How to add `System.Web.Extensions` assembly to .net core project in vscode

Here I just want to know how to add assembly.

In general, you would have nodes like this in your .csproj:

<Reference Include="System.Web.Extensions" />

However, since you are using .NET Core, you cannot reference assemblies that are targeted to the full .NET Framework. In this case, System.Web.Extensions is one of those, so you cannot use it in your .NET Core project.

System.Web.Extensions is part of full .net framework . If you want to serialize and deserialize object,You can use Newtonsoft.Json,

#using Newtonsoft.Json
....
JsonConvert.DeserializeObject(json);

Update

Just get package name and version number from NuGet and add to .csproj then save. You will be prompted to run restore that will import new packages.

<ItemGroup>
     <PackageReference Include="Newtonsoft.Json" Version="9.0.1" />
  </ItemGroup>

Replace System.Web.Extensions.Serialization with System.Text.Json.Serialization. The usage of the two is very similar.