this is already deprecated in win32, you should use ExpandEnvironmentStrings

        /// To expand the environment variable references, use the ExpandEnvironmentStrings function.

public static extern int ExpandEnvironmentStrings([MarshalAs(UnmanagedType.LPTStr)] String source, [Out] StringBuilder destination, int size);

<DllImport("kernel32.dll", EntryPoint:="ExpandEnvironmentStringsW", SetLastError:=true, CharSet:=CharSet.Unicode)> _

Public Function ExpandEnvironmentStrings(<MarshalAs(UnmanagedType.LPTStr)> lpSrc As String , _

Public Declare Function ExpandEnvironmentStrings Lib "kernel32.dll" Alias "ExpandEnvironmentStringsW" (ByVal lpSrc As String, ByVal lpDst As String, ByVal nSize As Long) As Long

This is basically the inverse of ExpandEnvironmentStrings or [Environment.ExpandEnvironmentVariables]. Only Microsoft knows why this inverse function didn't make it's way into the framework.