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

Unity2017升级到2019后，脚本全都不可用，报错后半段部分内容为“Feature `out variable declaration' cannot be used because it is not part of the C# 4.0 language specification”。

在Unity选择【Edit】——【Project Settings】——【Player】——【Other Settings】——【Configuration】大项下的【Scripting Runtime Version】选项选择【.NET 4.x Equivalent】即可解决问题。