How to rethrow InnerException without losing stack trace in C#?

It is possible to preserve the stack trace before rethrowing without reflection:

static void PreserveStackTrace (Exception e)
{
    var ctx = new StreamingContext  (StreamingContextStates.CrossAppDomain) ;
    var mgr = new ObjectManager     (null, ctx) ;
    var si  = new SerializationInfo (e.GetType (), new FormatterConverter ()) ;

    e.GetObjectData    (si, ctx)  ;
    mgr.RegisterObject (e, 1, si) ; // prepare for SetObjectData
    mgr.DoFixups       ()         ; // ObjectManager calls SetObjectData

    // voila, e is unmodified save for _remoteStackTraceString
}

This wastes a lot of cycles compared to calling InternalPreserveStackTrace via cached delegate, but has the advantage of relying only on public functionality. Here are a couple of common usage patterns for stack-trace preserving functions:

// usage (A): cross-thread invoke, messaging, custom task schedulers etc.
catch (Exception e)
{
    PreserveStackTrace (e) ;

    // store exception to be re-thrown later,
    // possibly in a different thread
    operationResult.Exception = e ;
}

// usage (B): after calling MethodInfo.Invoke() and the like
catch (TargetInvocationException tiex)
{
    PreserveStackTrace (tiex.InnerException) ;

    // unwrap TargetInvocationException, so that typed catch clauses 
    // in library/3rd-party code can work correctly;
    // new stack trace is appended to existing one
    throw tiex.InnerException ;
}

I think your best bet would be to just put this in your catch block:

throw;

And then extract the innerexception later.

In .NET 4.5 there is now the ExceptionDispatchInfo class.

This lets you capture an exception and re-throw it without changing the stack-trace:

using ExceptionDispatchInfo = 
    System.Runtime.ExceptionServices.ExceptionDispatchInfo;

try
{
    task.Wait();
}
catch(AggregateException ex)
{
    ExceptionDispatchInfo.Capture(ex.InnerException).Throw();
}

This works on any exception, not just AggregateException.

It was introduced due to the await C# language feature, which unwraps the inner exceptions from AggregateException instances in order to make the asynchronous language features more like the synchronous language features.