System.IO.FileStream class

This article provides supplementary remarks to the reference documentation for this API.

Use the FileStream class to read from, write to, open, and close files on a file system, and to manipulate other file-related operating system handles, including pipes, standard input, and standard output. You can use the Read, Write, CopyTo, and Flush methods to perform synchronous operations, or the ReadAsync, WriteAsync, CopyToAsync, and FlushAsync methods to perform asynchronous operations. Use the asynchronous methods to perform resource-intensive file operations without blocking the main thread. This performance consideration is particularly important in a Windows 8.x Store app or desktop app where a time-consuming stream operation can block the UI thread and make your app appear as if it is not working. FileStream buffers input and output for better performance.

Important

This type implements the IDisposable interface. When you have finished using the type, you should dispose of it either directly or indirectly. To dispose of the type directly, call its Dispose method in a try/catch block. To dispose of it indirectly, use a language construct such as using (in C#) or Using (in Visual Basic). For more information, see the "Using an Object that Implements IDisposable" section in the IDisposable interface topic.

The IsAsync property detects whether the file handle was opened asynchronously. You specify this value when you create an instance of the FileStream class using a constructor that has an isAsync, useAsync, or options parameter. When the property is true, the stream utilizes overlapped I/O to perform file operations asynchronously. However, the IsAsync property does not have to be true to call the ReadAsync, WriteAsync, or CopyToAsync method. When the IsAsync property is false and you call the asynchronous read and write operations, the UI thread is still not blocked, but the actual I/O operation is performed synchronously.

The Seek method supports random access to files. Seek allows the read/write position to be moved to any position within the file. This is done with byte offset reference point parameters. The byte offset is relative to the seek reference point, which can be the beginning, the current position, or the end of the underlying file, as represented by the three members of the SeekOrigin enumeration.

Note

Disk files always support random access. At the time of construction, the CanSeek property value is set to true or false depending on the underlying file type. If the underlying file type is FILE_TYPE_DISK, as defined in winbase.h, the CanSeek property value is true. Otherwise, the CanSeek property value is false.

If a process terminates with part of a file locked or closes a file that has outstanding locks, the behavior is undefined.

For directory operations and other file operations, see the File, Directory, and Path classes. The File class is a utility class that has static methods primarily for the creation of FileStream objects based on file paths. The MemoryStream class creates a stream from a byte array and is similar to the FileStream class.

For a list of common file and directory operations, see Common I/O Tasks.

Detection of stream position changes

When a FileStream object does not have an exclusive hold on its handle, another thread could access the file handle concurrently and change the position of the operating system's file pointer that is associated with the file handle. In this case, the cached position in the FileStream object and the cached data in the buffer could be compromised. The FileStream object routinely performs checks on methods that access the cached buffer to ensure that the operating system's handle position is the same as the cached position used by the FileStream object.

If an unexpected change in the handle position is detected in a call to the Read method, .NET discards the contents of the buffer and reads the stream from the file again. This can affect performance, depending on the size of the file and any other processes that could affect the position of the file stream.

If an unexpected change in the handle position is detected in a call to the Write method, the contents of the buffer are discarded and an IOException exception is thrown.

A FileStream object will not have an exclusive hold on its handle when either the SafeFileHandle property is accessed to expose the handle or the FileStream object is given the SafeFileHandle property in its constructor.