reading. For this specific instance, using the File.OpenText method is suitable. This method returns a StreamReader object for reading from the specified file.

    Dim reader As StreamReader

    ' Opens a reader the same as NewStreamReader does,
    ' but uses one of the File methods.
    Set reader = File.OpenText("lines.txt")

This method of opening a file for reading is identical to the previous example of calling NewStreamReader directly. File.OpenText merely wraps an identical call as the previous example.

The two previous examples show how to open a file directly with a file name. Sometimes a file may already be open as a FileStream, or may need to be opened in a more custom manor. A StreamReader can accept a FileStream object (or any object implementing the Stream interface) inplace of the file name.

    Dim reader As StreamReader
	
    ' Opens a reader using a FileStream object returned from File.OpenRead.
    Set reader = NewStreamReader(File.OpenRead("lines.txt"))

This opens a FileStream in read-only mode. The FileStream is then passed into the NewStreamReader method. The StreamReader is now ready for reading from the supplied FileStream.

Once a StreamReader is open, there are many methods provided to retrieve the text from the file. The lines of a file can be read using the ReadLine method. This method reads characters until a new-line character is read in. The returned line does not include the new-line character. The StreamReader defines both a Carriage-Return and a Line-Feed as valid new-line characters. If a Carriage-Return is immediately followed by a Line-Feed, then the Line-Feed is also consumed and not returned.

    Dim reader As StreamReader
    Dim strLine As String
    Dim lines As ArrayList

    Set reader = NewStreamReader("lines.txt")
    Set lines = New ArrayList

    Do
        strLine = reader.ReadLine

        ' ReadLine will return a null String if there is nothing left to read from the Stream.
        If cString.IsNull(strLine) Then Exit Do
        lines.Add strLine
    Loop

    reader.CloseReader

This example opens a file containing lines of text separated by either a Carriage-Return or Line-Feed. The StreamReader is then used to read each line individually and adds them to an ArrayList. Something to note is that strLine is defined as a Variant datatype. This is because the StreamReader.ReadLine will return a Null if there is nothing remaining in the Stream to be read. Once the lines have been read from the file and the loop is exited, the StreamReader.CloseReader is called. This will close the underlying Stream and release any resources.

Summary

There are many ways to create a new StreamReader. Each provides a methods for opening a file in a specific way. Once a StreamReader is created, it provides many methods for retrieving decoded text from the stream being read from.

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