NullReferenceException in Xamarin Firebase connection

Issue

I am connecting my xamarin app to firebase, I created a full CRUD function and implemented it to my app.

I think I did everything right, but apperantly I did not because the UPDATE and DELETE functions does not really work..

I can Add items to the database but not UPDATE or DELETE them. When I try to uppdate or delete an item I get error on my update and delete functions.

This is my FirebaseHelper.cs that contains all the CRUD functions:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Firebase.Database;
using Firebase.Database.Query;

namespace ElseWareApp.Models
{
    public class FirebaseHelper
    {
        FirebaseClient firebase = new FirebaseClient("https://urldb-4fcca-default-rtdb.firebaseio.com/");

        public async Task<List<Person>> GetAllPersons()
        {

            return (await firebase
                .Child("Persons")
                .OnceAsync<Person>()).Select(item => new Person
            {
                Name = item.Object.Name,
                PersonId = item.Object.PersonId
            }).ToList();
        }

        public async Task AddPerson(int personId, string name)
        {

            await firebase
                .Child("Persons")
                .PostAsync(new Person() { PersonId = personId, Name = name });
        }

        public async Task<Person> GetPerson(int personId)
        {
            var allPersons = await GetAllPersons();
            await firebase
                .Child("Persons")
                .OnceAsync<Person>();
            return allPersons.Where(a => a.PersonId == personId).FirstOrDefault();
        }

        public async Task UpdatePerson(int personId, string name)
        {
            var toUpdatePerson = (await firebase
                .Child("Persons")
                .OnceAsync<Person>()).Where(a => a.Object.PersonId == personId).FirstOrDefault();

            await firebase
                .Child("Persons")
                .Child(toUpdatePerson.Key)
                .PutAsync(new Person() { PersonId = personId, Name = name });
        }

        public async Task DeletePerson(int personId)
        {
            var toDeletePerson = (await firebase
                .Child("Persons")
                .OnceAsync<Person>()).Where(a => a.Object.PersonId == personId).FirstOrDefault();
    

    await firebase.Child("Persons").Child(toDeletePerson.Key).DeleteAsync();

    }
  }
}

The error I get is in DeletePerson class is in the last line: **await firebase.Child("Persons").Child(toDeletePerson.Key).DeleteAsync();**

The error I get in UpdatePerson class is also in last line

 await firebase
                    .Child("Persons")
                    .Child(toUpdatePerson.Key)
                    .PutAsync(new Person() { PersonId = personId, Name = name });

Both functions gives me this error:

System.NullReferenceException: 'Object reference not set to an instance of an object.'

Hope someone can help!

Solution

if this query is returning null

var toDeletePerson = (await firebase
            .Child("Persons")
            .OnceAsync<Person>()).Where(a => a.Object.PersonId == personId).FirstOrDefault();

then this will throw a null reference exception

await firebase.Child("Persons").Child(toDeletePerson.Key).DeleteAsync();

you should test for null first

if (toDeletePerson != null) 
{
    await firebase.Child("Persons").Child(toDeletePerson.Key).DeleteAsync();
}

Answered By - Jason