System.IO.FileStream

Use the FileStream class to read from, write to, open, and close files on a file system
This is one of the classes derived from the Stream class.
File streams are buffered
Data is not written to a file when you write to the FileStream.

Once you have a filestream of the data you want you need to attach it to a Reader or Writer to be able to use it.
There are two options:
Plain Text - Use StreamReader and StreamWriter classes
Binary Data - Use BinaryReader and BinaryWriter classes

BeginRead	start an asynchronous operation on the stream
BeginWrite	start an asynchronous operation on the stream
Close	Closes the stream
EndRead	To find out how many bytes were actually written
Flush	Empties the buferred stream and ensures all the contents are written
Handle	Returns the operating system file handle
Length	Returns the total size of the stream. You can change the length using SetLength
Lock	Lock a portion of the file
Position	determines the current position in the stream. You can change the position using Seek.
Read	Reads a number of bytes from the specified position into a Byte array. The ReadByte method reads and returns a single byte.
SetLength	Trims or extends the underlying file.
Unlock	Unlock a portion of the file
Write	Writes a number of bytes from an array into the stream. The WriteByte can write a single byte.

You can check which operations are allowed by using the CanRead, CanWrite and CanSeek properties.

System.IO.FileStream oFileStream; 
oFileStream = System.IO.FileStream("C:\Temp\textfile.txt", 
    System.IO.FileMode.Open, 
    System.IO.FileAccess.Read, 
    System.IO.FileShare.Read); 

   if (oFileStream.CanRead == true) 
   { 
   } 

System.IO.FileMode.Append | Create | CreateNew | Open | OpenOrCreate | Truncate 
System.IO.FileAccess.Read | Write | ReadWrite 
System.IO.FileShare.None | ReadWrite | Read | Write | Inheritable

Using File.OpenRead

This is another way of obtaining a FileStream object.

System.IO.FileStream oFileStream; 
oFileStream = System.IO.File.OpenRead("C:\Temp\textfile.txt");

Stream Base Class

System.IO.Stream