Test string DeSerilizeFn

main.cs

using System.Linq;
using ServiceStack;
using ServiceStack.Text;

JsConfig<string>.DeSerializeFn = str => str?.Trim();

public class Test
{
    public string Name { get; set; }
    public string Age { get; set; }
    public string Description { get; set; }
}

var dto = new Test { Name = " John Doe ", Age = "18" };
var json = dto.ToJson();

var fromDto = json.FromJson<Test>();

var isDescriptionNull = fromDto.Description == null;

packages.config

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="System.Memory" version="4.5.2" targetFramework="net45" />
  <package id="ServiceStack.Text" version="5.5.0" targetFramework="net45" />
  <package id="ServiceStack.Client" version="5.5.0" targetFramework="net45" />
  <package id="ServiceStack.Interfaces" version="5.5.0" targetFramework="net45" />
</packages>