Initial commit

.vscode/launch.json

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+{
+   // Use IntelliSense to find out which attributes exist for C# debugging
+   // Use hover for the description of the existing attributes
+   // For further information visit https://github.com/OmniSharp/omnisharp-vscode/blob/master/debugger-launchjson.md
+   "version": "0.2.0",
+   "configurations": [
+        {
+            "name": ".NET Core Launch (console)",
+            "type": "coreclr",
+            "request": "launch",
+            "preLaunchTask": "build",
+            // If you have changed target frameworks, make sure to update the program path.
+            "program": "${workspaceFolder}/bin/Debug/netcoreapp2.1/Playground.dll",
+            "args": [],
+            "cwd": "${workspaceFolder}",
+            // For more information about the 'console' field, see https://github.com/OmniSharp/omnisharp-vscode/blob/master/debugger-launchjson.md#console-terminal-window
+            "console": "internalConsole",
+            "stopAtEntry": false,
+            "internalConsoleOptions": "openOnSessionStart"
+        },
+        {
+            "name": ".NET Core Attach",
+            "type": "coreclr",
+            "request": "attach",
+            "processId": "${command:pickProcess}"
+        }
+    ,]
+}

.vscode/tasks.json

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+{
+    "version": "2.0.0",
+    "tasks": [
+        {
+            "label": "build",
+            "command": "dotnet",
+            "type": "process",
+            "args": [
+                "build",
+                "${workspaceFolder}/Playground.csproj"
+            ],
+            "problemMatcher": "$msCompile"
+        }
+    ]
+}

Program.cs

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+using System;
+
+namespace Playground
+{
+    internal static class Program
+    {
+        private static void Main()
+        {
+            Console.WriteLine("Small dependency injection example.");
+            var injector = new SmallInjector();
+            injector.RegisterType<ServiceOne, IServiceOne>(true);
+            injector.RegisterType<ServiceTwo, IServiceTwo>(true);
+            injector.RegisterType<UsefulClass>(false);
+
+            Console.WriteLine();
+            Console.WriteLine("Resolve class instances.");
+            var useful1 = injector.Resolve<UsefulClass>();
+            var useful2 = injector.Resolve<UsefulClass>();
+            var useful3 = injector.Resolve<UsefulClass>();
+            Console.WriteLine();
+            Console.WriteLine("Run test methods.");
+            useful1.TestTheServices();
+            useful2.TestTheServices();
+            useful3.TestTheServices();
+            Console.ReadLine();
+        }
+    }
+}

RegisteredType.cs

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+using System;
+
+namespace Playground
+{
+    public class RegisteredType
+    {
+        public RegisteredType(Type concreteType, bool isSingleton)
+        {
+            ConcreteType = concreteType;
+            IsSingleton = isSingleton;
+        }
+
+        public readonly bool IsSingleton;
+        public readonly Type ConcreteType;
+        public object SingletonInstance { get; set; }
+    }
+}

ServiceOne.cs

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+using System;
+
+namespace Playground
+{
+    public class ServiceOne : IServiceOne
+    {
+        private readonly Guid _guid;
+
+        public ServiceOne()
+        {
+            _guid = Guid.NewGuid();
+            Console.WriteLine(nameof(ServiceOne) + ".ctor\t\tId: " + _guid);
+        }
+
+        public override string ToString() => nameof(ServiceOne) + ".ToString()\tId: " + _guid;
+    }
+
+    public interface IServiceOne
+    {
+
+    }
+}

ServiceTwo.cs

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+using System;
+
+namespace Playground
+{
+    public class ServiceTwo : IServiceTwo
+    {
+        private readonly Guid _guid;
+
+        public ServiceTwo()
+        {
+            _guid = Guid.NewGuid();
+            Console.WriteLine(nameof(ServiceTwo) + ".ctor\t\tId: " + _guid);
+        }
+
+        public override string ToString() => nameof(ServiceTwo) + ".ToString()\tId: " + _guid;
+
+    }
+
+    public interface IServiceTwo
+    {
+    }
+}

