private static async Task WriteToFileAsync(string path, FileMode mode, string?contents, Encoding encoding, CancellationToken cancellationToken)
{
ReadOnlyMemory <byte> preamble = encoding.GetPreamble();
int preambleSize = preamble.Length;
using SafeFileHandle fileHandle = OpenHandle(path, mode, FileAccess.Write, FileShare.Read, FileOptions.Asynchronous, GetPreallocationSize(mode, contents, encoding, preambleSize));
long fileOffset = mode == FileMode.Append && fileHandle.CanSeek ? RandomAccess.GetLength(fileHandle) : 0;
if (string.IsNullOrEmpty(contents))
{
if (preambleSize > 0 && // even if the content is empty, we want to store the preamble
fileOffset == 0) // if we're appending to a file that already has data, don't write the preamble.
{
await RandomAccess.WriteAtOffsetAsync(fileHandle, preamble, fileOffset, cancellationToken).ConfigureAwait(false);
}
return;
}
byte[] bytes = ArrayPool <byte> .Shared.Rent(preambleSize + encoding.GetMaxByteCount(Math.Min(contents.Length, ChunkSize)));
try
{
if (fileOffset == 0)
{
preamble.CopyTo(bytes);
}
else
{
preambleSize = 0; // don't append preamble to a non-empty file
}
Encoder encoder = encoding.GetEncoder();
ReadOnlyMemory <char> remaining = contents.AsMemory();
while (!remaining.IsEmpty)
{
ReadOnlyMemory <char> toEncode = remaining.Slice(0, Math.Min(remaining.Length, ChunkSize));
remaining = remaining.Slice(toEncode.Length);
int encoded = encoder.GetBytes(toEncode.Span, bytes.AsSpan(preambleSize), flush: remaining.IsEmpty);
ReadOnlyMemory <byte> toStore = new ReadOnlyMemory <byte>(bytes, 0, preambleSize + encoded);
await RandomAccess.WriteAtOffsetAsync(fileHandle, toStore, fileOffset, cancellationToken).ConfigureAwait(false);
fileOffset += toStore.Length;
preambleSize = 0;
}
}
finally
{
ArrayPool <byte> .Shared.Return(bytes);
}
}
static async Task Core(string path, byte[] bytes, CancellationToken cancellationToken)
{
using SafeFileHandle sfh = OpenHandle(path, FileMode.Create, FileAccess.Write, FileShare.Read, FileOptions.Asynchronous);
await RandomAccess.WriteAtOffsetAsync(sfh, bytes, 0, cancellationToken).ConfigureAwait(false);
}