c# - Visual Studio Does not recognize System.Linq

Question

I created this program and C# and it worked correctly a few weeks ago. Now, I copied all of this code to a another project I am working on. I added it to a brand new C# Class inside of the project I am working on. Visual Studio didn't recognize the assembly references that say "not found" in the comments.

This is just bizarre since they were found in the previous program. Does anyone know why Visual Studio can't find these assembly references.

using System;

using System.Collections.Generic;

using System.Linq;    //not found
using System.Text;
using XML_Creator;     //not found
using System.Xml.Linq; //not found
using System.IO;

Answer 1

An educated guess: It's probably the .NET framework target version of the new project.

System.Linq was introduced on .NET Framework 3.5.

Go to Project -> Properties -> Application and check the Target Framework property. It's probably a target framework prior to 3.5