Angular – Value posted to .NET Framework API is always null

I am trying to post something on my API.
In the moment a test JSON would be enough for a little feeling of success.

Originally I want to post an object like so:

sendRezept(rezept:Rezept) {
  postHeader =  new HttpHeaders()
    .set('Content-Type', 'application/json')
  return this.http.post<string>(this.rUrl, rezept, {headers:postHeader})
}

The POST function of my API looks like that:

public string Post([FromBody] string value)
{
    if (value != null)
    {
        value = value.ToString();
        return "yay";
    }
    return "damned";
}

Each and everytime whatever I tried the value is null and "damned" is returned.

I read, that there is no ‘_’ allowed in JSON so I made an interface of rezept, which I recently tried pushing. It looks like this:

sendRezept(rezept:Rezept) {
  let medikamente = rezept.combineMedikamente();
  let meds:any[] = [];
  for (let i = 0; i<medikamente.length; i++){
    if (medikamente[i].name == '')
      meds[i] = null;
    else 
    meds[i] = medikamente[i]
  }
  let irezept:IRezept = {
    kunde:rezept.kunde,
    krankenkasse:rezept.krankenkasse,
    arzt:rezept.arzt,
    isGebFrei:rezept.isGebFrei,
    isGebPfl:rezept.isGebPfl,
    isNoctu:rezept.isNoctu,
    isSonstige:rezept.isSonstige,
    isUnfall:rezept.isUnfall,
    isArbeitsunfall:rezept.isArbeitsunfall,
    ausstelldatum:rezept.ausstelldatum,
    unfalldatum:rezept.unfalldatum,
    arbeitgeber:rezept.arbeitgeber,
    medikament1:meds[0],
    medikament2:meds[1],
    medikament3:meds[2],
    title:rezept.title
  }
  return this.http.post<string>(this.rUrl, irezept, {headers:this.postHeader})
}

>Solution :

Your controller API action should expect to receive an object, not a string.

[HttpPost]
public string Post([FromBody] Rezept irezept)
{
    ...
    // Access property: irezept.Kunde
}

public class Rezept
{
    public string Kunde { get; set; }
    // Properties
}