(B) Add entity base class

Clinics.Backend/Domain/Primitives/Entity.cs

+namespace Domain.Primitives;
+
+// To use nullable reference types within the file
+#nullable enable
+public class Entity
+{
+    protected Entity(int id)
+    {
+        Id = id;
+    }
+
+    #region Properties
+
+    // Init is to ensure that Id is unchangable
+    public int Id { get; private init; }
+
+    #endregion
+
+    #region Equality by matching Id
+
+    public override bool Equals(object? obj)
+    {
+        if (InvalidType(obj))
+        {
+            return false;
+        }
+
+        if (obj is not Entity entity)
+        {
+            return false;
+        }
+
+        return entity.Id == Id;
+
+    }
+
+    public bool Equals(Entity other)
+    {
+        if (InvalidType(other))
+        {
+            return false;
+        }
+        return other.Id == Id;
+    }
+
+    public static bool operator ==(Entity? first, Entity? second)
+    {
+        return (first is not null) && (second is not null) && first.Equals(second);
+    }
+
+    public static bool operator !=(Entity? first, Entity? second)
+    {
+        return !(first == second);
+    }
+
+    private bool InvalidType(object? obj)
+    {
+        return (obj is null) || (obj.GetType() != this.GetType());
+    }
+
+    #endregion
+
+    #region Custom hash code for collections
+
+    // For dealing with collections
+    public override int GetHashCode()
+    {
+        // It's a best practice to mult by prime number when defining custom hash code
+        return Id.GetHashCode() * 41;
+    }
+
+    #endregion
+}