// class
public class Res
{
    public int Code { get; set; }
    public string Msg { get; set; }

    public Res(int code = 200, string msg = "Success")
    {
        Code = code;
        Msg = msg;
    }
}

public class Res<T> : Res
{
    public T Body { get; set; }

    public Res(T body = default, int code = 200, string msg = "Success")
        : base(code, msg)
    {
        Body = body;
    }
}

// Extend function ToJson()
public static class Ext
{
    private static readonly JsonSerializerOptions _opts;

    static Ext()
    {
        _opts = new JsonSerializerOptions
        {
            // ... default configuration
        };
    }

    // Serialize to JSON string from entity
    public static string ToJson(this object data, JsonSerializerOptions options = null)
    {
        if (data == null)
        {
            return string.Empty;
        }

        return JsonSerializer.Serialize(data, options ?? _opts);   // throw exception when data has Value Type body
    }

    // Deserialize to entity from JSON string
    public Static T ToJson<T>(this string json, JsonSerializerOptions options = null)
    {
        if (string.IsNullOrWhiteSpace(json))
        {
            return default;
        }

        return JsonSerializer.Deserialize<T>(json, options ?? _opts);   // throw exception when data has Value Type body
    }
}

var res0 = new Res();
var res0Json = res0.ToJson();   // Success (Correct JSON string)

var res1 = new Res<string>("a");   // body is Reference Type
var res1Json = res1.ToJson();   // Success (Correct JSON string)

var res2 = new Res<int>(1);   // body is Value Type
var res2Json = res2.ToJson();   // Fail (Throw an exception: Object reference not set to an instance of an object.)