The type 'System.Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.

Today I was working on Asp.net 4.5 website and I got following error:

The type 'System.Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.

I have solved this issue by adding one line in web.config file:

    <compilation debug="true" targetFramework="4.5">
      <assemblies> 
        <add assembly="System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" /> 
      </assemblies>
    </compilation>

I got three of them, I have added all those assemblies into web.config like above and it works perfect.