The type or namespace name does not exist in the namespace system.windows the forms (if an assembly reference is missing) error

C # project:

Add "using System.Windows.Forms;" "The type or namespace name does not exist in the namespace System.Windows, the forms (is missing an assembly reference)" error,

The query knows that the System.Windows.Forms assembly is missing and adds a secondary assembly in VS2015, with the following steps:

Project Add Reference

Search for "System.Windows.Forms" in the search box in the upper right corner of the figure, and the following options appear:

Tick "System.Windows.Forms" item, OK, then.

The type or namespace name does not exist in the namespace system.windows the forms (if an assembly reference is missing) error