Unity "Feature 'out variable declaration' cannot be used because it is not part of the C# 4.0"

Unity2017升級到2019后，腳本全都不可用，報錯后半段部分內容為“Feature `out variable declaration' cannot be used because it is not part of the C# 4.0 language specification”。

在Unity選擇【Edit】——【Project Settings】——【Player】——【Other Settings】——【Configuration】大項下的【Scripting Runtime Version】選項選擇【.NET 4.x Equivalent】即可解決問題。