ReusableAwaiter.cs

using System;
using System.Runtime.CompilerServices;

public sealed class ReusableAwaiter<T> : INotifyCompletion
{
    private Action _continuation = null!;
    private T _result = default(T)!;
    private Exception _exception = null!;

    public bool IsCompleted
    {
        get;
        private set;
    }

    public T GetResult()
    {
        if (_exception != null)
            throw _exception;
        return _result;
    }

    public void OnCompleted(Action continuation)
    {
        if (_continuation != null)
            throw new InvalidOperationException("This ReusableAwaiter instance has already been listened");
        _continuation = continuation;
    }

    /// <summary>
    /// Attempts to transition the completion state.
    /// </summary>
    /// <param name="result"></param>
    /// <returns></returns>
    public bool TrySetResult(T result)
    {
        if (!this.IsCompleted)
        {
            this.IsCompleted = true;
            this._result = result;

            if (_continuation != null)
                _continuation();
            return true;
        }
        return false;
    }

    /// <summary>
    /// Attempts to transition the exception state.
    /// </summary>
    /// <param name="result"></param>
    /// <returns></returns>
    public bool TrySetException(Exception exception)
    {
        if (!this.IsCompleted)
        {
            this.IsCompleted = true;
            this._exception = exception;

            if (_continuation != null)
                _continuation();
            return true;
        }
        return false;
    }

    /// <summary>
    /// Reset the awaiter to initial status
    /// </summary>
    /// <returns></returns>
    public ReusableAwaiter<T> Reset()
    {
        this._result = default(T)!;
        this._continuation = null!;
        this._exception = null!;
        this.IsCompleted = false;
        return this;
    }

    public ReusableAwaiter<T> GetAwaiter()
    {
        return this;
    }
}