If using .Net Core 3.0 or later;
Default to using the built in System.Text.Json
parser implementation.
e.g.
using System.Text.Json;
var json = JsonSerializer.Serialize(aList);
alternatively, other, less mainstream options are available like Utf8Json parser and Jil: These may offer superior performance, if you really need it but, you will need to install their respective packages.
If stuck using .Net Core 2.2 or earlier;
Default to using Newtonsoft JSON.Net as your first choice JSON Parser.
e.g.
using Newtonsoft.Json;
var json = JsonConvert.SerializeObject(aList);
you may need to install the package first.
PM> Install-Package Newtonsoft.Json
For more details see and upvote the answer that is the source of this information.
For reference only, this was the original answer, many years ago;
// you need to reference System.Web.Extensions
using System.Web.Script.Serialization;
var jsonSerialiser = new JavaScriptSerializer();
var json = jsonSerialiser.Serialize(aList);