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File.OpenRead Method

Opens an existing file for reading.

[Visual Basic]
Public Shared Function OpenRead( _
   ByVal path As String _
) As FileStream
[C#]
public static FileStream OpenRead(
 string path
);
[C++]
public: static FileStream* OpenRead(
 String* path
);
[JScript]
public static function OpenRead(
   path : String
) : FileStream;

Parameters

path
The file to be opened for reading.

Return Value

A read-only FileStream on the specified path.

Exceptions

Exception Type Condition
UnauthorizedAccessException The caller does not have the required permission.
ArgumentException path is a zero-length string, contains only white space, or contains one or more invalid characters as defined by InvalidPathChars.
ArgumentNullException path is a null reference (Nothing in Visual Basic).
PathTooLongException 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.
DirectoryNotFoundException The specified path is invalid, such as being on an unmapped drive.
UnauthorizedAccessException path specified a directory.

-or-

The caller does not have the required permission.

FileNotFoundException The file specified in path was not found.
NotSupportedException path is in an invalid format.

Remarks

This method is equivalent to FileStream(String, FileMode.Open, FileAccess.Read, FileShare.Read).

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.

For an example of using this method, see the Example section below. The following table lists examples of other typical or related I/O tasks.

To do this... See the example in this topic...
Create a text file. Writing Text to a File
Write to a text file. Writing Text to a File
Read from a text file. Reading Text from a File
Append text to a file. Opening and Appending to a Log File

File.AppendText

FileInfo.AppendText

Rename or move a file. File.Move

FileInfo.MoveTo

Copy a file. File.Copy

FileInfo.CopyTo

Get the size of a file. FileInfo.Length
Get the attributes of a file. File.GetAttributes
Set the attributes of a file. File.SetAttributes
Determine if a file exists. File.Exists
Read from a binary file. Reading and Writing to a Newly Created Data File
Write to a binary file. Reading and Writing to a Newly Created Data File
Create a directory. CreateDirectory

Directory

Example

[Visual Basic, C#, C++] The following example opens a file for reading.

[Visual Basic] 
Imports System
Imports System.IO
Imports System.Text

Public Class Test
    Public Shared Sub Main()
        Dim path As String = "c:\temp\MyTest.txt"
        Dim fs As FileStream

        ' Delete the file if it exists.
        If File.Exists(path) = False Then
            ' Create the file.
            fs = File.Create(path)
            Dim info As Byte() = New UTF8Encoding(True).GetBytes("This is some text in the file.")

            ' Add some information to the file.
            fs.Write(info, 0, info.Length)
            fs.Close()
        End If

        ' Open the stream and read it back.
        fs = File.OpenRead(path)
        Dim b(1024) As Byte
        Dim temp As UTF8Encoding = New UTF8Encoding(True)

        Do While fs.Read(b, 0, b.Length) > 0
            Console.WriteLine(temp.GetString(b))
        Loop
        fs.Close()
    End Sub
End Class

[C#] 
using System;
using System.IO;
using System.Text;

class Test 
{
    public static void Main() 
    {
        string path = @"c:\temp\MyTest.txt";

        // Delete the file if it exists.
        if (!File.Exists(path)) 
        {
            // Create the file.
            using (FileStream fs = File.Create(path)) 
            {
                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.OpenRead(path)) 
        {
            byte[] b = new byte[1024];
            UTF8Encoding temp = new UTF8Encoding(true);

            while (fs.Read(b,0,b.Length) > 0) 
            {
                Console.WriteLine(temp.GetString(b));
            }
        }
    }
}

[C++] 
#using <mscorlib.dll>
using namespace System;
using namespace System::IO;
using namespace System::Text;

int main() {
    String* path = S"c:\\temp\\MyTest.txt";

    // Delete the file if it exists.
    if (!File::Exists(path)) {
        // Create the file.
        FileStream* fs = File::Create(path);
        try {
            Byte info[] = (new UTF8Encoding(true))->GetBytes(S"This is some text in the file.");

            // Add some information to the file.
            fs->Write(info, 0, info->Length);
        } __finally {
            if (fs) __try_cast<IDisposable*>(fs)->Dispose();
        }
    }

    // Open the stream and read it back.
    FileStream* fs = File::OpenRead(path);
    try {
        Byte b[] = new Byte[1024];
        UTF8Encoding* temp = new UTF8Encoding(true);

        while (fs->Read(b,0,b->Length) > 0) {
            Console::WriteLine(temp->GetString(b));
        }
    } __finally {
        if (fs) __try_cast<IDisposable*>(fs)->Dispose();
    }
}

[JScript] No example is available for JScript. To view a Visual Basic, C#, or C++ example, click the Language Filter button Language Filter in the upper-left corner of the page.

Requirements

Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family, .NET Compact Framework, Common Language Infrastructure (CLI) Standard

.NET Framework Security: 

See Also

File Class | File Members | System.IO Namespace | Working with I/O | Reading Text from a File | Writing Text to a File | Basic File I/O | Reading and Writing to a Newly Created Data File

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