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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
using System.Buffers;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;
namespace System.IO
{
/// <summary>Provides methods to help in the implementation of Stream-derived types.</summary>
internal static partial class StreamHelpers
{
/// <summary>
/// Provides an implementation usable as an override of Stream.CopyToAsync but that uses the shared
/// ArrayPool for the intermediate buffer rather than allocating a new buffer each time.
/// </summary>
/// <param name="source">The source stream from which to read.</param>
/// <param name="destination">The destination stream to which to write.</param>
/// <param name="bufferSize">The buffer size to use.</param>
/// <param name="cancellationToken">The cancellation token to use to cancel the operation.</param>
/// <remarks>
/// If/when the base CopyToAsync implementation is changed to use a pooled buffer,
/// this will no longer be necessary.
/// </remarks>
public static Task ArrayPoolCopyToAsync(Stream source, Stream destination, int bufferSize, CancellationToken cancellationToken)
{
Debug.Assert(source != null);
ValidateCopyToArgs(source, destination, bufferSize);
return ArrayPoolCopyToAsyncCore(source, destination, bufferSize, cancellationToken);
}
/// <summary>Standard read/write loop using ReadAsync on the source and WriteAsync on the destination.</summary>
private static async Task ArrayPoolCopyToAsyncCore(Stream source, Stream destination, int bufferSize, CancellationToken cancellationToken)
{
byte[] buffer = ArrayPool<byte>.Shared.Rent(bufferSize);
bufferSize = 0; // reuse same field for high water mark to avoid needing another field in the state machine
try
{
while (true)
{
int bytesRead = await source.ReadAsync(buffer, 0, buffer.Length, cancellationToken).ConfigureAwait(false);
if (bytesRead == 0)
{
break;
}
if (bytesRead > bufferSize)
{
bufferSize = bytesRead;
}
await destination.WriteAsync(buffer, 0, bytesRead, cancellationToken).ConfigureAwait(false);
}
}
finally
{
Array.Clear(buffer, 0, bufferSize); // clear only the most we used
ArrayPool<byte>.Shared.Return(buffer, clearArray: false);
}
}
}
}
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