Add further test coverage

Also remove a condition which can never be exercised.
ServiceDescriptor's own constructor makes this impossible.

CSF.Screenplay.SpecFlow/ServiceCollectionAdapter.cs

@@ -84,10 +84,8 @@ public void Add(ServiceDescriptor item)
                 OpenGenericRegisterFactory.MakeGenericMethod(item.ServiceType).Invoke(this, new[] { item });
             else if (item.ImplementationInstance != null)
                 OpenGenericRegisterInstance.MakeGenericMethod(item.ServiceType).Invoke(this, new[] { item });
-            else if (item.ImplementationType != null)
-                OpenGenericRegisterType.MakeGenericMethod(item.ServiceType, item.ImplementationType).Invoke(this, Array.Empty<object>());
             else
-                throw new ArgumentException($"Unsupported {nameof(ServiceDescriptor)}; one of implementation factory, instance or type must not be null", nameof(item));
+                OpenGenericRegisterType.MakeGenericMethod(item.ServiceType, item.ImplementationType).Invoke(this, Array.Empty<object>());
         }
 
         void RegisterType<TSvc,TImpl>() where TImpl : class,TSvc

Tests/CSF.Screenplay.SpecFlow.Tests/ServiceCollectionAdapterTests.cs

@@ -0,0 +1,68 @@
+using System;
+using System.Collections;
+using BoDi;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace CSF.Screenplay;
+
+[TestFixture,Parallelizable]
+public class ServiceCollectionAdapterTests
+{
+    [Test]
+    public void UnsupportedFunctionalityShouldThrowNotSupportedException()
+    {
+        var container = new ObjectContainer();
+        var sut = new ServiceCollectionAdapter(container);
+
+        Assert.Multiple(() =>
+        {
+            Assert.That(() => sut[0], Throws.InstanceOf<NotSupportedException>(), "Indexer get");
+            Assert.That(() => sut[0] = null, Throws.InstanceOf<NotSupportedException>(), "Indexer set");
+            Assert.That(() => sut.Clear(), Throws.InstanceOf<NotSupportedException>(), nameof(IServiceCollection.Clear));
+            Assert.That(() => sut.Contains(null), Throws.InstanceOf<NotSupportedException>(), nameof(IServiceCollection.Contains));
+            Assert.That(() => sut.CopyTo(null, default), Throws.InstanceOf<NotSupportedException>(), nameof(IServiceCollection.CopyTo));
+            Assert.That(() => sut.GetEnumerator(), Throws.InstanceOf<NotSupportedException>(), nameof(IServiceCollection.GetEnumerator));
+            Assert.That(() => ((IEnumerable) sut).GetEnumerator(), Throws.InstanceOf<NotSupportedException>(), nameof(IServiceCollection.GetEnumerator) + ": " + nameof(IEnumerable));
+            Assert.That(() => sut.IndexOf(default), Throws.InstanceOf<NotSupportedException>(), nameof(IServiceCollection.IndexOf));
+            Assert.That(() => sut.Insert(default, null), Throws.InstanceOf<NotSupportedException>(), nameof(IServiceCollection.Insert));
+            Assert.That(() => sut.Remove(null), Throws.InstanceOf<NotSupportedException>(), nameof(IServiceCollection.Remove));
+            Assert.That(() => sut.RemoveAt(default), Throws.InstanceOf<NotSupportedException>(), nameof(IServiceCollection.RemoveAt));
+        });
+    }
+
+    [Test]
+    public void AddTypeShouldAddToObjectContainer()
+    {
+        var container = new ObjectContainer();
+        var sut = new ServiceCollectionAdapter(container);
+
+        sut.AddSingleton<ISampleInterface, SampleType>();
+        Assert.That(container.Resolve<ISampleInterface>, Is.InstanceOf<SampleType>());
+    }
+
+    [Test]
+    public void AddInstanceShouldAddToObjectContainer()
+    {
+        var container = new ObjectContainer();
+        var sut = new ServiceCollectionAdapter(container);
+
+        var instance = new SampleType();
+        sut.AddSingleton<ISampleInterface>(instance);
+        Assert.That(container.Resolve<ISampleInterface>(), Is.SameAs(instance));
+    }
+
+    [Test]
+    public void AddFactoryShouldAddToObjectContainer()
+    {
+        var container = new ObjectContainer();
+        var sut = new ServiceCollectionAdapter(container);
+
+        var instance = new SampleType();
+        sut.AddSingleton<ISampleInterface>(_ => instance);
+        Assert.That(container.Resolve<ISampleInterface>(), Is.SameAs(instance));
+    }
+
+    interface ISampleInterface {}
+
+    class SampleType : ISampleInterface {}
+}