Null reference exception thrown when using List.Add<T>

[VoidPlayz510]

Library/API/IoT binding

nanoFramework.System.Collections

Visual Studio version

VS2026

.NET nanoFramework extension version

2.0.0-preview.15

Target name(s)

ESP32

Firmware version

N/A

Device capabilities

No response

Description

When trying to add either type double or type custom class to a list the program throws a null reference exception even if the collection itself isn't null and was defined. (Sometimes works, sometimes does not)

How to reproduce

1. Setup the pins for RX/TX on DS18b20 device
2. Modify the esp32 configuration to match that in program.cs
3. Run the program

Expected behaviour

The item(s) to be added to the list without the compiler thinking the list is null

Screenshots

Sample project or code

`using Iot.Device.Ds18b20;
using nanoFramework.Device.OneWire;
using nanoFramework.Hardware.Esp32;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Threading;
using UnitsNet;

namespace GPSMinimal;

public class Program
{
private static OneWireHost OneWire;
private static readonly object TempLock = new();

private static readonly List<double> LatestTemps = new List<double>();
private static readonly List<Ds18b20> Sensors = new List<Ds18b20>();

public static void InitialiseAll()
{
    OneWire = new OneWireHost();

    var deviceList = OneWire.FindAllDevices();

    if (deviceList == null || deviceList.Count == 0)
    {
        Console.WriteLine("No DS18B20 sensors found");
        return;
    }

    lock (TempLock)
    {
        Sensors.Clear();

        foreach (byte[] deviceAddr in deviceList)
        {
            if (deviceAddr[0] != 0x28)
                continue;

            Console.WriteLine("Creating sensor");

            var sensor = new Ds18b20(
                OneWire,
                deviceAddr,
                false,
                TemperatureResolution.VeryHigh);

            Console.WriteLine("Configuring sensor");

            sensor.IsAlarmSearchCommandEnabled = false;

            Console.WriteLine("Adding sensor");

            Sensors.Add(sensor);

            Console.WriteLine("Added sensor");
        }
    }

    Console.WriteLine($"Total sensors: {Sensors.Count}");
}

public static void GetTemperatureFromSensorsInCelcius()
{
    lock (TempLock)
    {
        LatestTemps.Clear();

        foreach (var sensor in Sensors)
        {
            if (!sensor.TryReadTemperature(out Temperature temp))
                continue;

            LatestTemps.Add(temp.DegreesCelsius);
        }
    }
}

public static List<double> GetSnapshot()
{
    lock (TempLock)
    {
        return new List<double>(LatestTemps);
    }
}

public void Dispose()
{
    if (OneWire != null)
    {
        OneWire.Dispose();
        OneWire = null;
    }
}

private static void TemperaturePollingLoop()
{
    while (true)
    {
        GetTemperatureFromSensorsInCelcius();
        var snapshot = GetSnapshot();
        foreach (var temp in snapshot)
        {
            Console.WriteLine($"Temp {temp:F2}");
        }

        Thread.Sleep(1000);
    }
}

public static void Main()
{
    Debug.WriteLine("Hello from nanoFramework!");

    // temp pins
    Configuration.SetPinFunction(27, DeviceFunction.COM3_RX);
    Configuration.SetPinFunction(27, DeviceFunction.COM3_TX);

    InitialiseAll();
    new Thread(TemperaturePollingLoop).Start();

    Thread.Sleep(Timeout.Infinite);
}

}`

Aditional information

No response

[josesimoes]

double is only supported on specific fw builds. None of the official fw packages has this enabled.
Can you please double check if this is what is causing the exception? Change type to float.

[VoidPlayz510]

Hii thanks for responding! I changed the type to float and then casted the degrees as float since it's a double for the property type and this still resulted with the same problem. :((

286ABF7400000094
28FD247600000060
28F790730000007E
Creating sensor
Configuring sensor
Adding sensor
++++ Exception System.NullReferenceException - CLR_E_NULL_REFERENCE (1) ++++
++++ Message:
++++ System.Collections.Generic.List`1::Add [IP: 0017] ++++
++++ GPSMinimal.Program::InitialiseAll [IP: 0090] ++++
++++ GPSMinimal.Program::Main [IP: 0021] ++++
Exception thrown: 'System.NullReferenceException' in nanoFramework.System.Collections.dll
An unhandled exception of type 'System.NullReferenceException' occurred in nanoFramework.System.Collections.dll

It seems to be throwing it before even getting to the temperature though as it's failing to add the sensors

private static readonly List<Ds18b20> Sensors = new List<Ds18b20>();

Sensors.Add(sensor); -> throws here