поправила везде методы создания и чтобы комментарии могли быть null

Polyclinic/PolyclinicContracts/BindingModels/CourseBindingModel.cs

@@ -6,7 +6,7 @@ namespace PolyclinicContracts.BindingModels
     {
         public int DaysCount { get; set; }
         public int PillsPerDay { get; set; }
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
         public Dictionary<int, IDiagnoseModel> CourseDiagnoses { get; set; } = new();
         public int Id { get; set; }
     }

Polyclinic/PolyclinicContracts/BindingModels/MedicamentBindingModel.cs

@@ -8,6 +8,6 @@ namespace PolyclinicContracts.BindingModels
         public int? SymptomId { get; set; }
         public int ProcedureId { get; set; }
         public string Name { get; set; } = string.Empty;
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
     }
 }

Polyclinic/PolyclinicContracts/BindingModels/ProcedureBindingModel.cs

@@ -7,7 +7,7 @@ namespace PolyclinicContracts.BindingModels
         public int Id { get; set; }
         public int UserId { get; set; }
         public string Name { get; set; } = string.Empty;
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
         public DateTime DateStartProcedure { get; set; } = DateTime.Now;
         public DateTime? DateStopProcedure { get; set; } 
 

Polyclinic/PolyclinicContracts/BindingModels/RecipeBindingModel.cs

@@ -7,7 +7,7 @@ namespace PolyclinicContracts.BindingModels
         public int Id { get; set; }
         public int? CourseId { get; set; }
         public int ProceduresCount { get; set; }
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
         public Dictionary<int, IProcedureModel> RecipeProcedures { get; set; } = new();
     }
 }

Polyclinic/PolyclinicContracts/BindingModels/SymptomBindingModel.cs

@@ -5,7 +5,7 @@ namespace PolyclinicContracts.BindingModels
     public class SymptomBindingModel : ISymptomModel
     {
         public string Name { get; set; } = string.Empty;
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
         public Dictionary<int, IDiagnoseModel> SymptomDiagnoses { get; set; } = new();
         public int Id { get; set; }
     }

Polyclinic/PolyclinicContracts/ViewModels/CourseViewModel.cs

@@ -10,7 +10,7 @@ namespace PolyclinicContracts.ViewModels
         [DisplayName("Препарата в день")]
         public int PillsPerDay { get; set; }
         [DisplayName("Комментарий")]
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
         public Dictionary<int, IDiagnoseModel> CourseDiagnoses { get; set; } = new();
         public int Id { get; set; }
     }

Polyclinic/PolyclinicContracts/ViewModels/MedicamentViewModel.cs

@@ -14,7 +14,7 @@ namespace PolyclinicContracts.ViewModels
 
 
         [DisplayName("Комментарий")]
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
 
 
         [DisplayName("Название симптома")]

Polyclinic/PolyclinicContracts/ViewModels/ProcedureViewModel.cs

@@ -12,7 +12,7 @@ namespace PolyclinicContracts.ViewModels
         public string Name { get; set; } = string.Empty;
 
         [DisplayName("Комментарий")]
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
 
         [DisplayName("Дата 'от'")]
         public DateTime DateStartProcedure { get; set; } = DateTime.Now;

Polyclinic/PolyclinicContracts/ViewModels/RecipeViewModel.cs

@@ -11,7 +11,7 @@ namespace PolyclinicContracts.ViewModels
         public int ProceduresCount { get; set; }
 
         [DisplayName("Комментарий")]
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
 
         [DisplayName("Номер курса")]
         public int? CourseId { get; set; }

Polyclinic/PolyclinicContracts/ViewModels/SymptomViewModel.cs

@@ -8,7 +8,7 @@ namespace PolyclinicContracts.ViewModels
         [DisplayName("Название")]
         public string Name { get; set; } = string.Empty;
         [DisplayName("Комментарий")]
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
         public Dictionary<int, IDiagnoseModel> SymptomDiagnoses { get; set; } = new();
         public int Id { get; set; }
     }

Polyclinic/PolyclinicDataModels/Models/ICourseModel.cs

@@ -4,7 +4,7 @@
     {
         int DaysCount { get; }
         int PillsPerDay { get; }
-        string Comment { get; }
+        string? Comment { get; }
         Dictionary<int, IDiagnoseModel> CourseDiagnoses { get; }
     }
 }

Polyclinic/PolyclinicDataModels/Models/IMedicamentModel.cs

@@ -3,7 +3,7 @@
     public interface IMedicamentModel : IId
     {
         string Name { get; }
-        string Comment { get; }
+        string? Comment { get; }
         int ProcedureId { get; }
         int? SymptomId { get; }
     }

