In my case, target Framework of Assembly Project and the target Framework of the project where i want to use this assembly are different.
Target framework of assembly project was 4.5 and target framework of my project was 4.0.
When I have change the target framework of my project to the target framework of assembly project i.e. from 4.0 to 4.5 , it works fine.
For change the Target Framework of project,
Right Click the project -> Properties -> Application -> Target Framework
This Target Framework is drop-down list, select your required Framework from this list.