C#编译问题'System.Collections.Generic.IEnumerable' does not contain a definition for 'Where' and no extension method 'Where' accepting a first argument

'System.Collections.Generic.IEnumerable<string>' does not contain a definition for 'Where' and no extension method 'Where' accepting a first argument of type 'System.Collections.Generic.IEnumerable<string>' could be found (are you missing a using directive or an assembly reference?)

在.cs头文件加上using System.Linq;