Using VS 15, C# with .Net 4.5.2
The computer is on an AD network, with the ad name "AD".
This problem happens with AD normal-user rights, AD admin rights, and local admin rights. It doesn't matter what rights the program gets, the same problem occurs.

Our test file is "C:/windows/system32/conhost.exe".
The file above exists, it is very much existing. I can see it with explorer.

This is the file in explorer:

This is the file properties:

You can see that it is there, right?
The following cmd command checks if the file exists:

IF EXIST "C:\windows\system32\conhost.exe" (echo does exist) ELSE (echo doesnt exist)

It returns "does exist" as promised.

The following C# code checks if the file exists:

FileInfo file = new FileInfo("C:/windows/system32/conhost.exe");
MessageBox.Show(file.Exists + "");

This returns "False".

This code also returns "False":

MessageBox.Show(File.Exists("C:/windows/system32/conhost.exe") + "");

This code also doesn't find it:

foreach (string file in Directory.GetFiles("C:/windows/system32/"))
{
    //conhost is NEVER mentioned, like it doesn't exist
}

This code also doesn't find it:

foreach (string file in Directory.EnumerateFiles("C:/windows/system32/"))
{
    //conhost is NEVER mentioned, like it doesn't exist
}

False, False, False:

MessageBox.Show(File.Exists("C:/windows/system32/conhost.exe") + "");
MessageBox.Show(File.Exists("C:\\windows\\system32\\conhost.exe") + "");
MessageBox.Show(File.Exists(@"C:\windows\system32\conhost.exe") + "");

What am I doing wrong?
Extra note: I copied conhost to C:\conhost.exe, and my program can find that without problem. My program also finds other files in system32, just not conhost and a few others. For example, it finds "connect.dll" which is in system32, so it's not the directory's read permission.
More extra notes: conhost.exe and connect.dll has the same security attributes (Security tab in the file properties).