File

This class can be used as an interface to a particular file but not to the actual data within the file.
You can use this class to open files but if you want to read and write the contents then you must use the FileStream class.
One drawback of using these methods is that the text streams returned use the UTF-8 encoding.
This is fine for a lot of applications but if the file contains any characters that are not recognised then these are usually just ignored.
Using the StreamReader and StreamWriter constructors directly avoids any possible encoding issues.
These classes also allow you to set the buffer size (useful for optimisation) as well as being able to specify the byte order (to guarantee the correct encoding).

FileMode enumeration

FileMode is an enumeration of values used to specify the type of file access.
FileMode.CreateNew - creates a new file assuming it doesn't already exist.
FileMode.Open -
FileMode.Append -
FileMode.OpenOrCreate - Opens a file if it exists or creates it if it doesn't exist
FileMode.Create - Creates a new file silently overwriting if it already exists.

Open

Opens a file with a specified mode, access and share returning a FileStream object
The sharing mode lets you control whether or not other processes can read from or write to the file while you have it open.
A lot of the arguments correspond to overloads of the FileStream constructor.
This method gives you greater (than the Create, OpenRead and OpenWrite methods) when creating new files.
If you want total control when opening and creating files then you should use the FileStream constructor.

System.IO.File.Open(file [,mode [,access [,share ]]]) 
System.IO.File.Open("C:\temp\file1.txt", _ 
                    FileMode.Append | Create | CreateNew | Open | OpenOrCreate | Truncate, _ 
                    FileAccess.Read | Write | ReadWrite, _ 
                    FileShare.None | ReadWrite | Read | Write | Inheritable 

OpenRead

Opens an existing file in read mode returning a FileStream object
The file is opened using the default sharing mode
The FileStream is positioned at the beginning of the file.

System.IO.File.OpenRead(file) 

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

OpenWrite

Opens a file in write mode returning a FileStream object
Any existing data in the file will be overwritten.
If the file you are trying to open does not exist then it will be created.

System.IO.File.OpenWrite(file); 

OpenText

Opens a text file for reading returning a StreamReader object
This method returns a StreamReader that is positioned at the beginning of the file.

System.IO.File.OpenText(file); 

CreateText

Creates a text file and opens it for reading. This returns a StreamWriter object.
If the file you want to create already exists then it is overwritten.

System.IO.File.CreateText(file); 

AppendText

This method can be used if you want to append data to a text file.
Opens an existing text file in append mode returning a StreamWriter object
If the file you want to open does not exist then a new file is created.

System.IO.File.AppendText(file); 

Copy

Copies a file from one location to another.
This method will fail if the file already exists and you have not explicitly stated to overwrite the file.
You also have the option to overwrite the file it it already exists.

System.IO.File.Copy(source, dest [,overwrite]); 
System.IO.File.Copy("file1.txt","file2.txt",false); 
System.IO.File.Copy("file1.txt","file2.txt",true); 

Delete

Deletes an existing file
This method will throw an exception if the file is in use or if you do not have access (or permission) to delete it.
It will not throw an exception if the file does not exist.

System.IO.File.Delete(path); 

GetCreationTime

Returns a DateTime telling you when the file was created.

System.IO.File.GetCreationTime(path); 

SetCreatiionTime

System.IO.File.SetCreatiionTime(path, datetime); 

GetLastAccessTime

System.IO.File.GetLastAccessTime(path); 

SetLastAccessTime

System.IO.File.SetLastAccessTime(path, datetime); 

GetLastWriteTime

System.IO.File.GetLastWriteTime(path); 

SetLastWriteTime

System.IO.File.SetLastWriteTime(path, datetime);