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File.Open Method (String, FileMode, FileAccess)

Opens a FileStream on the specified path, with the specified mode and access.

Namespace:  System.IO
Assembly:  mscorlib (in mscorlib.dll)

public static FileStream Open(
	string path,
	FileMode mode,
	FileAccess access


Type: System.String
The file to open.
Type: System.IO.FileMode
A FileMode value that specifies whether a file is created if one does not exist, and determines whether the contents of existing files are retained or overwritten.
Type: System.IO.FileAccess
A FileAccess value that specifies the operations that can be performed on the file.

Return Value

Type: System.IO.FileStream
An unshared FileStream that provides access to the specified file, with the specified mode and access.


path is a zero-length string, contains only white space, or contains one or more invalid characters as defined by InvalidPathChars.


access specified Read and mode specified Create, CreateNew, Truncate, or Append.


path is null.


The specified path, file name, or both exceed the system-defined maximum length. For example, on Windows-based platforms, paths must be less than 248 characters, and file names must be less than 260 characters.


The specified path is invalid, (for example, it is on an unmapped drive).


An I/O error occurred while opening the file.


path specified a file that is read-only and access is not Read.


path specified a directory.


The caller does not have the required permission.


mode is Create and the specified file is a hidden file.


mode or access specified an invalid value.


The file specified in path was not found.


path is in an invalid format.

The path parameter is permitted to specify relative or absolute path information. Relative path information is interpreted as relative to the current working directory. To obtain the current working directory, see GetCurrentDirectory.

The following example opens a file with read-only access.

using System;
using System.IO;
using System.Text;

class Test 
    public static void Main() 
        // This sample assumes that you have a folder named "c:\temp" on your computer.
        string filePath = @"c:\temp\MyTest.txt";

        // Delete the file if it exists.
        if (File.Exists(filePath)) 

        // Create the file.
        using (FileStream fs = File.Create(filePath)) 
            Byte[] info = new UTF8Encoding(true).GetBytes("This is some text in the file.");
            // Add some information to the file.
            fs.Write(info, 0, info.Length);

        // Open the stream and read it back.
        using (FileStream fs = File.Open(filePath, FileMode.Open, FileAccess.Read)) 
            byte[] b = new byte[1024];
            UTF8Encoding temp = new UTF8Encoding(true);

            while (fs.Read(b,0,b.Length) > 0) 

                // Try to write to the file.
            catch (Exception e) 
                Console.WriteLine("Writing was disallowed, as expected: {0}", e.ToString());

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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