diff --git a/Syrette.Tests/ServiceContainerConstructorSelectionTests.cs b/Syrette.Tests/ServiceContainerConstructorSelectionTests.cs
new file mode 100644
index 0000000..e5dc555
--- /dev/null
+++ b/Syrette.Tests/ServiceContainerConstructorSelectionTests.cs
@@ -0,0 +1,127 @@
+namespace Syrette.Tests;
+
+public class ServiceContainerConstructorSelectionTests
+{
+    public class CtorWithOptional
+    {
+        public ITestService? A { get; }
+        public string? Name { get; }
+
+        public CtorWithOptional(ITestService a, string? name = "default")
+        {
+            A = a;
+            Name = name;
+        }
+    }
+
+    public class CtorWithExactMatch
+    {
+        public int Value { get; }
+
+        public CtorWithExactMatch(int value) { Value = value; }
+        public CtorWithExactMatch(int value, string label) { Value = value; }
+    }
+
+    public class NoSatisfiableCtor
+    {
+        public NoSatisfiableCtor(int value) { }
+    }
+
+    [Fact]
+    public void Greedy_picks_most_satisfiable_constructor()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+        container.AddSingleton<ITestServiceB, TestServiceBImpl>();
+
+        container.AddSingleton<TestMultiCtor>();
+        var instance = container.GetService<TestMultiCtor>();
+
+        Assert.NotNull(instance.A);
+        Assert.NotNull(instance.B);
+        Assert.Null(instance.C);
+    }
+
+    [Fact]
+    public void Greedy_picks_all_satisfied_over_partial()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+        container.AddSingleton<ITestServiceB, TestServiceBImpl>();
+        container.AddSingleton<ITestServiceC, TestServiceCImpl>();
+
+        container.AddSingleton<TestMultiCtor>();
+        var instance = container.GetService<TestMultiCtor>();
+
+        Assert.NotNull(instance.A);
+        Assert.NotNull(instance.B);
+        Assert.NotNull(instance.C);
+    }
+
+    [Fact]
+    public void Greedy_falls_back_to_1_param_when_only_1_satisfiable()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+
+        container.AddSingleton<TestMultiCtor>();
+        var instance = container.GetService<TestMultiCtor>();
+
+        Assert.NotNull(instance.A);
+        Assert.Null(instance.B);
+        Assert.Null(instance.C);
+    }
+
+    [Fact]
+    public void Optional_parameter_used_when_not_registered()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+        container.AddSingleton<CtorWithOptional>();
+
+        var instance = container.GetService<CtorWithOptional>();
+        Assert.NotNull(instance.A);
+        Assert.Equal("default", instance.Name);
+    }
+
+    [Fact]
+    public void Registration_args_satisfy_constructor()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<CtorWithExactMatch, CtorWithExactMatch>(42);
+
+        var instance = container.GetService<CtorWithExactMatch>();
+        Assert.Equal(42, instance.Value);
+    }
+
+    [Fact]
+    public void Resolution_args_satisfy_constructor()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<CtorWithExactMatch, CtorWithExactMatch>();
+
+        var instance = (CtorWithExactMatch)container.GetService(
+            typeof(CtorWithExactMatch), new object[] { 99 });
+        Assert.Equal(99, instance.Value);
+    }
+
+    [Fact]
+    public void Resolution_args_override_registration_args()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<CtorWithExactMatch, CtorWithExactMatch>(10);
+
+        var instance = (CtorWithExactMatch)container.GetService(
+            typeof(CtorWithExactMatch), new object[] { 20 });
+        Assert.Equal(20, instance.Value);
+    }
+
+    [Fact]
+    public void No_suitable_constructor_throws()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<NoSatisfiableCtor>();
+        Assert.Throws<InvalidOperationException>(() =>
+            container.GetService<NoSatisfiableCtor>());
+    }
+}
diff --git a/Syrette.Tests/ServiceContainerDisposalTests.cs b/Syrette.Tests/ServiceContainerDisposalTests.cs
new file mode 100644
index 0000000..3d0770e
--- /dev/null
+++ b/Syrette.Tests/ServiceContainerDisposalTests.cs
@@ -0,0 +1,40 @@
+namespace Syrette.Tests;
+
+public class ServiceContainerDisposalTests
+{
