深入理解 c# 第一章 使用异步函数在windows form 中显示产品 c#5

        private async void CheckProduct(object sender, EventArgs e)
        {
            try
            {
                // Only permit one lookup at a time
                productCheckButton.Enabled = false;
                statusLabel.Text = "Checking...";
                nameValue.Text = "";
                priceValue.Text = "";
                stockValue.Text = "";

                string id = idInput.Text;
                Task<Product> productLookup = directory.LookupProductAsync(id);
                Task<int> stockLookup = warehouse.LookupStockLevelAsync(id);

                Product product = await productLookup;
                if (product == null)
                {
                    statusLabel.Text = "Product not found";
                    // We don't care about the result of the stock check
                    return;
                }
                nameValue.Text = product.Name;
                priceValue.Text = product.Price.ToString("c");

                int stock = await stockLookup;
                stockValue.Text = stock.ToString();
                statusLabel.Text = "Ready";
            }
            finally
            {
                // However we finish this method, allow another lookup
                productCheckButton.Enabled = true;
            }
        }
    }

c#5 特性：可以中断代码的执行，但不阻塞线程

产品和库存查找是异步的

在等待结果时，并没有真正阻塞UI线程  即使代码在UI线程里

线程从他离开的地方继续执行

没有任何启动线程的 /BackgroundWorker, 调用Control.BeginInvoke 或对异步事件附加 回调函数

async/await 去掉了模板代码。

        public async Task<Product> LookupProductAsync(string productId)
        {
            await Task.Delay(Math.Abs(productId.GetHashCode() % 4000));
            return Products.FirstOrDefault(p => p.Id == productId);
        }

输入

        private static readonly List<Product> Products = new List<Product>
        {
            new Product(id: "WS", name: "West Side Story", price: 9.99m),
            new Product(id: "AS", name: "Assassins", price: 14.99m),
            new Product(id: "FR", name: "Frogs", price: 13.99m),
            new Product(id: "ST", name: "Sweeney Todd", price: 10.99m),
        };

输出