答案在于构建输出：

  

2> C：\ WINDOWS \ Microsoft.NET \框架\ v4.0.30319 \ Microsoft.Common.targets（1605,5）：警告MSB3274：主要参考PaymillWrapper无法得到解决，因为它是建立对.NETFramework，版本= V4.5的框架。这是一个更高的版本比当前目标框架.NETFramework，版本= 4.0版。

     

2> C：\ WINDOWS \ Microsoft.NET \框架\ v4.0.30319 \ Microsoft.Common.targets（1605,5）：警告MSB3268：主要参考PaymillWrapper无法得到解决，因为它有一个间接的依赖在框架组装System.Net.Http，版本= 4.0.0.0，文化=中性公钥= b03f5f7f11d50a3a，它无法在当前的目标框架内解决。 .NETFramework，版本= V4.0。要解决此问题，要么删除提及PaymillWrapper或重新定位你的应用程序的框架版本，其中包含System.Net.Http，版本= 4.0.0.0，文化=中性公钥= b03f5f7f11d50a3a。

在换句话说，的NuGet包使用的引用，.NET 4.5，所以你只能用它在.NET 4.5 +项目。

您可能要问Paymill包装的作者，看看他们是否能公布其面向.NET 4，而不是一个版本。

请注意，的NuGet包页面连州这样的：

  

依赖
   - .NETFramework 4.5

I've got a project that uses a library (from nuget)
The target framework for my project is currently 4.0

I'm using objects and methods from the library, I get intellisense etc...

However, when I build, compilation fails with

The type or namespace could not be found. Are you missing a using directive or assembly reference?

If I change the target framework of the project to 4.5, it compiles.

Is there a way round this?

EDIT

As a specific example, here are the steps to reproduce this problem in one particular case.

• File -> New Project
• New Console application

• Set Target framework to 4.0

• Nuget install paymill wrapper

Use one of the types in the Paymill wrapper. For example:

using PaymillWrapper.Models;
using PaymillWrapper.Service;  

public class MyClass
{
    private readonly PaymentService _paymentService;
}

VS doesn't complain.
Compile

Receive error:

The type or namespace name 'ClientService' could not be found (are you missing a using directive or an assembly reference?)

The answer lies in the build output:

2>C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1605,5): warning MSB3274: The primary reference "PaymillWrapper" could not be resolved because it was built against the ".NETFramework,Version=v4.5" framework. This is a higher version than the currently targeted framework ".NETFramework,Version=v4.0".

2>C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(1605,5): warning MSB3268: The primary reference "PaymillWrapper" could not be resolved because it has an indirect dependency on the framework assembly "System.Net.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" which could not be resolved in the currently targeted framework. ".NETFramework,Version=v4.0". To resolve this problem, either remove the reference "PaymillWrapper" or retarget your application to a framework version which contains "System.Net.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".

In other words, the NuGet package uses a reference to .NET 4.5, so you can only use it on .NET 4.5+ projects.

You might want to ask the authors of Paymill Wrapper to see if they could publish a version which targets .NET 4 instead.

Note that the NuGet package page even states this:

Dependencies
- .NETFramework 4.5