+    [Fact]
+    public void Dispose_disposes_singletons_implementing_IDisposable()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<TestDisposableService>();
+        var instance = container.GetService<TestDisposableService>();
+
+        container.Dispose();
+        Assert.True(instance.IsDisposed);
+    }
+
+    [Fact]
+    public void Dispose_clears_singleton_cache()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<TestDisposableService>();
+        container.GetService<TestDisposableService>();
+
+        container.Dispose();
+
+        Assert.Throws<InvalidOperationException>(() =>
+            container.GetService<TestDisposableService>());
+    }
+
+    [Fact]
+    public void Multiple_dispose_is_safe()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<TestDisposableService>();
+        container.GetService<TestDisposableService>();
+
+        container.Dispose();
+        container.Dispose();
+        Assert.True(true);
+    }
+}
diff --git a/Syrette.Tests/ServiceContainerRegistrationTests.cs b/Syrette.Tests/ServiceContainerRegistrationTests.cs
new file mode 100644
index 0000000..e9bbd8a
--- /dev/null
+++ b/Syrette.Tests/ServiceContainerRegistrationTests.cs
@@ -0,0 +1,100 @@
+namespace Syrette.Tests;
+
+public class ServiceContainerRegistrationTests
+{
+    [Fact]
+    public void AddSingleton_TInterface_TImplementation_registers_successfully()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+        Assert.NotNull(container.GetService<ITestService>());
+    }
+
+    [Fact]
+    public void AddSingleton_TInterface_TImplementation_with_args_registers_successfully()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestDeepService>(Guid.NewGuid());
+        Assert.NotNull(container.GetService<ITestService>());
+    }
+
+    [Fact]
+    public void AddSingleton_TClass_registers_successfully()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<TestServiceImpl>();
+        Assert.NotNull(container.GetService<TestServiceImpl>());
+    }
+
+    [Fact]
+    public void AddSingleton_TClass_with_args_registers_successfully()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<TestDeepService>(Guid.NewGuid());
+        Assert.NotNull(container.GetService<TestDeepService>());
+    }
+
+    [Fact]
+    public void AddTransient_TInterface_TImplementation_registers_successfully()
+    {
+        var container = new ServiceContainer();
+        container.AddTransient<ITestService, TestServiceImpl>();
+        Assert.NotNull(container.GetService<ITestService>());
+    }
+
+    [Fact]
+    public void AddTransient_TInterface_TImplementation_with_args_registers_successfully()
+    {
+        var container = new ServiceContainer();
+        container.AddTransient<ITestService, TestDeepService>(Guid.NewGuid());
+        Assert.NotNull(container.GetService<ITestService>());
+    }
+
+    [Fact]
+    public void AddTransient_TClass_registers_successfully()
+    {
+        var container = new ServiceContainer();
+        container.AddTransient<TestServiceImpl>();
+        Assert.NotNull(container.GetService<TestServiceImpl>());
+    }
+
+    [Fact]
+    public void AddTransient_TClass_with_args_registers_successfully()
+    {
+        var container = new ServiceContainer();
+        container.AddTransient<TestDeepService>(Guid.NewGuid());
+        Assert.NotNull(container.GetService<TestDeepService>());
+    }
+
+    [Fact]
+    public void Fluent_chaining_returns_same_container()
+    {
+        var container = new ServiceContainer();
+        var result = container
+            .AddSingleton<ITestService, TestServiceImpl>()
+            .AddTransient<TestServiceImpl>();
+        Assert.Same(container, result);
+    }
+
+    [Fact]
+    public void Duplicate_registration_same_pair_throws()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+        Assert.Throws<InvalidOperationException>(() =>
+            container.AddSingleton<ITestService, TestServiceImpl>());
+    }
+
+    [Fact]
+    public void Duplicate_registration_different_impl_for_same_service_allowed()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+        container.AddSingleton<ITestService, TestServiceAlt>();
+
