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https://github.com/dotnet/corefx/blob/master/src/System.Net.Http/src/System/Net/Http/HttpMessageHandler.cs#L20corefx如何与HttpMessageHandler一起使用?
protected internal abstract Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken);
Task<HttpResponseMessage> task = _handler.SendAsync(request, cancellationToken);
如何调用保护方法?