.NET 2.0 Workaround for PathTooLongException

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Introduction

File operations like File.OpenRead or the FileStream constructor would throw a PathTooLongException when presented with an absolute path of length greater than 260 characters.

Looking at the Win32 API, I found that the CreateFileW (Unicode) version of the function supports long file names up to 32000 characters if the file path is prefixed with a \\?\. I went ahead and created this class that has Open methods similar to File.Open/OpenRead/OpenWrite to make the interface consistent.

Background

In ANSI mode of the CreateFile API, Windows restricts the total file length to 260 characters. You can see this effect when you try to create a new file and rename to something more than 260 characters. Windows will automatically limit you to the total file path=260, starting from c:\.

Using the code

I have attached a class in the System.IO namespace called Win32File. It has the same interface as File.Open/OpenRead/OpenWrite.

//Sample Usage

FileStream fs1= Win32File.Open( "Long file name of greater " + 
                "than 260 char length", FileMode.Open);
StreamWriter sw = new StreamWriter(fs1);
sw.WriteLine("Hello world");
sw.Close();
sw.Dispose();

History

Version 1.0.
Version 1.1 -- Updated code to accommodate UNC paths. The prefix needs to be \\?\UNC\.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Balaji Lakshmanan

Software Developer (Senior)

United States

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Great code! and a question

langalibalele

0:23 19 Mar '12

First Implementation of Win32 API for long file names which actually worked, thanks!
I am getting the Message "Error opening file Win32 Error:123" when trying to run your method Open on

\\\\?\\UNC\\?\\UNC\\hts10077\\fs000000076\\Exe_Off\\GlobalTechI_CE00975\\Gen\\02_Project-Execution\\02-02_Design\\02_Clients-Documents\\03_M5000\\01_M5000 Installation Manual\\05_Installation_Manual_110830\\Document List (Falldorf)\\47_Plug & Play Elektrische Verbindungen komplett\\47_MS-~2.PDF

Does it not like "~" in the filename?

My vote of 5

hoernchenmeister

21:02 1 Feb '12

Hi Balaji, thanks for sharing this, I could successfully fix a problem when compressing files from a UNC path using your code. You saved me quite a lot of hasseling Cheers Andy
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Alternative: SUBST

Juy Juka

22:51 28 Nov '11

Hello @All, Hello Blaji Lakshmanan,

Sometimes your solution can not be used, because it isn't possible to replace every call where a File is opened (for example when using 3rd-Party products).
If you have acces to the system you could use a Windows-Command to map the too long path to a shorter path:
SUBST (see CMD->HELP SUBST for details)
You can create a virtual drive for the big part of the path and access the file(s) over that virtual drive with all the normal File-Methods. Of cause this is more usefull when you have many files in the same path (and subdirectories) and of no uses if the filename itself is too long.

Greetings
Juy Juka

Thanks for posting, but a few tips

Geert van Horrik

21:26 21 Jun '10

Great, this way, I don't have to write this myself. However, you could have saved yourself from a lot of redundant code:

public static System.IO.FileStream Open(string filepath, FileMode mode)
{
return Open(filepath, mode, FileAccess.ReadWrite);
}

public static System.IO.FileStream Open(string filepath, FileMode mode, FileAccess access)
{
return Open(filepath, mode, access, FileShare.None);
}

Then, you only have to implement the final Open method, which saves you about 50 % of your file (and redundant code).

Geert

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FileMode.Create

Henk Meijerink

5:38 19 Aug '09

Thanks for this very useful code Balaji.

I found I needed to change one of the if statements to include a test for "|| mode == FileMode.Create" as follows:

public static FileStream Open(string filepath, FileMode mode)
{
    ...
    if ((iError > 0 && !((mode == FileMode.Append || mode == FileMode.Create)
        && iError == ERROR_ALREADY_EXISTS)) || sh.IsInvalid)
    ...
}

This then allows overwiting the file if it already exists, as per the intent of FileMode.Create.

Thanks, Henk.

CreateFileW For browsing directories with more than 256

diego jose

12:23 25 Apr '08

Hi: I was wondering if you could give me a hand with INTPR, im trying to use createFileW to get all the files in the directories that have more than 256 characters in the path. Im stuck at the call on that returns the pointer, how do you convert it or read from it? Regards, Diego d
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Re: CreateFileW For browsing directories with more than 256

Chintan.Desai

23:34 21 Sep '10

Hi, I am stuck in the same problem since 2 days. I need to naviage all files under folders. However, there are many file having more than 260char and getting PathTooLongException. If you got the solution, Please provide me or mail me (desai.chintan.d@gmail.com) thanks. Thanks in advance, Chintan.
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can't create file with really long name

mezhaka

1:38 4 Apr '08

hello, i've been trying to create the file with really long name using WinApi CreateFileW directly, but failed to do that. so googling brought me to this page.
i've tried the following code:



        static void Main(string[] args) {

            StringBuilder name = new StringBuilder();

            for (int i = 0; i <= 261; i++)

                name.Append('f');

            name.Append(".out");

            FileStream fs1 = Win32File.Open(name.ToString(), FileMode.Open);

        }

it throws exception with iError == 2.

i'd be grateful for any comments.
thanks,
anton.

nice tip

dmihailescu

10:53 12 Dec '07

I did not know about this limitation. You can make your solution more elegant by using extension methods that are a new feature in c# 3.5. Your code could become something like: object.OpenFileLong(....) where object is a FileStream. Unfortunately it does not work on static methods so FileStream.OpenFileLong won't work.
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