+        var services = container.GetServiceTypes<ITestService>();
+        Assert.Equal(2, services.Count);
+        Assert.Contains(typeof(TestServiceImpl), services);
+        Assert.Contains(typeof(TestServiceAlt), services);
+    }
+}
diff --git a/Syrette.Tests/ServiceContainerResolutionTests.cs b/Syrette.Tests/ServiceContainerResolutionTests.cs
new file mode 100644
index 0000000..28e9e59
--- /dev/null
+++ b/Syrette.Tests/ServiceContainerResolutionTests.cs
@@ -0,0 +1,141 @@
+namespace Syrette.Tests;
+
+public class ServiceContainerResolutionTests
+{
+    [Fact]
+    public void GetService_T_resolves_singleton()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+        var instance = container.GetService<ITestService>();
+        Assert.NotNull(instance);
+        Assert.IsType<TestServiceImpl>(instance);
+    }
+
+    [Fact]
+    public void GetService_T_resolves_transient()
+    {
+        var container = new ServiceContainer();
+        container.AddTransient<ITestServiceB, TestServiceBImpl>();
+        var instance = container.GetService<ITestServiceB>();
+        Assert.NotNull(instance);
+        Assert.IsType<TestServiceBImpl>(instance);
+    }
+
+    [Fact]
+    public void Singleton_returns_same_instance()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+        var a = container.GetService<ITestService>();
+        var b = container.GetService<ITestService>();
+        Assert.Same(a, b);
+    }
+
+    [Fact]
+    public void Transient_returns_new_instance()
+    {
+        var container = new ServiceContainer();
+        container.AddTransient<ITestServiceB, TestServiceBImpl>();
+        var a = container.GetService<ITestServiceB>();
+        var b = container.GetService<ITestServiceB>();
+        Assert.NotSame(a, b);
+    }
+
+    [Fact]
+    public void GetService_Type_non_generic_resolves()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+        var instance = container.GetService(typeof(ITestService));
+        Assert.NotNull(instance);
+        Assert.IsType<TestServiceImpl>(instance);
+    }
+
+    [Fact]
+    public void GetService_Type_with_args_resolves()
+    {
+        var container = new ServiceContainer();
+        container.AddTransient<ITestService, TestDeepService>();
+        var instance = container.GetService(typeof(ITestService), new object[] { Guid.NewGuid() });
+        Assert.NotNull(instance);
+    }
+
+    [Fact]
+    public void GetService_T_throws_for_unregistered()
+    {
+        var container = new ServiceContainer();
+        Assert.Throws<InvalidOperationException>(() =>
+            container.GetService<ITestService>());
+    }
+
+    [Fact]
+    public void GetService_Type_throws_for_unregistered()
+    {
+        var container = new ServiceContainer();
+        Assert.Throws<InvalidOperationException>(() =>
+            container.GetService(typeof(ITestService)));
+    }
+
+    [Fact]
+    public void TryGetService_returns_null_for_unregistered()
+    {
+        var container = new ServiceContainer();
+        var result = container.TryGetService(typeof(ITestService));
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void TryGetService_returns_instance_when_registered()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+        var result = container.TryGetService(typeof(ITestService));
+        Assert.NotNull(result);
+    }
+
+    [Fact]
+    public void Resolve_by_implementation_type_when_not_registered_directly()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+        var instance = container.GetService(typeof(TestServiceImpl));
+        Assert.NotNull(instance);
+    }
+
+    [Fact]
+    public void Resolution_time_args_override_registration_time_args()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestDeepService>(Guid.Empty);
+        var resolved = container.GetService(typeof(ITestService), new object[] { Guid.NewGuid() });
+        Assert.NotNull(resolved);
+        var deep = (TestDeepService)resolved;
+        Assert.NotEqual(Guid.Empty, deep.Id);
+    }
+
+    [Fact]
+    public void GetServices_returns_all_registered_implementations()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+        container.AddSingleton<ITestService, TestServiceAlt>();
