Other File I/O

Not all files are plain text files — often we need to read and/or write binary data. .NET provides the FileStream class for this purpose.

The FileStream class provides constructors for creating a FileStream for reading, writing, or both. These constructors can be used to specify how the file is to be opened or created, the type of access to be allowed (i.e., reading/writing), and how the file is to be locked. In most cases, however, a simpler way to construct an appropriate FileStream is to use one of the following static methods provided by the the File class:

  • File.OpenRead(string fn): returns a FileStream for reading the file with the given path name. A FileNotFoundException is thrown if the file does not exist.
  • File.OpenWrite(string fn): returns a FileStream for writing to the file with the given path name. If the file exists, it will be replaced; otherwise, it will be created.

Two of the most commonly-used methods of a FileStream are ReadByte and WriteByte. The ReadByte method takes no parameters and returns an int. If there is at least one byte available to read, the next one is read and its value (a nonnegative integer less than 256) is returned; otherwise, the value returned is -1 (this is the only way to detect when the end of the stream has been reached). The WriteByte method takes a byte as its only parameter and writes it to the file. It returns nothing.

Because a FileStream has no EndOfStream property, we must code a loop to read to the end of the stream somewhat differently from what we have seen before. We can take advantage of the fact that in C#, an assignment statement can be used within an expression. When this is done, the value of the assignment statement is the value that it assigns. Thus, if input is a FileStream opened for input, we can set up a loop to read a byte at a time to the end of the stream as follows:

int k;
while ((k = input.ReadByte()) != -1)
{
    byte b = (byte)k;
    . . .
}

In the above code, the ReadByte method reads a byte from the file as long as there is one to read, and assigns it to the int variable k. If there is no byte to read, it assigns -1 to k. In either case, the value of the assignment statement is the value assigned to k. Thus, if the ReadByte method is at the end of the stream, it returns -1, which is assigned to k, and the loop terminates. Otherwise, the loop iterates, assigning k to b as a byte. The remainder of the iteration can then use the byte read, which is in b.