C# compiling error: 'System.Array' does not contain a definition for 'Select' and no extension method 'Select' accepting a first argument of type 'System.Array' could be found (are you missing a using directive or an assembly reference?)

Question

Hello experts,
I'm totally new to C#. I'm trying to modify existing code to automatically rename a file if exists. I found a solution online as follows:
string[] allFiles = Directory.GetFiles(folderPath).Select(filename => Path.GetFileNameWithoutExtension(filename)).ToArray();
        string tempFileName = fileName;
        int count = 1;
        while (allFiles.Contains(tempFileName ))
            tempFileName = String.Format("{0} ({1})", fileName, count++); 
        output = Path.Combine(folderPath, tempFileName );
        string fullPath=output + ".xml";
However, it gives the following compilation errors
for the Select and Contain methods respectively.:
'System.Array' does not contain a definition for 'Select' and no extension method 'Select' accepting a first argument of type 'System.Array' could be found
(are you missing a using directive or an assembly reference?)
'System.Array' does not contain a definition for 'Contains' and no extension method 'Contains' accepting a first argument of type 'System.Array' could be
found (are you missing a using directive or an assembly reference?)
I googled on these errors, and people suggested to add using System.Linq;
I did, but the errors persist. 
Any help and information is greatly appreciated.
P. S. Here are the using clauses I have:
using System;
using System.Data;
using System.Windows.Forms;
using System.IO;
using System.Collections.Generic;
using System.Text;
using System.Linq;

Answer 1

Besides your issue with System.Core, you also have a problem with the logic of our code, particularly your variables. It is confusing what your variables represent. You have an infinite loop, so the last section of code is never reached. Take a look 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;
namespace consAppFileManipulation
class Program
static void Main(string[] args)
string fullPath = @"c:\temp\trace.log";
string folderPath = @"c:\temp\";
string fileName = "trace.log";
string output = "";
string fileNameOnly = Path.GetFileNameWithoutExtension(fullPath);
string extension = Path.GetExtension(fullPath);
string path = Path.GetDirectoryName(fullPath);
string newFullPath = fullPath;
string[] allFiles = Directory.GetFiles(folderPath).Select(filename => Path.GetFileNameWithoutExtension(filename)).ToArray();
string tempFileName = fileName;
int count = 1;
//THIS IS AN INFINITE LOOP
while (allFiles.Contains(fileNameOnly))
tempFileName = String.Format("{0} ({1})", fileName, count++);
//THIS CODE IS NEVER REACHED
output = Path.Combine(folderPath, tempFileName);
fullPath = output + ".xml";
//string fullPath = output + ".xml";
UML, then code