正如汉斯的回答中所说，.NET 实现Progress<T>用于SynchronizationContext.Post发送其请求。您可以像在Yves 的回答中那样直接报告它，或者您可以使用SynchronizationContext.Send这样请求将被阻塞，直到接收者处理它。
因为引用源是可用的，所以它就像复制源并将其更改Post为 aSend和更改SynchronizationContext.CurrentNoFlow为一样简单，SynchronizationContext.Current因为它是CurrentNoFlow一个内部属性。
/// <summary>
/// Provides an IProgress{T} that invokes callbacks for each reported progress value.
/// </summary>
/// <typeparam name="T">Specifies the type of the progress report value.</typeparam>
/// <remarks>
/// Any handler provided to the constructor or event handlers registered with
/// the <see cref="ProgressChanged"/> event are invoked through a
/// <see cref="System.Threading.SynchronizationContext"/> instance captured
/// when the instance is constructed. If there is no current SynchronizationContext
/// at the time of construction, the callbacks will be invoked on the ThreadPool.
/// </remarks>
public class SynchronousProgress<T> : IProgress<T>
{
/// <summary>The synchronization context captured upon construction. This will never be null.</summary>
private readonly SynchronizationContext m_synchronizationContext;
/// <summary>The handler specified to the constructor. This may be null.</summary>
private readonly Action<T> m_handler;
/// <summary>A cached delegate used to post invocation to the synchronization context.</summary>
private readonly SendOrPostCallback m_invokeHandlers;
/// <summary>Initializes the <see cref="Progress{T}"/>.</summary>
public SynchronousProgress()
{
// Capture the current synchronization context. "current" is determined by Current.
// If there is no current context, we use a default instance targeting the ThreadPool.
m_synchronizationContext = SynchronizationContext.Current ?? ProgressStatics.DefaultContext;
Contract.Assert(m_synchronizationContext != null);
m_invokeHandlers = new SendOrPostCallback(InvokeHandlers);
}
/// <summary>Initializes the <see cref="Progress{T}"/> with the specified callback.</summary>
/// <param name="handler">
/// A handler to invoke for each reported progress value. This handler will be invoked
/// in addition to any delegates registered with the <see cref="ProgressChanged"/> event.
/// Depending on the <see cref="System.Threading.SynchronizationContext"/> instance captured by
/// the <see cref="Progress"/> at construction, it's possible that this handler instance
/// could be invoked concurrently with itself.
/// </param>
/// <exception cref="System.ArgumentNullException">The <paramref name="handler"/> is null (Nothing in Visual Basic).</exception>
public SynchronousProgress(Action<T> handler) : this()
{
if (handler == null) throw new ArgumentNullException("handler");
m_handler = handler;
}
/// <summary>Raised for each reported progress value.</summary>
/// <remarks>
/// Handlers registered with this event will be invoked on the
/// <see cref="System.Threading.SynchronizationContext"/> captured when the instance was constructed.
/// </remarks>
public event EventHandler<T> ProgressChanged;
/// <summary>Reports a progress change.</summary>
/// <param name="value">The value of the updated progress.</param>
protected virtual void OnReport(T value)
{
// If there's no handler, don't bother going through the [....] context.
// Inside the callback, we'll need to check again, in case
// an event handler is removed between now and then.
Action<T> handler = m_handler;
EventHandler<T> changedEvent = ProgressChanged;
if (handler != null || changedEvent != null)
{
// Post the processing to the [....] context.
// (If T is a value type, it will get boxed here.)
m_synchronizationContext.Send(m_invokeHandlers, value);
}
}
/// <summary>Reports a progress change.</summary>
/// <param name="value">The value of the updated progress.</param>
void IProgress<T>.Report(T value) { OnReport(value); }
/// <summary>Invokes the action and event callbacks.</summary>
/// <param name="state">The progress value.</param>
private void InvokeHandlers(object state)
{
T value = (T)state;
Action<T> handler = m_handler;
EventHandler<T> changedEvent = ProgressChanged;
if (handler != null) handler(value);
if (changedEvent != null) changedEvent(this, value);
}
}
/// <summary>Holds static values for <see cref="Progress{T}"/>.</summary>
/// <remarks>This avoids one static instance per type T.</remarks>
internal static class ProgressStatics
{
/// <summary>A default synchronization context that targets the ThreadPool.</summary>
internal static readonly SynchronizationContext DefaultContext = new SynchronizationContext();
}