The namespace "System" does not contain a type or namespace name "Linq ". Is Assembly reference missing ?, Namespace

The namespace "System" does not contain a type or namespace name "Linq ". Is Assembly reference missing ?, Namespace
Complete error message

Error 1: The namespace "System" does not contain a type or namespace name "Linq ". Is Assembly reference missing? F: \ CsProjects \ CSharp implement SPY ++ \ Form1.cs 6 14 CSharp implement SPY ++

Error cause

At the beginning, the Framework Version selected for creating a project was 4.0. However, to adapt to the platform adaptability of the project, the Framework version was changed to 2.0. This problem occurs when the project is re-compiled.

Solution

Double-click the error message to open the System. Linq file, as shown below:

Delete "using System. Linq;" in the file, save the file, and re-compile it.