Add project files.

Author: ovinokurov

DesignPatterns.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

DesignPatterns.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.3.32825.248
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "DesignPatterns", "DesignPatterns.csproj", "{884C2FD3-F204-4DE2-B246-C0843039D7D9}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{884C2FD3-F204-4DE2-B246-C0843039D7D9}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{884C2FD3-F204-4DE2-B246-C0843039D7D9}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{884C2FD3-F204-4DE2-B246-C0843039D7D9}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{884C2FD3-F204-4DE2-B246-C0843039D7D9}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {26F319F6-0A76-4330-934F-660C59AB743F}
+	EndGlobalSection
+EndGlobal

IProduct.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace DesignPatterns
+{
+    public interface IProduct
+    {
+        string GetUsername(string emailAddress);
+    }
+
+}

Product.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace DesignPatterns
+{
+    public class Product : IProduct
+    {
+        public string GetUsername(string emailAddress)
+        {
+            int atIndex = emailAddress.IndexOf('@');
+            return (atIndex > 0) ? emailAddress.Substring(0, atIndex) : string.Empty;
+        }
+    }
+
+}

ProductFactory.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace DesignPatterns
+{
+    public class ProductFactory
+    {
+        public IProduct CreateProduct()
+        {
+            return new Product();
+        }
+    }
+
+}

Program.cs

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+using DesignPatterns;
+
+//No Pattern 
+Console.WriteLine("No Pattern");
+Console.WriteLine($"Hello, {GetUsername("[email protected]")}!");
+
+static string GetUsername(string emailAddress)
+{
+    int atIndex = emailAddress.IndexOf('@');
+    return (atIndex > 0) ? emailAddress.Substring(0, atIndex) : string.Empty;
+}
+
+//Singleton Pattern
+Console.WriteLine("Singleton Pattern");
+Console.WriteLine($"Hello, {Singleton.Instance.GetUsername("[email protected]")}!");
+
+//Factory Pattern
+ProductFactory factory = new ProductFactory();
+IProduct product = factory.CreateProduct();
+Console.WriteLine("Factory Pattern");
+Console.WriteLine($"Hello, {product.GetUsername("[email protected]")}!");

Singleton.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace DesignPatterns
+{
+    public class Singleton
+    {
+        private static Singleton instance;
+
+        private Singleton()
+        {
+            // Private constructor to prevent instantiation from outside the class
+        }
+
+        public static Singleton Instance
+        {
+            get
+            {
+                if (instance == null)
+                {
+                    instance = new Singleton();
+                }
+                return instance;
+            }
+        }
+
+        public string GetUsername(string emailAddress)
+        {
+            int atIndex = emailAddress.IndexOf('@');
+            return (atIndex > 0) ? emailAddress.Substring(0, atIndex) : string.Empty;
+        }
+    }
+}