System.InvalidOperationException: No service for type 'X' has been registered

Workarounds

1. 95% success Register the missing service in Program.cs with the appropriate lifetime

   Add the missing registration in Program.cs (or Startup.cs for older patterns):
   
   var builder = WebApplication.CreateBuilder(args);
   
   // Register interface -> implementation mapping:
   builder.Services.AddScoped<IMyService, MyService>();
   
   // Or register concrete type directly:
   builder.Services.AddTransient<MyService>();
   
   // For services with dependencies, ensure ALL transitive dependencies are registered:
   builder.Services.AddScoped<IRepository, SqlRepository>();
   builder.Services.AddScoped<IMyService, MyService>(); // MyService depends on IRepository
   
   // Common registrations often missed:
   builder.Services.AddDbContext<AppDbContext>(options =>
       options.UseSqlServer(builder.Configuration.GetConnectionString("Default")));
   builder.Services.AddHttpClient<IApiClient, ApiClient>();
   builder.Services.AddMemoryCache();
   builder.Services.AddScoped<ICurrentUserService, CurrentUserService>();
   
   var app = builder.Build();

   Sources: https://learn.microsoft.com/en-us/dotnet/core/extensions/dependency-injection https://learn.microsoft.com/en-us/aspnet/core/fundamentals/dependency-injection

2. 88% success Use assembly scanning to auto-register services by convention

   Use Scrutor or a custom convention to auto-register services and avoid missing registrations:
   
   // Install Scrutor: dotnet add package Scrutor
   
   builder.Services.Scan(scan => scan
       .FromAssemblyOf<MyService>()
       .AddClasses(classes => classes.AssignableTo<IScopedService>())
       .AsImplementedInterfaces()
       .WithScopedLifetime()
       .AddClasses(classes => classes.AssignableTo<ITransientService>())
       .AsImplementedInterfaces()
       .WithTransientLifetime()
   );
   
   // Or use a simple custom convention:
   var serviceTypes = typeof(Program).Assembly.GetTypes()
       .Where(t => t.IsClass && !t.IsAbstract && t.Name.EndsWith("Service"));
   foreach (var type in serviceTypes)
   {
       var iface = type.GetInterfaces().FirstOrDefault(i => i.Name == $"I{type.Name}");
       if (iface != null)
           builder.Services.AddScoped(iface, type);
   }

   Sources: https://github.com/khellang/Scrutor https://learn.microsoft.com/en-us/dotnet/core/extensions/dependency-injection

3. 85% success Validate DI registrations at startup to catch missing registrations early

   Enable scope validation in development to catch DI issues at startup instead of at runtime:
   
   // In Program.cs:
   var builder = WebApplication.CreateBuilder(args);
   
   // Enable scope validation (already on by default in Development):
   if (builder.Environment.IsDevelopment())
   {
       builder.Host.UseDefaultServiceProvider(options =>
       {
           options.ValidateScopes = true;
           options.ValidateOnBuild = true; // Catches missing registrations at startup!
       });
   }
   
   // ValidateOnBuild will throw at app startup if any registered service
   // has a dependency that is not registered, giving you an immediate error
   // instead of a runtime exception when the service is first requested.
   
   var app = builder.Build(); // Throws here if DI graph is incomplete

   Sources: https://learn.microsoft.com/en-us/dotnet/core/extensions/dependency-injection#scope-validation