[API Proposal]: Introduce RandomAccess.SetLength(SafeFileHandle handle) method

[adamsitnik]

Background and motivation

Recently, we have introduced new RandomAccess type (#24847) which exposes some convenient methods for File IO. One of them is RandomAccess.GetLength(SafeFileHandle) that allows for reading file length.

When it comes to setting file length, users can:

• use FileMode.Truncate to truncate given file when opening it.
• specify preallocationSize to set initial size when opening the file.

The problem is that by using the new APIs, they can't set file length after the file has been opened.

FileStream exposes SetLength method, so should RandomAccess.

API Proposal

namespace System.IO
{
    public static class RandomAccess
    {
        public static void SetLength(SafeFileHandle handle, long length);
    }
}

API Usage

void WriteToFile(string path, string contents, Encoding encoding)
{
    long estimatedFileSize = encoding.GetMaxByteCount(contents.Length); // estimated file length
    using SafeFileHandle fileHandle = OpenHandle(path, FileMode.Create, FileAccess.Write, preallocationSize: estimatedFileSize);

    byte[] bytes = encoding.GetBytes(contents);
    RandomAccess.Write(fileHandle, bytes, fileOffset: 0);

    if (bytes.Length < estimatedFileSize) // shrink the size if initial estimation was wrong
    {
        RandomAccess.SetLength(fileHandle, bytes.Length);
    }
}

Risks

I can't see any.

[ghost]

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Issue Details

---

Background and motivation

Recently, we have introduced new RandomAccess type (#24847) which exposes some convenient methods for File IO. One of them is RandomAccess.GetLength(SafeFileHandle) that allows for reading file length.

When it comes to setting file length, users can:

• use FileMode.Truncate to truncate given file when opening it.
• specify preallocationSize to set initial size when opening the file.

The problem is that by using the new APIs, they can't set file length after the file has been opened.

FileStream exposes SetLength method, so should RandomAccess.

API Proposal

namespace System.IO
{
    public static class RandomAccess
    {
        public static void SetLength(SafeFileHandle handle, long length);
    }
}

API Usage

void WriteToFile(string path, string contents, Encoding encoding)
{
    long estimatedFileSize = encoding.GetMaxByteCount(contents.Length); // estimated file length
    using SafeFileHandle fileHandle = OpenHandle(path, FileMode.Create, FileAccess.Write, preallocationSize: estimatedFileSize);

    byte[] bytes = encoding.GetBytes(contents);
    RandomAccess.Write(fileHandle, bytes, fileOffset: 0);

    if (bytes.Length < estimatedFileSize) // shrink the size if initial estimation was wrong
    {
        RandomAccess.SetLength(fileHandle, bytes.Length);
    }
}

Risks

I can't see any.

Author:	adamsitnik
Assignees:	adamsitnik
Labels:	area-System.IO
Milestone:	7.0.0

[Thealexbarney]

Oh, I didn't realize this API was missing from RandomAccess until it was too late. I'd just assumed it would be included in a lower-level file operations API set.

What's the best way to work around this in .NET 6 if one always needs to have the possibility of setting the file length available? Open a FileStream, get its SafeFileHandle, use that for reads/writes and call FileStream.SetLength() to set the file length?

[adamsitnik]

Open a FileStream, get its SafeFileHandle, use that for reads/writes and call FileStream.SetLength() to set the file length?

Yes. The alternative it to specify the exact size as preallocationSize when opening file

[bartonjs]

Video

Looks good as proposed

namespace System.IO
{
    public partial static class RandomAccess
    {
        public static void SetLength(SafeFileHandle handle, long length);
    }
}