A C# Xamarin.Android activity base class that offers StartActivityForResultAsync as a much simpler means to kick off activities and resume execution when they are done.

ActivityBase.cs

using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using Android.App;
using Android.Content;
using Android.OS;

public abstract class ActivityBase : Activity {
	private int activityResultRegistrationCounter = 10000;

	private Dictionary<int, TaskCompletionSource<Tuple<Result, Intent>>> activityResultRegistrations = new Dictionary<int, TaskCompletionSource<Tuple<Result, Intent>>>();

	/// <summary>
	/// Starts another activity and allows awaiting on its return.
	/// </summary>
	/// <param name="intent">The intent that should be used to launch the new activity.</param>
	/// <returns>A task that completes with the finishing of the activity, providing the result of the activity.</returns>
	/// <example>
	/// var result = await this.StartActivityForResultAsync(Intent.CreateChooser(intent, "Select picture"));
	/// if (result.Item1 == Result.Ok) {
	///     // User chose a picture. Get the Uri
	///     Android.Net.Uri imageSource = result.Item2.Data;
	/// }
	/// </example>
	public Task<Tuple<Result, Intent>> StartActivityForResultAsync(Intent intent) {
		int requestCode = activityResultRegistrationCounter++;
		var completionSource = new TaskCompletionSource<Tuple<Result, Intent>>();
		this.activityResultRegistrations[requestCode] = completionSource;
		this.StartActivityForResult(intent, requestCode);
		return completionSource.Task;
	}

	protected override void OnActivityResult(int requestCode, Result resultCode, Intent data) {
		base.OnActivityResult(requestCode, resultCode, data);

		TaskCompletionSource<Tuple<Result, Intent>> completionSource;
		if (this.activityResultRegistrations.TryGetValue(requestCode, out completionSource)) {
			this.activityResultRegistrations.Remove(requestCode);
			completionSource.SetResult(Tuple.Create(resultCode, data));
		}
	}
}