Added ServiceCollectionVerify example

Author: linkdotnet

README.md

@@ -4,6 +4,7 @@ Contains all of my examples from various blog posts. You can find a comprehensiv
 
 | BlogPost                                                                                                       | Publish Date |
 | -------------------------------------------------------------------------------------------------------------- | ------------ |
+| [Verifying your DI Container](ServiceCollectionVerify/)                                                        | 30.04.2023   |
 | [.NET 8 Performance Edition](BenchmarkDotNet8/)                                                                | 12.04.2023   |
 | [Creating a ToolTip Component in Blazor](BlazorToolTip/)                                                       | 31.03.2023   |
 | [C# Source Generators: How to get build information?](BuildInformation/)                                       | 26.03.2023   |

ServiceCollectionVerify/Program.cs

@@ -0,0 +1,10 @@
+using Microsoft.Extensions.DependencyInjection;
+using ServiceCollectionVerify;
+
+var services = new ServiceCollection();
+
+services.AddScoped<TransientService>();
+services.AddSingleton<SingletonService>();
+services.AddScoped<ServiceWithMissingDependency>();
+
+services.Verify();

ServiceCollectionVerify/README.md

@@ -0,0 +1,5 @@
+# Verifying your DI Container
+
+Microsoft's integrated dependency injection (short DI) container is very powerful, but there are also certain pitfalls. In this article, I will show you what some of the pitfalls are and how you can verify them.
+
+Found [here](https://steven-giesel.com/blogPost/ce948083-974a-4c16-877f-246b8909fa6d)

ServiceCollectionVerify/Service.cs

@@ -0,0 +1,26 @@
+namespace ServiceCollectionVerify;
+
+// Ohoh - a singleton with a dependency on a scoped service -> captive dependency
+public class SingletonService
+{
+    private readonly TransientService _transientService;
+
+    public SingletonService(TransientService transientService)
+    {
+        _transientService = transientService;
+    }
+}
+public class TransientService { }
+
+// Service with a missing dependency
+public class ServiceWithMissingDependency
+{
+    public ServiceWithMissingDependency(MissingDependency missingDependency)
+    {
+    }
+}
+
+// Does not get registered in the service collection
+public class MissingDependency
+{
+}

ServiceCollectionVerify/ServiceCollectionExtensions.cs

@@ -0,0 +1,79 @@
+using System.Text;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace ServiceCollectionVerify;
+
+public class VerifyResult
+{
+    public bool IsValid => Errors.Count == 0;
+    public List<string> Errors { get; } = new();
+
+    public override string ToString()
+    {
+        var sb = new StringBuilder();
+        sb.AppendLine($"Found {Errors.Count} error(s):");
+        for (var i = 0; i < Errors.Count; i++)
+        {
+            sb.AppendLine($"{i + 1}. {Errors[i]}");
+        }
+
+        return sb.ToString();
+    }
+}
+
+public static class ServiceCollectionExtensions
+{
+    public static void Verify(this IServiceCollection services)
+    {
+var result = new VerifyResult();
+using var serviceProvider = services.BuildServiceProvider();
+
+result.Errors.AddRange(CheckServicesCanBeResolved(serviceProvider, services));
+result.Errors.AddRange(CheckForCaptiveDependencies(services));
+
+if (!result.IsValid)
+{
+    throw new InvalidOperationException(result.ToString());
+}
+    }
+
+    private static List<string> CheckServicesCanBeResolved(IServiceProvider serviceProvider, IServiceCollection services)
+    {
+        var unresolvedTypes = new List<string>();
+        foreach (var serviceDescriptor in services)
+        {
+            try
+            {
+                serviceProvider.GetRequiredService(serviceDescriptor.ServiceType);
+            }
+            catch
+            {
+                unresolvedTypes.Add($"Unable to resolve '{serviceDescriptor.ServiceType.FullName}'");
+            }
+        }
+
+        return unresolvedTypes;
+    }
+
+private static IEnumerable<string> CheckForCaptiveDependencies(IServiceCollection services)
+{
+    var singletonServices = services
+        .Where(descriptor => descriptor.Lifetime == ServiceLifetime.Singleton)
+        .Select(descriptor => descriptor.ServiceType);
+
+    foreach (var singletonService in singletonServices)
+    {
+        var captiveScopedServices = singletonService
+            .GetConstructors()
+            .SelectMany(property => property.GetParameters())
+            .Where(propertyType => services.Any(descriptor => descriptor.ServiceType == propertyType.ParameterType
+                                                              && descriptor.Lifetime == ServiceLifetime.Scoped
+                                                              || descriptor.Lifetime == ServiceLifetime.Transient));
+
+        foreach (var captiveService in captiveScopedServices)
+        {
+            yield return $"Singleton service '{singletonService.FullName}' has one or more captive dependencies: {string.Join(", ", captiveService.ParameterType.FullName)}";
+        }
+    }
+}
+}

ServiceCollectionVerify/ServiceCollectionVerify.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <OutputType>exe</OutputType>
+        <TargetFramework>net7.0</TargetFramework>
+        <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+    </PropertyGroup>
+
+    <ItemGroup>
+      <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="7.0.0" />
+    </ItemGroup>
+
+</Project>

ServiceCollectionVerify/ServiceCollectionVerify.sln

@@ -0,0 +1,16 @@
+
+Microsoft Visual Studio Solution File, Format Version 12.00
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ServiceCollectionVerify", "ServiceCollectionVerify.csproj", "{A9DA8B90-C0A7-4616-811E-CA34D7D7A873}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{A9DA8B90-C0A7-4616-811E-CA34D7D7A873}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{A9DA8B90-C0A7-4616-811E-CA34D7D7A873}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{A9DA8B90-C0A7-4616-811E-CA34D7D7A873}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{A9DA8B90-C0A7-4616-811E-CA34D7D7A873}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+EndGlobal