Getting NullReferenceException when attempting to play an ISampleProvider in Naudio (c#)

Question

I am creating my own software synthesizer in C#, using Naudio, and am starting off by generating some simple sine waves and then playing them. I would like to do this myself, rather than using Naudio's inbuilt SignalGenerator. When passing my custom object sine to generate a sine wave (that implements ISampleProvider) to WaveOutEvent.Init, I get thrown a NullReferenceException. I have created an instance of the class SineWave prior to calling init, but I still get that NullReferenceException.

I have tested whether both my WaveOutEvent (wo) and SineWave (sine) objects are null, with if commands, like this:

if (sineWave != null)
{
    Console.WriteLine("sine is not null");
}
if (waveOut != null)
{
    Console.WriteLine("wo is not null");
}

Both of these statements pass, and both sine is not null and wo is not null are written to the console.

namespace AddSynth
{
    public class SineWave : ISampleProvider
    {
        public WaveFormat WaveFormat { get; }
        int frequency = 440;
        int sampleRate = 44100;
        double amp = 0.25;
        int phase = 0;
        public int Read(float[] buffer, int offset, int count)
        {
            int sampleCount = sampleRate / frequency;
            for (int i = 0; i < buffer.Length; i++)
            {
                buffer[i] = (float)(amp * Math.Sin(2 * Math.PI * frequency * i + phase));
            }
            return sampleCount;
        }
    }

    public class Playback
    {
        static void Main()
        {
            Playback playBack = new Playback();
            playBack.playAudio();
        }

        public void playAudio()
        {
            WaveOutEvent waveOut = new WaveOutEvent();
            SineWave sineWave = new SineWave();
            if (sineWave != null)
            {
                Console.WriteLine("sine is not null");
            }
            if (waveOut != null)
            {
                Console.WriteLine("wo is not null");
            }
            waveOut.Init(sineWave.ToWaveProvider());
            waveOut.Play();
            Console.ReadKey();
        }
    }
}

I expect for a sine wave to be played through my computer's audio. I hope I've added enough info.

EDIT: Just realised that I probably should have added the stack trace as well, so here it is, if it helps:

   at NAudio.Wave.SampleProviders.SampleToWaveProvider..ctor(ISampleProvider source)
   at NAudio.Wave.WaveExtensionMethods.ToWaveProvider(ISampleProvider sampleProvider)
   at AddSynth.Playback.playAudio() in C:\Users\User1\source\repos\AddSynth\AddSynth\Program.cs:line 44
   at AddSynth.Playback.Main() in C:\Users\User1\source\repos\AddSynth\AddSynth\Program.cs:line 29

Answer 1

As Nkosi said, the problem was that I never assigned the WaveFormat variable.