1. are you missing an assembly reference
给项目添加新控件的时候，经常发现这种错误
Error 21 The type or namespace name 'CustomControls' does not exist in the namespace '***.**MS' (are you missing an assembly reference?)
解决方法： Please go to your project properties and check the targeting framework version. I believe, your's would be 3.5. Change it to 4.0 and compile. The error will get resolved by automatically, by adding the necessary referance for the project to be build.