переопределила метод то стринг в классах мастер, прием, услуга

BeautySalon/BeautySalonDBModels/Models/Master.cs

@@ -5,5 +5,10 @@
         public int MasterId { get; set; }
         public int SpecialisationId { get; set; }
         public string FIO { get; set; } = string.Empty;
+
+        public override string ToString()
+        {
+            return FIO;
+        }
     }
 }

BeautySalon/BeautySalonDBModels/Models/Reception.cs

@@ -1,4 +1,6 @@
-namespace BeautySalonDBModels.Models
+using System.Data.SqlTypes;
+
+namespace BeautySalonDBModels.Models
 {
     public class Reception
     {

BeautySalon/BeautySalonDBModels/Models/Service.cs

@@ -6,5 +6,10 @@
         public string ServiceName { get; set; } = string.Empty;
         public double Price { get; set; }
         public int SpecialisationId { get; set; }
+
+        public override string ToString()
+        {
+            return ServiceName;
+        }
     }
 }