C sharp Namespace : I am pretty new to C# but I have worked a little bit for my classes. When I worked in C sharp for classes, our teacher used to tell us the namespaces that we needed for completing the assignment. 

I wanted to know where do I find all the collection of the namespace for C sharp. 
One of my friends directed me to this site 
https://docs.microsoft.com/en-us/dotnet/api/index?view=netframework-4.0

but some of the namespaces did not work for example,

[enter image description here][1]


  [1]: https://i.stack.imgur.com/RlmA5.jpg

I am working on a small project for ASP.Net MVC and wanted to try different namespaces 