Polyclinic/PolyclinicDataModels/Models/IProcedureModel.cs

@@ -3,7 +3,7 @@
     public interface IProcedureModel : IId
     {
         string Name { get; }
-        string Comment { get; }
+        string? Comment { get; }
         int UserId { get; }
         DateTime DateStartProcedure { get; }
         DateTime? DateStopProcedure { get; }   

Polyclinic/PolyclinicDataModels/Models/IRecipeModel.cs

@@ -3,7 +3,7 @@
     public interface IRecipeModel : IId
     {
         int ProceduresCount { get; set; }
-        string Comment { get; set; }
+        string? Comment { get; set; }
         int? CourseId { get; set; }
         Dictionary<int, IProcedureModel> RecipeProcedures { get; }
     }

Polyclinic/PolyclinicDataModels/Models/ISymptomModel.cs

@@ -3,7 +3,7 @@
     public interface ISymptomModel : IId
     {
         string Name { get; }
-        string Comment { get; }
+        string? Comment { get; }
         Dictionary<int, IDiagnoseModel> SymptomDiagnoses { get; }
     }
 }

Polyclinic/PolyclinicDatabaseImplement/Models/Course.cs

@@ -14,7 +14,7 @@ namespace PolyclinicDatabaseImplement.Models
         public int DaysCount { get; set; }
         [Required]
         public int PillsPerDay { get; set; }
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
         [ForeignKey("CourseId")]
         public virtual List<CourseDiagnose> Diagnoses { get; set; } = new();
         private Dictionary<int, IDiagnoseModel>? _courseDiagnoses = null;
@@ -34,7 +34,7 @@ namespace PolyclinicDatabaseImplement.Models
             }
         }
 
-        public static Course Create(PolyclinicDatabase context, CourseBindingModel model)
+        public static Course? Create(PolyclinicDatabase context, CourseBindingModel model)
         {
             return new Course()
             {

Polyclinic/PolyclinicDatabaseImplement/Models/Medicament.cs

@@ -13,7 +13,7 @@ namespace PolyclinicDatabaseImplement.Models
         [Required]
         public string Name { get; set; } = string.Empty;
 
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
 
         [Required]
         public int ProcedureId { get; set; }
@@ -24,7 +24,7 @@ namespace PolyclinicDatabaseImplement.Models
         public virtual Symptom? Symptom { get; set; }
         public virtual Procedure? Procedure { get; set; }
 
-        public static Medicament Create(MedicamentBindingModel bindingModel)
+        public static Medicament? Create(MedicamentBindingModel bindingModel)
         {
             return new Medicament()
             {

Polyclinic/PolyclinicDatabaseImplement/Models/Procedure.cs

@@ -21,9 +21,9 @@ namespace PolyclinicDatabaseImplement.Models
         public DateTime? DateStopProcedure { get; set; }
         public virtual User User { get; set; } = new();
 
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
 
-        public static Procedure Create(ProcedureBindingModel bindingModel)
+        public static Procedure? Create(ProcedureBindingModel bindingModel)
         {
             return new Procedure()
             {

Polyclinic/PolyclinicDatabaseImplement/Models/Recipe.cs

@@ -14,7 +14,7 @@ namespace PolyclinicDatabaseImplement.Models
         [Required]
         public int ProceduresCount { get; set; }
 
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
         public int? CourseId { get; set; }
         public virtual Course Course { get; set; } = new();
 
@@ -36,7 +36,7 @@ namespace PolyclinicDatabaseImplement.Models
             }
         }
 
-        public static Recipe Create(PolyclinicDatabase database, RecipeBindingModel bindingModel)
+        public static Recipe? Create(PolyclinicDatabase database, RecipeBindingModel bindingModel)
         {
             return new Recipe()
             {

Polyclinic/PolyclinicDatabaseImplement/Models/Symptom.cs

@@ -11,7 +11,7 @@ namespace PolyclinicDatabaseImplement.Models
     {
         [Required]
         public string Name { get; set; } = string.Empty;
-        public string Comment { get; set; } = string.Empty;
+        public string? Comment { get; set; } = string.Empty;
         [ForeignKey("SymptomId")]
         public virtual List<SymptomDiagnose> Diagnoses { get; set; } = new();
         private Dictionary<int, IDiagnoseModel>? _symptomDiagnoses = null;
@@ -32,7 +32,7 @@ namespace PolyclinicDatabaseImplement.Models
         }
         public int Id { get; set; }
 
-        public static Symptom Create(PolyclinicDatabase context, SymptomBindingModel model)
+        public static Symptom? Create(PolyclinicDatabase context, SymptomBindingModel model)
         {
             return new Symptom()
             {