/// <summary>
/// Set new task custom field value
/// </summary>
/// <param name="projectDS">Checked out single project</param>
/// <param name="taskRow">Task for update</param>
/// <param name="lookupTableDS">Lookup tables</param>
/// <param name="mdPropRow">Enterprise task custom field</param>
/// <param name="fieldValue">New value for enterprise task custom field</param>
static internal void SetTaskCustomField(ProjectDataSet projectDS, ProjectDataSet.TaskRow taskRow, LookupTableMultiLangDataSet lookupTableDS, CustomFieldDataSet.CustomFieldsRow mdPropRow, string fieldValue)
{
    var taskCustomFieldsTable = projectDS.TaskCustomFields;
    var projUid = taskRow.PROJ_UID;
    var taskUid = taskRow.TASK_UID;
    //...
    //Getting current custom field value
    var customFieldRow = taskCustomFieldsTable.Rows.Cast<ProjectDataSet.TaskCustomFieldsRow>().Where(x => x.MD_PROP_UID == mdPropRow.MD_PROP_UID && x.PROJ_UID == projUid && x.TASK_UID == taskUid).FirstOrDefault();

    if (!string.IsNullOrEmpty(fieldValue))
    {
        //If new value for task custom field is non empty -> update custom field value. It's WORKING correct
    }
    else
    {
        if (customFieldRow != null)
        {
            try
            {
                taskCustomFieldsTable.RemoveTaskCustomFieldsRow(customFieldRow);
            }
            catch (Exception e)
            {
                //Custom logging
                Logger.WriteError(String.Format("SetTaskCustomField TaskUID = "{0}" Deleting Task custom field value Error", taskRow.TASK_UID.ToString()), e);
            }
        }
    }
}