ASP .Net Framework API gets empty objects

Question

I have got an API with a RezeptController in that I have got Put and Post functions, which work fine from the Angular Tool. In my C# Tool which works in a similar way I am trying to Post my object like that:

public async Task UpdateRezept(Rezept rezept)
{
    HttpResponseMessage response = await client.PutAsJsonAsync(
       rezeptUrl + "/" + rezept.id, rezept);
    response.EnsureSuccessStatusCode();
} 

When I look into that object everything looks just great, but in my API, which looks like that:

public void Put(int id, [FromBody] Rezept iRezept)
{
    SerializerService serializerService = new SerializerService();
    string stringifiedObject = serializerService.getXml(typeof(Rezept), iRezept);
    SharepointService ss = new SharepointService();
    ss.UpdateList(id, stringifiedObject);
}

Is only a empty shell. The Output looks like that:

<?xml version="1.0" encoding="utf-16"?>
<Rezept xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <id>0</id>
    <ausstelldatum xsi:nil="true" />
    <isGebFrei>false</isGebFrei>
    <isGebPfl>false</isGebPfl>
    <isNoctu>false</isNoctu>
    <isSonstige>false</isSonstige>
    <isUnfall>false</isUnfall>
    <isArbeitsunfall>false</isArbeitsunfall>
    <unfalldatum xsi:nil="true" />
</Rezept>

while the JSON in my app looks like that:

{
  "Zuzahlung": 22.595813239527445,
  "GesamtBrutto": 217.85619214798743,
  "IsValid": true,
  "kunde": {
    "IsValid": true,
    "vorname": "test",
    "name": "test",
    "strasse": "test",
    "ort": "test",
    "plz": "12345",
    "versichertennummer": "K987456321",
    "versichertenstatus1": {
      "kennung": 1,
      "name": "Mitglied"
    },
    "versichertenstatus2": {
      "kennung": 0,
      "name": "Keine besondere Personengruppe"
    },
    "versichertenstatus3": {
      "kennung": 0,
      "name": "Kein DMP"
    },
    "gebDatum": "1998-03-25T00:00:00"
  },
  "ausstelldatum": "2022-08-01T00:00:00",
  "arzt": {
    "IsValid": true,
    "betriebsstaettennummer": "963258741",
    "lanr": "963258741"
  },
  "medikament1": {
    "name": "Ibuprofen 800 Ratiopharm",
    "taxe": 93.400614110045723,
    "pzn": null,
    "autIdem": true
  },
  "medikament2": {
    "name": "adwrewfdsfsd",
    "taxe": 82.55751828522871,
    "pzn": null,
    "autIdem": true
  },
  "medikament3": {
    "name": "dsgsdgfg",
    "taxe": 41.898059752712982,
    "pzn": null,
    "autIdem": false
  },
  "isGebFrei": false,
  "isGebPfl": true,
  "isNoctu": false,
  "isSonstige": false,
  "isUnfall": false,
  "isArbeitsunfall": false,
  "unfalldatum": null,
  "arbeitgeber": null,
  "id": 45,
  "title": "Rezept 45",
  "krankenkasse": {
    "name": "AOK Baden Württemberg",
    "kennung": "108018132"
  }
}

The Model in my API looks like that:

    [Serializable]
public class Rezept
{
    public Krankenkasse krankenkasse;
    public int id;
    public string title;

    public Kunde kunde;
    public DateTime? ausstelldatum;
    public Arzt arzt;

    public Medikament medikament1;
    public Medikament medikament2;
    public Medikament medikament3;

    public bool isGebFrei;
    public bool isGebPfl;
    public bool isNoctu;
    public bool isSonstige;
    public bool isUnfall;
    public bool isArbeitsunfall;

    public DateTime? unfalldatum;
    public string arbeitgeber;

    public Rezept()
    {
    }
}

Do you guys see my mistake?