Вроде усе

ClassLibrary1/IAuthorStorageContract.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace ClassLibrary1;
+
+public interface IAuthorStorageContract
+{
+    List<Author> GetList();
+    Author? GetElementById(string id);
+    Author? GetElementByFIO(string fio);
+    Author? GetElementByEmail(string email);
+    void AddElement(Author author);
+    void UpdElement(Author author);
+    void DelElement(string id);
+}