PathCombine

[DllImport("shlwapi.dll", CharSet = CharSet.Unicode)]
public static extern IntPtr PathCombine(
   StringBuilder pszDest,
   [MarshalAs(UnmanagedType.LPWStr)] string pszDir,
   [MarshalAs(UnmanagedType.LPWStr)] string pszFile
);

Microsoft documentation: Link