thank's a lot Bonnie,

I Found the problem...
That's what it should like...

OleDbConnection oCon = new OleDbConnection(@"provider=vfpoledb;data source=C:\Devellopement\Data\Video.dbc");
		
OleDbCommand poCmd = new OleDbCommand();
poCmd.Connection = oCon;
poCmd.CommandText = "test(?)";
poCmd.CommandType = CommandType.Text;


OleDbParameter x = new OleDbParameter("psParam",OleDbType.VarChar);
x.Value = "Salut";
poCmd.Parameters.Add(x);
			
poCmd.Connection.Open();
string y  = (string)poCmd.ExecuteScalar();

Vfp SP code...

PROCEDURE Test() as String
	LPARAMETERS psXmlParam as String
	RETURN TRANSFORM(psXmlParam)
ENDPROC