Advanced Text File I/O

Though the File.ReadAllText and File.WriteAllText methods provide simple mechanisms for reading and writing text files, they are not always the best choices. For one reason, files can be very large — too large to fit into memory, or possibly even larger than the maximum length of a string in C# (2,147,483,647 characters). Even when it is possible to store the entire contents of a file as a string, it may not be desirable, as the high memory usage may degrade the overall performance of the system.

For the purpose of handling a sequence of input or output data in more flexible ways, .NET provides streams. These streams are classes that provide uniform access to a wide variety of sequences of input or output data, such as files, network connections, other processes, or even blocks of memory. The StreamReader and StreamWriter classes (in the System.IO namespace) provide read and write, respectively, access to text streams, including text files.

Some of the more useful public members of the StreamReader class are:

  • A constructor that takes a string giving a file name as its only parameter and constructs a StreamReader to read from that file.
  • A Read method that takes no parameters. It reads the next character from the stream and returns it as an int. If it cannot read a character because it is already at the end of the stream, it returns -1 (it returns an int because -1 is outside the range of char values).
  • A ReadLine method that takes no parameters. It reads the next line from the stream and returns it as a string?. If it cannot read a line because it is already at the end of the stream, it returns null.
  • An EndOfStream property that gets a bool indicating whether the end of the stream has been reached.

With these members, we can read a text file either a character at a time or a line at a time until we reach the end of the file. The StreamWriter class has similar public members:

  • A constructor that takes a string giving a file name as its only parameter and constructs a StreamWriter to write to this file. If the file already exists, it is replaced by what is written by the StreamWriter; otherwise, a new file is created.
  • A Write method that takes a char as its only parameter. It writes this char to the end of the stream.
  • Another Write method that takes a string? as its only parameter. It writes this string? to the end of the stream. If the given string? is null, nothing is written.
  • A WriteLine method that takes no parameters. It writes a line terminator to the end of the stream (i.e., it ends the current line of text).
  • Another WriteLine method that takes a char as its only parameter. It writes this char to the end of the stream, then terminates the current line of text.
  • Yet another WriteLine method that takes a string? as its only parameter. It writes this string? to the end of the stream, then terminates the current line of text. If the string? is null, only the line terminator is written.

Thus, with a StreamWriter, we can build a text file a character at a time, a line at a time, or an arbitrary string at a time. In fact, a number of other Write and WriteLine methods exist, providing the ability to write various other types, such as int or double. In each case, the given value is first converted to a string, then written to the stream.

Streams are different from other classes, such as strings or arrays, in that they are unmanaged resources. When a managed resource, such as a string or an array, is no longer being used by the program, the garbage collector will reclaim the space that it occupies so that it can be allocated to new objects that may need to be constructed. However, after a stream is constructed, it remains under the control of the program until the program explicitly releases it. This has several practical ramifications. For example, the underlying file remains locked, restricting how other programs may use it. In fact, if an output stream is not properly closed by the program, some of the data written to it may not actually reach the underlying file. This is because output streams are typically buffered for efficiency — when bytes are written to the stream, they are first accumulated in an internal array, then written as a single block when the array is full. When the program is finished writing, it needs to make sure that this array is flushed to the underlying file.

Both the StreamReader and StreamWriter classes have Dispose methods to release them properly; however, because I/O typically requires exception handling, it can be tricky to ensure that this method is always called when the I/O is finished. Specifically, the try-catch may be located in a method that does not have access to the stream. In such a case, the catch-block cannot call the stream’s Dispose method.

To handle this difficulty, C# provides a using statement. A using statement is different from a using directive, such as

using System.IO;

A using statement occurs within a method definition, not at the top of a code file. Its recommended form is as follows:

using ( /* declaration and initialization of disposable variable(s) */ )

    /* Code that uses the disposable variables(s) */


Thus, if we want to read and process a text file whose name is given by the string variable fileName, we could use the following code structure:

using (StreamReader input = new StreamReader(fileName))

    /* Code that reads and process the file accessed by the
     * StreamReader input */


This declares the variable input to be of type StreamReader and initializes it to a new StreamReader to read the given file. This variable is only visible with the braces; furthermore, it is read-only — its value cannot be changed to refer to a different StreamReader. The using statement then ensures that whenever control exits the code within the braces, input’s Dispose method is called.

More than one variable of the same type may be declared and initialized within the parentheses of a using statement; for example:

using (StreamReader input1 = new StreamReader(fileName1),
    input2 = new StreamReader(fileName2))

    /* Code that reads from input1 and input2 */


The type of variable(s) declared must be a subtype of IDisposable . This ensures that the variables each have a Dispose method.

As a complete example of the use of a StreamReader and a StreamWriter, together with a using statement for each, suppose we want to write a method that takes as its parameters two strings giving the name of an input file and the name of an output file. The method is to reproduce the input file as the output file, but with each line prefixed by a line number and a tab. We will start numbering lines with 1. The following method accomplishes this:

/// <summary>
/// Copies the file at inFileName to outFileName with each line
/// prefixed by its line number followed by a tab.
/// </summary>
/// <param name="inFileName">The path name of the input file.</param>
/// <param name="outFileName">The path name of the output file.</param>
private void AddLineNumbers(string inFileName, string outFileName)
    using (StreamReader input = new StreamReader(inFileName))
        using (StreamWriter output = new StreamWriter(outFileName))
            int count = 0;
            while (!input.EndOfStream)
                // Because input is not at the end of the stream, its ReadLine
                // method won't return null.
                string line = input.ReadLine()!;
                output.Write('\t');   // The tab character

As noted above, a StreamReader’s ReadLine method has a return type of string? because it will return null if the end of the stream has already been reached. Furthermore, the compiler is unable to determine that the loop condition will prevent the call to ReadLine from returning null. Thus, in order to suppress the compiler warning when the returned string? is assigned to a string, we include a ! following the call to ReadLine, and document the reason with a comment above this line.

We can call the above method within a try-block to handle any exceptions that may be thrown during the I/O. The catch-block will not have access to either input or output, but it doesn’t need it. If an exception is thrown during the I/O, the two using statements will ensure that the Dispose methods of both the StreamReader and the StreamWriter are called.