+        var services = container.GetServices<ITestService>();
+        Assert.Equal(2, services.Count);
+        var types = services.Select(s => s.GetType()).ToList();
+        Assert.Contains(typeof(TestServiceImpl), types);
+        Assert.Contains(typeof(TestServiceAlt), types);
+    }
+
+    [Fact]
+    public void GetServiceTypes_returns_all_implementation_types()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+        container.AddSingleton<ITestService, TestServiceAlt>();
+        var types = container.GetServiceTypes<ITestService>();
+        Assert.Equal(2, types.Count);
+        Assert.Contains(typeof(TestServiceImpl), types);
+        Assert.Contains(typeof(TestServiceAlt), types);
+    }
+}
diff --git a/Syrette.Tests/ServiceContainerThreadingTests.cs b/Syrette.Tests/ServiceContainerThreadingTests.cs
new file mode 100644
index 0000000..39c845c
--- /dev/null
+++ b/Syrette.Tests/ServiceContainerThreadingTests.cs
@@ -0,0 +1,61 @@
+namespace Syrette.Tests;
+
+public class ServiceContainerThreadingTests
+{
+    [Fact]
+    public void Concurrent_singleton_resolution_returns_same_instance()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+
+        var results = new ITestService[10];
+
+        Parallel.For(0, 10, i =>
+        {
+            results[i] = container.GetService<ITestService>();
+        });
+
+        for (int i = 1; i < results.Length; i++)
+        {
+            Assert.Same(results[0], results[i]);
+        }
+    }
+
+    [Fact]
+    public void Concurrent_transient_resolution_returns_unique_instances()
+    {
+        var container = new ServiceContainer();
+        container.AddTransient<ITestService, TestServiceImpl>();
+
+        var results = new ITestService[10];
+
+        Parallel.For(0, 10, i =>
+        {
+            results[i] = container.GetService<ITestService>();
+        });
+
+        for (int i = 0; i < results.Length; i++)
+        {
+            for (int j = i + 1; j < results.Length; j++)
+            {
+                Assert.NotSame(results[i], results[j]);
+            }
+        }
+    }
+
+    [Fact]
+    public void Concurrent_registration_and_resolution_does_not_crash()
+    {
+        var container = new ServiceContainer();
+
+        Parallel.Invoke(
+            () => { try { container.AddSingleton<ITestService, TestServiceImpl>(); } catch { } },
+            () => { try { container.AddSingleton<ITestService, TestServiceImpl>(); } catch { } },
+            () => { try { container.GetService<ITestService>(); } catch { } },
+            () => { try { container.AddSingleton<ITestService, TestServiceImpl>(); } catch { } },
+            () => { try { container.GetService<ITestService>(); } catch { } }
+        );
+
+        Assert.NotNull(container);
+    }
+}
diff --git a/Syrette.Tests/Syrette.Tests.csproj b/Syrette.Tests/Syrette.Tests.csproj
new file mode 100644
index 0000000..83ef3fa
--- /dev/null
+++ b/Syrette.Tests/Syrette.Tests.csproj
@@ -0,0 +1,25 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+    <TargetFrameworks>net10.0;net8.0</TargetFrameworks>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.2" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+    <PackageReference Include="xunit" Version="2.9.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Syrette\Syrette.csproj" />
+  </ItemGroup>
+
+</Project>
diff --git a/Syrette.Tests/TestTypes.cs b/Syrette.Tests/TestTypes.cs
new file mode 100644
index 0000000..7d12ad4
--- /dev/null
+++ b/Syrette.Tests/TestTypes.cs
@@ -0,0 +1,35 @@
+namespace Syrette.Tests;
+
+public interface ITestService { }
+public class TestServiceImpl : ITestService { }
+public class TestServiceAlt : ITestService { }
+
+public interface ITestServiceB { }
+public class TestServiceBImpl : ITestServiceB { }
+
+public interface ITestServiceC { }
+public class TestServiceCImpl : ITestServiceC { }
+
+public class TestMultiCtor
+{
+    public ITestService? A { get; }
+    public ITestServiceB? B { get; }
+    public ITestServiceC? C { get; }
+
+    public TestMultiCtor(ITestService a) { A = a; }
+    public TestMultiCtor(ITestService a, ITestServiceB b) { A = a; B = b; }
