Import deserialized System.Text.Json data into current type

Question

I want to import json data from within the class which is the target of the deserialization. Is this possible with System.Text.Json without additional mapping? Ideally I would use "this" instead of the generic type parameter. I know that is impossible, but is there a similar option? Here is my test code which works, because it creates the data object only to map it to the property. Ideally, I would not need to instantiate "Test" twice.

public class Test
{
    public string? Bar { get; set; }
    
    public void ImportJson(string payload)
    {
        var data = System.Text.Json.JsonSerializer.Deserialize<Test>(payload);
        Bar = data?.Bar; // Don't want to map
    }
}

string foo = "{ \"Bar\": \"baz\" }";
var t = new Test();
t.ImportJson(foo);
Console.WriteLine(t.Bar);

No, this isn't possible. `Deserialize` can only ever create new instances, there are no overloads that can fill existing instances. As such it makes little sense to offer `ImportJson` as an instance method, just make it `static Test FromJson(string payload)` or suchlike. (Of course when you have that it's a trivial wrapper around `Deserialize`, so probably of no added vlaue.) — Jeroen Mostert, Mar 08 '22 at 17:28
You are looking for a `Populate()` method, but, unlike Newtonsoft, there's [no such method built in to System.Text.Json](https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-migrate-from-newtonsoft-how-to#populate-existing-objects). See: [.Net Core 3.0 JsonSerializer populate existing object](https://stackoverflow.com/q/56835040/3744182). In fact that looks to be a duplicate, agree? — dbc, Mar 08 '22 at 18:31
Agree, it is a duplicate to this question, I did not find it lacking to know the Json.net "Populate()" method. — TvdH, Mar 08 '22 at 21:05

Serge · Accepted Answer · 2022-03-08T19:36:26.210

2

you can try something like this

    string foo = "{ \"Bar\": \"baz\" }";
    var t = new Test();
    t.Deserialize(foo);

    Console.WriteLine(t.Instance.Bar);

classes

public static class Util
{
    public static void Deserialize<T>(this T obj, string json) where T : IImportJson<T>
    {
        obj.Instance=System.Text.Json.JsonSerializer.Deserialize<T>(json);
    }
}

public class Test : ImportJson<Test>
{
    public string? Bar { get; set;}
    
}

public interface IImportJson<T>
{
    public T Instance { get; set; }
}
public class ImportJson<T>: IImportJson<T>
{
    public T Instance { get; set; }
}

if class dosn't have many properies, it could be like this too

public interface IImportJson<T>
{
    public void ImportJson (T obj);
}
public class Test : IImportJson<Test>
{
    public string? Bar { get; set; }
    
    public void ImportJson(Test test)
    {
        Bar=test.Bar;
    }
}

edited Mar 08 '22 at 19:36

answered Mar 08 '22 at 19:14

Serge

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This still requires an additional instance, but looking at this made me realize that I should reorganize my code and use the deserializer to create the only instance, something along the lines of JsonSerializer.Deserialize(foo, type) since type is dynamic in my case. – TvdH Mar 08 '22 at 21:04
What do you mean about additional instance? In this demo case you just assign one property and don't keep anything else. – Serge Mar 08 '22 at 21:16
I will have "Test" and the encapsulated "Test.Instance" with this. "Test" instantiated with "new" and "Test.Instance" instantiated by the JsonSerializer. However, it does fulfill my original requirement. – TvdH Mar 09 '22 at 06:54

Import deserialized System.Text.Json data into current type

1 Answers1