<System.Runtime.InteropServices.DllImport("shlwapi")> _
Public Function StrFormatByteSizeA(ByVal dw As UInteger, _
  ByVal pszBuf As System.Text.StringBuilder, _
  ByVal cchBuf As Integer) As System.IntPtr
End Function

Public Function TailleFichier(ByVal TailleEnOctets As UInteger) As String  Dim mem As New System.Text.StringBuilder(256)
  If StrFormatByteSizeA(TailleEnOctets, mem, mem.Capacity) <> System.IntPtr.Zero Then
    Return mem.ToString()
  Else
    Return Nothing
  End If
End Function  

        
        
    

function StrFormatByteSize(dw: DWORD; szBuf: PChar; uiBufSize: UINT): PChar; stdcall;
  external 'shlwapi.dll' name 'StrFormatByteSizeA';
 
function FormatByteSize(Value: DWORD): string;
var
  Buff: array[0..255] of Char;
begin
  StrFormatByteSize(Value, Buff, SizeOf(Buff));
  Result := Buff;
end;
 
// exemple d'utilisation :
var 
  FmtSize: string;
  Value: Longword;
 
FmtSize := FormatByteSize(Value);
 

Private Declare Function StrFormatByteSize Lib "shlwapi" Alias "StrFormatByteSizeA" (ByVal dw As Long, ByVal pszBuf As String, ByVal cchBuf As Long) As Long
'retourne la taille d'un fichier se trouvant en sPath
Public Function TailleFichier(ByVal sPath As String) As String
Dim StrOut As String
Dim lSize As Long
    'On vérifie si le fichier existe
    If Dir(sPath) <> vbNullString Then
        'on récupère sa taille
        lSize = FileLen(sPath)
        StrOut = Space(64)
        'on la formate avec StrFormatByteSize
        Call StrFormatByteSize(lSize, StrOut, Len(StrOut) - 1)
        TailleFichier = Trim$(StrOut)
    End If
End Function

// using System.Text;
// using System.Runtime.InteropServices;

[DllImport("shlwapi")]
private static extern string StrFormatByteSize64(long qdw, StringBuilder pszBuf, uint cchBuf);
private static string StrFormatByteSize64(long qdw) {
  StringBuilder StrSize = new StringBuilder(64);
    StrFormatByteSize64(qdw, StrSize, 64U);
  return StrSize.ToString();
}

// Exemples
StrFormatByteSize64(532L); // "532 bytes"
StrFormatByteSize64(1340L); // "1,30 KB"
StrFormatByteSize64(23506L); // "22,9 KB"
StrFormatByteSize64(2400016L); // "2,28 MB"
StrFormatByteSize64(2400000000L); // "2,23 GB"
StrFormatByteSize64(5466554984654L); // "4,97 TB"
StrFormatByteSize64(11874650654198465L); // "10,5 PB"
StrFormatByteSize64(7266167416426541069L); // "6,30 EB"
StrFormatByteSize64(long.MaxValue); // "7,99 EB"