当您在Blend 4打开VS2012工程，像这样的消息显示：

When you open a VS2012 project in blend 4, a message like this is showed:

的事情是，如果你点击Yes按钮，然后它会打开该项目，但如果项目的目标框架是4.5则混合物只将显示您的XAML代码，并没有更多的东西。但是如果你的项目目标框架是4.0或更低，那么你将可以编辑为通常你以前做过的项目。

The thing is that if you click the yes button then it will open the project, but if the target framework of the project is 4.5 then the blend only will show you the Xaml code, and no more else. But if the target framework of your project is 4.0 or lower, then you will may edit the project as normally you have done before.

希望这可以帮助你......

Hope this could helps you...