SmallInjector.cs

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+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Linq;
+using System.Linq.Expressions;
+using System.Reflection;
+namespace Playground
+{
+    public class SmallInjector
+    {
+        private readonly Dictionary<Type, RegisteredType> _container = new Dictionary<Type, RegisteredType>();
+
+        public void RegisterType<TService, TInterface>(bool isSingleton)
+        {
+            _container.Add(typeof(TInterface), new RegisteredType(typeof(TService), isSingleton));
+        }
+
+        public void RegisterType<TService>(bool isSingleton) => RegisterType<TService, TService>(isSingleton);
+
+        public TService Resolve<TService>() => (TService)Resolve(typeof(TService));
+
+        public object Resolve(Type serviceInterface)
+        {
+            if (!_container.TryGetValue(serviceInterface, out var registeredType))
+            {
+                throw new Exception("Can't resolve dependency " + serviceInterface);
+            }
+
+            if (registeredType.IsSingleton && registeredType.SingletonInstance != null)
+                return registeredType.SingletonInstance;
+            var constructor = registeredType.ConcreteType.GetConstructors()[0];
+            var parameters = constructor.GetParameters();
+            var constructorParameters = new object[parameters.Length];
+            for (var i = 0; i < parameters.Length; i++)
+            {
+                constructorParameters[i] = Resolve(parameters[i].ParameterType);
+            }
+
+            if (registeredType.IsSingleton)
+            {
+                return registeredType.SingletonInstance ?? (registeredType.SingletonInstance = constructor.Invoke(constructorParameters));
+            }
+            return constructor.Invoke(constructorParameters);
+
+        }
+    }
+}

SmallInjectorDemo.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <AssemblyName>SmallInjectorDemo</AssemblyName>
+    <RootNamespace>SmallInjectorDemo</RootNamespace>
+  </PropertyGroup>
+</Project>

SmallInjectorDemo.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio 15
+VisualStudioVersion = 15.0.26124.0
+MinimumVisualStudioVersion = 15.0.26124.0
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "SmallInjectorDemo", "SmallInjectorDemo.csproj", "{620CC001-7DF9-4233-AFC2-187FD9144835}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Debug|x64 = Debug|x64
+		Debug|x86 = Debug|x86
+		Release|Any CPU = Release|Any CPU
+		Release|x64 = Release|x64
+		Release|x86 = Release|x86
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{620CC001-7DF9-4233-AFC2-187FD9144835}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{620CC001-7DF9-4233-AFC2-187FD9144835}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{620CC001-7DF9-4233-AFC2-187FD9144835}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{620CC001-7DF9-4233-AFC2-187FD9144835}.Debug|x64.Build.0 = Debug|Any CPU
+		{620CC001-7DF9-4233-AFC2-187FD9144835}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{620CC001-7DF9-4233-AFC2-187FD9144835}.Debug|x86.Build.0 = Debug|Any CPU
+		{620CC001-7DF9-4233-AFC2-187FD9144835}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{620CC001-7DF9-4233-AFC2-187FD9144835}.Release|Any CPU.Build.0 = Release|Any CPU
+		{620CC001-7DF9-4233-AFC2-187FD9144835}.Release|x64.ActiveCfg = Release|Any CPU
+		{620CC001-7DF9-4233-AFC2-187FD9144835}.Release|x64.Build.0 = Release|Any CPU
+		{620CC001-7DF9-4233-AFC2-187FD9144835}.Release|x86.ActiveCfg = Release|Any CPU
+		{620CC001-7DF9-4233-AFC2-187FD9144835}.Release|x86.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {1A146B09-F1B0-4A1E-B396-739788B6A68E}
+	EndGlobalSection
+EndGlobal

UsefulClass.cs

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+using System;
+
+namespace Playground
+{
+    public class UsefulClass
+    {
+        private readonly IServiceOne _service;
+        private readonly IServiceTwo _service2;
+        private readonly Guid _guid;
+
+        public UsefulClass(IServiceOne service, IServiceTwo service2)
+        {
+            _service = service;
+            _service2 = service2;
+            _guid = Guid.NewGuid();
+            Console.WriteLine(nameof(UsefulClass) + ".ctor\tId: " + _guid);
+        }
+
+        public void TestTheServices()
+        {
+            Console.WriteLine(ToString());
+            Console.WriteLine(_service.ToString());
+            Console.WriteLine(_service2.ToString());
+        }
+
+        public override string ToString() => nameof(UsefulClass) + ".ToString()\tId: " + _guid;
+
+    }
+}