StoreWithMutex.cs

class Store
{
  private static Mutex mutex = new Mutex();
  private List<string> storeProducts;
  
  public Store()
  {
    storeProducts = new List<string>();
  }
  
  async Task<string> GetOrAddProduct(Product product)
  {
    try
    {
      mutex.WaitOne(); // controls access to code that isn't thread-safe
    
      if(storeProducts.Contains(product.Name))
      {
        return product;
      }
      var token = HelperClass.GenerateTokenOrSomething();
      await HelperClass.UploadProductDetails(token, product);
      storeProducts.Add(product.Name);
    }
    finally
    {
      mutex.ReleaseMutex();
      return product.Name;
    }
  }
}