WinRT的IBuffer

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ReadAsync要用到的IBuffer表示一个字节数组，接口如下：

public interface IBuffer {UInt32 Capacity { get; } // Maximum size of the buffer (in bytes)UInt32 Length { get; set; } // Number of bytes currently in use by the buffer}

看起来很奇怪，因为它不能访问字节的内容，其实所有的IBuffer对象都实现了一具IBufferByteAccess接口，此接口是COM接口，但不是WinRT接口。

namespace System.Runtime.InteropServices.WindowsRuntime {[ComImport][InterfaceType(ComInterfaceType.InterfaceIsIUnknown)][Guid("905a0fef-bc53-11df-8c49-001e4fc686da")]internal interface IBufferByteAccess {unsafe Byte* Buffer { get; }}}

CLR用了一些扩展方法将字节转换成IBuffer:

// Defined in System.Runtime.WindowsRuntime.dllnamespace System.Runtime.InteropServices.WindowsRuntime {public static class WindowsRuntimeBufferExtensions {public static IBuffer AsBuffer(this Byte[] source);public static IBuffer GetWindowsRuntimeBuffer(this MemoryStream stream);public static Byte[] ToArray(this IBuffer source);public static Stream AsStream(this IBuffer source);// Not shown: other overloads, CopyTo, GetByte, & IsSameData}}

例子如下：

private async void SimpleWriteAndRead(StorageFile file) {using (IRandomAccessStream raStream = await file.OpenAsync(FileAccessMode.ReadWrite)) {Byte[] bytes = new Byte[] { 1, 2, 3, 4, 5 };UInt32 bytesWritten = await raStream.WriteAsync(bytes.AsBuffer()); // Byte[] -> IBufferusing (var ms = new MemoryStream())using (var sw = new StreamWriter(ms)) {sw.Write("A string in a stream");sw.Flush(); // Required: Flushes StreamWriter's contents to underlying MemoryStreambytesWritten =await raStream.WriteAsync(ms.GetWindowsRuntimeBuffer()); // Stream -> IBuffer}} // Close the stream

using (IRandomAccessStream raStream = await file.OpenAsync(FileAccessMode.Read)) {// NOTE: This is the most efficient way to allocate, populate, & access data:Byte[] data = new Byte[5]; // Allocate the Byte[]IBuffer proposedBuffer = data.AsBuffer(); // Wrap it in an object that implements IBufferIBuffer returnedBuffer = await raStream.ReadAsync(proposedBuffer,proposedBuffer.Capacity, InputStreamOptions.None);if (returnedBuffer != proposedBuffer) {// The proposed & returned IBuffers are not the same.// Copy the returned bytes into the original Byte[]returnedBuffer.CopyTo(data);} else {// The proposed & returned IBuffers are the same.// The returned bytes are already in the original Byte[]}// TODO: Put code here to access the read bytes from the data array...data = new Byte[raStream.Size - 5]; // Allocate Byte[] for remainderproposedBuffer = data.AsBuffer(); // Wrap it in an object that implements IBufferreturnedBuffer = await raStream.ReadAsync(proposedBuffer,proposedBuffer.Capacity, InputStreamOptions.None);// We just use the returned IBuffer hereusing (var sr = new StreamReader(returnedBuffer.AsStream())) {String str = sr.ReadToEnd();}} // Close the stream}

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