+    public TestMultiCtor(ITestService a, ITestServiceB b, ITestServiceC c) { A = a; B = b; C = c; }
+}
+
+public class TestDeepService : ITestService
+{
+    public Guid Id { get; }
+
+    public TestDeepService(Guid id) { Id = id; }
+}
+
+public class TestDisposableService : IDisposable
+{
+    public bool IsDisposed { get; private set; }
+    public void Dispose() => IsDisposed = true;
+}
diff --git a/Syrette.Tests/UnitTest1.cs b/Syrette.Tests/UnitTest1.cs
new file mode 100644
index 0000000..80655dd
--- /dev/null
+++ b/Syrette.Tests/UnitTest1.cs
@@ -0,0 +1,10 @@
+namespace Syrette.Tests;
+
+public class UnitTest1
+{
+    [Fact]
+    public void Test1()
+    {
+
+    }
+}
diff --git a/Syrette.sln b/Syrette.sln
index d547e9a..0d36704 100644
--- a/Syrette.sln
+++ b/Syrette.sln
@@ -2,16 +2,44 @@
 Microsoft Visual Studio Solution File, Format Version 12.00
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Syrette", "Syrette\Syrette.csproj", "{4730ABA2-3979-4B74-A3FF-042F6C3C47D6}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Syrette.Tests", "Syrette.Tests\Syrette.Tests.csproj", "{CE07A0AF-1EAB-4AFF-B04F-16E0A8A5FC24}"
 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
 		{4730ABA2-3979-4B74-A3FF-042F6C3C47D6}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{4730ABA2-3979-4B74-A3FF-042F6C3C47D6}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{4730ABA2-3979-4B74-A3FF-042F6C3C47D6}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{4730ABA2-3979-4B74-A3FF-042F6C3C47D6}.Debug|x64.Build.0 = Debug|Any CPU
+		{4730ABA2-3979-4B74-A3FF-042F6C3C47D6}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{4730ABA2-3979-4B74-A3FF-042F6C3C47D6}.Debug|x86.Build.0 = Debug|Any CPU
 		{4730ABA2-3979-4B74-A3FF-042F6C3C47D6}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{4730ABA2-3979-4B74-A3FF-042F6C3C47D6}.Release|Any CPU.Build.0 = Release|Any CPU
+		{4730ABA2-3979-4B74-A3FF-042F6C3C47D6}.Release|x64.ActiveCfg = Release|Any CPU
+		{4730ABA2-3979-4B74-A3FF-042F6C3C47D6}.Release|x64.Build.0 = Release|Any CPU
+		{4730ABA2-3979-4B74-A3FF-042F6C3C47D6}.Release|x86.ActiveCfg = Release|Any CPU
+		{4730ABA2-3979-4B74-A3FF-042F6C3C47D6}.Release|x86.Build.0 = Release|Any CPU
+		{CE07A0AF-1EAB-4AFF-B04F-16E0A8A5FC24}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{CE07A0AF-1EAB-4AFF-B04F-16E0A8A5FC24}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{CE07A0AF-1EAB-4AFF-B04F-16E0A8A5FC24}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{CE07A0AF-1EAB-4AFF-B04F-16E0A8A5FC24}.Debug|x64.Build.0 = Debug|Any CPU
+		{CE07A0AF-1EAB-4AFF-B04F-16E0A8A5FC24}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{CE07A0AF-1EAB-4AFF-B04F-16E0A8A5FC24}.Debug|x86.Build.0 = Debug|Any CPU
+		{CE07A0AF-1EAB-4AFF-B04F-16E0A8A5FC24}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{CE07A0AF-1EAB-4AFF-B04F-16E0A8A5FC24}.Release|Any CPU.Build.0 = Release|Any CPU
+		{CE07A0AF-1EAB-4AFF-B04F-16E0A8A5FC24}.Release|x64.ActiveCfg = Release|Any CPU
+		{CE07A0AF-1EAB-4AFF-B04F-16E0A8A5FC24}.Release|x64.Build.0 = Release|Any CPU
+		{CE07A0AF-1EAB-4AFF-B04F-16E0A8A5FC24}.Release|x86.ActiveCfg = Release|Any CPU
+		{CE07A0AF-1EAB-4AFF-B04F-16E0A8A5FC24}.Release|x86.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
 	EndGlobalSection
 EndGlobal
diff --git a/Syrette/ServiceContainer.cs b/Syrette/ServiceContainer.cs
index c9b0d04..8d023bf 100644
--- a/Syrette/ServiceContainer.cs
+++ b/Syrette/ServiceContainer.cs
@@ -165,12 +165,12 @@ public class ServiceContainer : IDisposable {
 
         if (descriptor.Lifetime == ServiceLifetime.Singleton) {
             lock (singletonLock) {
-                if (singletons.TryGetValue(descriptor.ServiceType, out var singleton)) {
+                if (singletons.TryGetValue(descriptor.ImplementationType, out var singleton)) {
                     return singleton;
                 }
 
                 var instance = Instantiate(descriptor.ImplementationType, bestCtor, mergedArgs);
-                singletons[descriptor.ServiceType] = instance;
+                singletons[descriptor.ImplementationType] = instance;
                 return instance;
             }
         }