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 List<clsPatientModel> searchResult = new List<clsPatientModel>();

                searchResult = searchResult
                                   .Concat(
                                           from patient in patients
                                           where patient.DateEntry.EntryDate.Date == Convert.ToDateTime(dptEntryDate.Value.Date)
                                           select patient)
                                   .ToList();

                searchResult = searchResult
                                    .Concat(
                                            from patient in patients
                                            where patient.PatientDetails.DOB.Date == Convert.ToDateTime(dpDOB.Value.Date)
                                            select patient)
                                    .ToList();

                if (!string.IsNullOrWhiteSpace(txtSurname.Text))
                {
                    searchResult = searchResult
                                    .Concat(
                                            from patient in patients
                                            where patient.PatientDetails.Surname.ToLowerInvariant().Contains(txtSurname.Text.ToLowerInvariant().Trim())
                                            select patient)
                                    .ToList();
                }

                if (!string.IsNullOrWhiteSpace(txtName.Text))
                {
                    searchResult = searchResult
                                    .Concat(
                                            from patient in patients
                                            where patient.PatientDetails.Name.ToLowerInvariant().Contains(txtName.Text.ToLowerInvariant().Trim())
                                            select patient)
                                    .ToList();
                }