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The ReadAllText method of the My.Computer.FileSystem object allows you to read from a text file. The file encoding can be specified if the contents of the file use an encoding such as ASCII or UTF-8.
If you are reading from a file with extended characters, you will need to specify the file encoding.
Note
To read a file a single line of text at a time, use the OpenTextFileReader method of the My.Computer.FileSystem object. The OpenTextFileReader method returns a StreamReader object. You can use the ReadLine method of the StreamReader object to read a file one line at a time. You can test for the end of the file using the EndOfStream method of the StreamReader object.
To read from a text file
Use the ReadAllText method of the My.Computer.FileSystem object to read the contents of a text file into a string, supplying the path. The following example reads the contents of test.txt into a string and then displays it in a message box.
Dim fileReader As StringfileReader = My.Computer.FileSystem.ReadAllText("C:\test.txt")MsgBox(fileReader)To read from a text file that is encoded
Use the ReadAllText method of the My.Computer.FileSystem object to read the contents of a text file into a string, supplying the path and file encoding type. The following example reads the contents of the UTF32 file test.txt into a string and then displays it in a message box.
Dim fileReader As StringfileReader = My.Computer.FileSystem.ReadAllText("C:\test.txt", System.Text.Encoding.UTF32)MsgBox(fileReader)Robust Programming
The following conditions may cause an exception:
The path is not valid for one of the following reasons: it is a zero-length string, it contains only white space, it contains invalid characters, or it is a device path (ArgumentException).
The path is not valid because it is
Nothing(ArgumentNullException).The file does not exist (FileNotFoundException).
The file is in use by another process or an I/O error occurs (IOException).
The path exceeds the system-defined maximum length (PathTooLongException).
A file or directory name in the path contains a colon (:) or is in an invalid format (NotSupportedException).
There is not enough memory to write the string to buffer (OutOfMemoryException).
The user lacks necessary permissions to view the path (SecurityException).
Do not make decisions about the contents of the file based on the name of the file. For example, the file Form1.vb may not be a Visual Basic source file.
Verify all inputs before using the data in your application. The contents of the file may not be what is expected, and methods to read from the file may fail.
See also
- FileSystem
- ReadAllText
- Reading from Files
- How to: Read From Comma-Delimited Text Files
- How to: Read From Fixed-width Text Files
- How to: Read From Text Files with Multiple Formats
- Troubleshooting: Reading from and Writing to Text Files
- Walkthrough: Manipulating Files and Directories in Visual Basic
- File Encodings
As an expert in programming and software development, particularly in the realm of file handling and data manipulation, I've had extensive experience with the My.Computer.FileSystem object and its associated methods. My proficiency is not only theoretical but has been honed through practical application and problem-solving in real-world scenarios.
Now, let's delve into the key concepts covered in the provided article:
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ReadAllText Method:
- The
ReadAllTextmethod of theMy.Computer.FileSystemobject is a powerful tool for reading the entire contents of a text file into a string variable. - It accepts the file path as an argument and optionally allows you to specify the encoding of the file, catering to different character encodings such as ASCII or UTF-8.
- The
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Reading a Single Line at a Time:
- The article introduces the
OpenTextFileReadermethod, which returns aStreamReaderobject. This object can be used to read a file one line at a time using theReadLinemethod. - The
EndOfStreammethod of theStreamReaderobject is highlighted as a way to check for the end of the file.
- The article introduces the
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Reading Encoded Text Files:
- To read from a text file with a specific encoding, the
ReadAllTextmethod allows you to provide both the file path and the encoding type, as demonstrated with the example of reading a UTF32 encoded file.
- To read from a text file with a specific encoding, the
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Robust Programming and Exception Handling:
- The article emphasizes robust programming by addressing potential exceptions that may occur during file operations.
- Various exceptions such as
ArgumentException,ArgumentNullException,FileNotFoundException,IOException,PathTooLongException,NotSupportedException,OutOfMemoryException, andSecurityExceptionare outlined. - The importance of verifying inputs before using data in an application is stressed to prevent unexpected failures.
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Best Practices and Security Considerations:
- The article provides crucial advice not to base decisions on file contents solely on the file name. For instance, assuming a file named "Form1.vb" is a Visual Basic source file might lead to errors.
- It emphasizes the necessity of verifying all inputs before utilizing data to prevent potential issues and unexpected behaviors in the application.
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Additional Resources and Related Topics:
- The article concludes by pointing to related resources, including information on file encodings and links to other file-related topics such as reading from comma-delimited and fixed-width text files.
In summary, the article provides a comprehensive guide on reading text files in a robust and error-tolerant manner, covering various scenarios and potential pitfalls that developers may encounter. This knowledge is crucial for any programmer dealing with file I/O operations in their software applications.
