你能在3.5项目中使用.NET4.0DLL吗?

不。您可以在4.0项目中使用.NET3.5程序集,但不能相反。

https://code.msdn.microsoft.com/Using-a-NET-4-Based-DLL-bb141db3/

通过COM使用我们的.NET 4 DLL

using System; 
using Net4ToNet2Adapter; 

namespace Net2Assembly 
{ 
    class Program 
    { 
        static void Main(string[] args) 
        { 
            Console.WriteLine("CLR version from EXE: {0}", Environment.Version); 
            Type myClassAdapterType = Type.GetTypeFromProgID("Net4ToNet2Adapter.MyClassAdapter"); 
            object myClassAdapterInstance = Activator.CreateInstance(myClassAdapterType); 
            IMyClassAdapter myClassAdapter = (IMyClassAdapter)myClassAdapterInstance; 
            myClassAdapter.DoNet4Action(); 
        } 
    } 

}