using (MDPartsDatabase partsDatabase = new MDPartsManagement().OpenDatabase())
{
    //Get all parts which part number starts on "SIE"
    MDObjectFilter mDObjectFilter = new MDObjectFilter();           
    mDObjectFilter.AddPropertyCondition(22001, MDObjectFilter.CompareOperator.OperatorEqual, "SIE*"); //22001 - enum Properties.MDPartsDatabaseItem
    MDPart[] arrParts = partsDatabase.GetParts(mDObjectFilter);       
    partsDatabase.ExportParts("C:\\exportDirectory\\exportFile.xml", MDPartsDatabase.DataFormat.XML, arrParts);
}

using (MDPartsDatabase partsDatabase = new MDPartsManagement().OpenDatabase())
{
    //Export only parts modified today
    var today = DateTime.Today;
    var partsModifiedToday = partsDatabase.Parts.Where(item => item.Properties.PART_LASTCHANGE_DATE.ToTime() > today);
    partsDatabase.ExportPartsDatabaseItems("C:\\exportDirectory\\exportFile.xml", MDPartsDatabase.DataFormat.XML, partsModifiedToday);
}