Could not load assembly Microsoft.Threading.Tasks.dll - xamarin.forms

Question

I am receiving the following error on iOS and Android project. Anybody has any workaround for this issue? This post recommends a workaround but the steps are vague.
C:\Program Files (x86)\MSBuild\Xamarin\Android\Xamarin.Android.Common.targets(2,2): Error: Exception while loading assemblies: System.IO.FileNotFoundException: Could not load assembly 'Microsoft.Threading.Tasks, Version=1.0.12.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. Perhaps it doesn't exist in the Mono for Android profile?
File name: 'Microsoft.Threading.Tasks.dll'
at Xamarin.Android.Tuner.DirectoryAssemblyResolver.Resolve(AssemblyNameReference reference, ReaderParameters parameters)
at Xamarin.Android.Tasks.ResolveAssemblies.AddAssemblyReferences(ICollection1 assemblies, AssemblyDefinition assembly, Boolean topLevel)
at Xamarin.Android.Tasks.ResolveAssemblies.AddAssemblyReferences(ICollection1 assemblies, AssemblyDefinition assembly, Boolean topLevel)
at Xamarin.Android.Tasks.ResolveAssemblies.Execute() (Braemar.Vers.Mobile.Droid)
I think that these two paths should have this file but I couldn't find it.
C:\Program Files (x86)\Reference
Assemblies\Microsoft\Framework\MonoAndroid\v1.0
C:\Program Files (x86)\Reference
Assemblies\Microsoft\Framework\MonoTouch\v1.0
Xamarin Studio:
Version 5.10.2 (build 56)
Installation UUID: 88f72eda-5030-43e9-8d1b-7658e8a17627
Runtime:
Microsoft .NET 4.0.30319.42000
GTK+ 2.24.23 (MS-Windows theme)
GTK# 2.12.30
Xamarin.Android:
Version: 6.0.1.10 (Business Edition)
Android SDK: C:\Android\android-sdk
Supported Android versions:
2.3 (API level 10)
4.0.3 (API level 15)
4.1 (API level 16)
4.2 (API level 17)
4.3 (API level 18)
4.4 (API level 19)
4.4.87 (API level 20)
5.0 (API level 21)
5.1 (API level 22)
6.0 (API level 23)
SDK Tools Version: 24.4.1
SDK Platform Tools Version: 23.1.0 rc1
SDK Build Tools Version: 23.0.2
Java SDK: C:\Program Files (x86)\Java\jdk1.7.0_71
java version "1.7.0_71"
Java(TM) SE Runtime Environment (build 1.7.0_71-b14)
Java HotSpot(TM) Client VM (build 24.71-b01, mixed mode, sharing)
Build Information:
Release ID: 510020056
Git revision: bb74ff467c62ded42b7b7ac7fdd2edc60f8647b0
Build date: 2016-01-26 15:49:39-05
Xamarin addins: 8b797d7ba24d5abab226c2cf9fda77f666263f1b
Build lane: monodevelop-windows-cycle6-c6sr1
Operating System:
Windows 10.0.10586.0 (64-bit)
Project Information:
PCL 4.5 - Profile 7

Answer 1

I had such problem. It was caused by missing nuget packages in Droid project. Some of my subprojects referenced packages which had "Microsoft.Threading.Tasks" in references (PCLStorage, Restsharp portable). But in main Droid project I did not added this packages. After adding missing packages (PCLStorage, Restsharp portable) to Droid everything started work.

Answer 2

Is there any reason you are using Profile7?
I would recommend you create a project with profile 259, that is the default one for Xamarin Forms projects.
Otherwise you need to get a reference to that DLL. Normally I try installing the BCL NuGet package as my first port of call, but it might not be included in there: https://www.nuget.org/packages/Microsoft.Bcl/
Update
If you are having linker issues, you can skip certain assemblies: https://developer.xamarin.com/guides/ios/advanced_topics/linker/#Skipping_Assemblies
Sometimes the linker is more aggressive than anticipated.

Answer 3

I had the same problem.
Right click and select Options for the Android project.
Go to Build->General and uncheck "Use MSBuild build engine (recommended for this project type)"
Now I could build again!