The predefined type 'System.Threading.Tasks.Task' is defined in multiple assemblies in the global alias

I just have set up a new asp.net mvc 4 project, using the latest .net framework (4.5) in combination with Visual studio 2012. After I added some classes, side projects etc. I suddently notice that I'm receiving 5 times the same warning when I compile the project. The warning states:

ASPNETCOMPILER : warning CS1685: The predefined type 'System.Threading.Tasks.Task' is defined in multiple assemblies in the global alias; using definition from 'c:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.5\mscorlib.dll'

So now I'm wondering in which assemblies are this type defined than? All my projects in the solution are using version 4.5 of the .net framework, so 4.5\mscorlib.dll should be the only one or am I missing something?

My solutions consists of 4 projects ( 3 class libraries, and 1 mvc-4 web project). The 3 class libraries don't show any warnings when I compile them indepently. It's only the web project which throws the warnings, but the class library projects are referenced by the web project. The references of the web project:

Antlr3.Runtime
Microsoft.CSharp
Microsoft.Web.Infrastructure
System
System.Data
System.Web.DynamicData
System.Web.Entity
System.Web.ApplicationServices
System.ComponentModel.DataAnnotations
System.Core
System.Data.DataSetExtensions
System.Web.Optimization
System.Xml.Linq
System.Web
System.Web.Extensions
System.Web.Abstractions
System.Web.Routing
System.Xml
System.Configuration
System.Web.Services
System.EnterpriseServices
Newtonsoft.Json
System.Net.Http
System.Net.Http.Formatting
System.Net.Http.WebRequest
System.Web.Helpers
System.Web.Http
System.Web.Http.WebHost
System.Web.Mvc
System.Web.Razor
System.Web.WebPages
System.Web.WebPages.Deploymen
System.Web.WebPages.Razor
WebGrease

Does anyone have a suggestion where to look?

Thanks in advance

BHD

UPDATE It seems that more people have similar issues like this, in my case the answer was: We are using here some company libraries, which were indeed using older versions of the framework (3.5) and caused the warnings.

Solution

Similar to article C# Compiler Warning 1685. Check if any of the referenced assemblies are themselves referencing different versions of mscorlib.dll.

Get hold of DotPeek or .NET Reflector and you should be able check what versions of assemblies the referenced assemblies are referencing.