Rework IEquatable<T> example (#11077)

Author: antonfirsov

snippets/csharp/System/IEquatableT/Equals/Equals.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

snippets/csharp/System/IEquatableT/Equals/EqualsEx1.cs

@@ -0,0 +1,56 @@
+// <PersonSample>
+List<Person> applicants = new List<Person>()
+{
+    new Person("Jones", "099-29-4999"),
+    new Person("Jones", "199-29-3999"),
+    new Person("Jones", "299-49-6999")
+};
+
+// Create a Person object for the final candidate.
+Person candidate = new Person("Jones", "199-29-3999");
+bool contains = applicants.Contains(candidate);
+Console.WriteLine($"{candidate.LastName} ({candidate.NationalId}) is on record: {contains}");
+// The example prints the following output:
+// Jones (199-29-3999) is on record: True
+// </PersonSample>
+
+// <Person>
+public class Person : IEquatable<Person>
+{
+    public Person(string lastName, string ssn)
+    {
+        LastName = lastName;
+        NationalId = ssn;
+    }
+
+    public string LastName { get; }
+
+    public string NationalId { get; }
+
+    public bool Equals(Person? other) => other is not null && other.NationalId == NationalId;
+
+    public override bool Equals(object? obj) => Equals(obj as Person);
+
+    public override int GetHashCode() => NationalId.GetHashCode();
+
+    public static bool operator ==(Person person1, Person person2)
+    {
+        if (person1 is null)
+        {
+            return person2 is null;
+        }
+
+        return person1.Equals(person2);
+    }
+
+    public static bool operator !=(Person person1, Person person2)
+    {
+        if (person1 is null)
+        {
+            return person2 is not null;
+        }
+
+        return !person1.Equals(person2);
+    }
+}
+// </Person>