I have installed c# support for vscode(version is 1.15.0) and created a HelloWorld project via dotnet new console.

Now in Program.cs, I would like to use JavaScriptSerializer class which depends on System.Web.Extensions assembly. I typed using System.Web.Script.Serialization; and run dotnet restore ,but vscode cannot resolve it. The error is,

The type or namespace name 'Web' does not exist in the namespace
'System' (are you missing an assembly reference?)

It seems that System.Web is not part of .net core, but is there any way to add the assembly to the project?

I cannot find a project.json file which is refered in other posts since it is a csproj project.