【WCF】The 'await' operator can only be used within an async method.

在嘗試用 WCF 內建的 Async 方法時卡在
The 'await' operator can only be used within an async method. Consider marking this method with the 'async' modifier and changing its return type to 'Task<?>'.
'await' 運算子只能在非同步方法使用。請考慮以 'async' 修飾詞標記此方法...