Call method on UI thread from background in .net 2.0

Question

I am using MonoDevelop (.net 2.0) to develop a iOS and Android app. I use BeginGetResponse and EndGetResponse to asynchronously do a webrequest in a background thread.

IAsyncResult result = request.BeginGetResponse(new AsyncCallback(onLogin), state);

However, the callback onLogin does seem to still be running on a background thread, no allowing me to interact with the UI. How do I solve this?

Can see that there are Android and iOS specific solutions but want a cross-platform solution.

Edit: From mhutch answer I've got this far:

IAsyncResult result = request.BeginGetResponse(o => {
            state.context.Post(() => { onLogin(o); });
        }, state);

Where state contains a context variable of type SynchronizationContext set to SynchronizationContext.Current

It complains that Post requires two arguments, the second one being Object state. Inserting state gives the error

Argument `#1' cannot convert `anonymous method' expression to type `System.Threading.SendOrPostCallback' (CS1503) (Core.Droid)

Answer 1

Both Xamarin.iOS and Xamarin.Android set a SynchronizationContext for the GUI thread.

This means you get the SynchronizationContext.Current from the GUI thread and pass it to your callback (e.g via the state object or captured in a lambda). Then you can use the context's Post method to invoke things on the main thread.

For example:

//don't inline this into the callback, we need to get it from the GUI thread
var ctx = SynchronizationContext.Current;

IAsyncResult result = request.BeginGetResponse(o => {
    // calculate stuff on the background thread
    var loginInfo = GetLoginInfo (o);
    // send it to the GUI thread
    ctx.Post (_ => { ShowInGui (loginInfo); }, null);
}, state);