+semver:minor - feat(mocks): runtime auto-stubs for interfaces the source generator cannot see

docs/docs/writing-tests/mocking/advanced.md

@@ -137,6 +137,47 @@ var value = serviceB.GetValue(); // 42
 
 Use `Mock.Get(obj)` to retrieve the `Mock<T>` wrapper for any mock object — auto-mocked return values, or any object created by `T.Mock()`. Auto-mocks are cached — calling the same method returns the same mock instance.
 
+### Runtime Auto-Stubs
+
+Source-generated auto-mocks cover every interface the generator can name at compile time. Some
+types it structurally cannot see — most commonly a generic method like `T Get<T>()` invoked
+*inside* a third-party SDK with a `T` that is `internal` to that SDK, so your test assembly
+cannot even write the type name:
+
+```csharp
+// Inside the Azure Functions Worker SDK — not your code:
+//   features.Get<IFunctionBindingsFeature>()   // IFunctionBindingsFeature is internal to the SDK
+
+var features = IInvocationFeatures.Mock();
+
+// The SDK's internal Get<T>() call receives a functional runtime stub instead of null.
+```
+
+In loose mode, when no source-generated mock exists for a requested interface, TUnit.Mocks emits
+a functional stub at runtime. Stubs are recursive and use the same defaults you'd expect from
+runtime-proxy libraries like NSubstitute: strings return `""`, tasks come back completed,
+collections are empty, value types are zeroed, and interface-returning members return further
+stubs (or a real configurable `Mock<T>` when a source-generated factory exists for that type).
+Properties round-trip values set on them, and member results are cached for stable identity.
+
+The stub assembly is named `DynamicProxyGenAssembly2` and carries Castle DynamicProxy's public
+key — the exact identity SDKs already grant `InternalsVisibleTo` for NSubstitute/Moq
+compatibility — so any internal interface reachable by those libraries is reachable by
+TUnit.Mocks stubs too.
+
+Notes and limits:
+
+- Runtime stubs are not configurable or verifiable — by definition you cannot name their types
+  in test code. Nameable interfaces still get real, configurable mocks.
+- Strict mode is unaffected: unconfigured calls still throw.
+- On Native AOT (where `Reflection.Emit` does not exist) the feature is inert and unconfigured
+  calls keep returning default values.
+- The `netstandard2.0` asset (used by .NET Framework test projects) does not include the
+  runtime emitter — unconfigured calls return default values there too. Runtime stubs require
+  the `net8.0`+ assets.
+- Opt out globally with `settings.Mocks.RuntimeAutoStubs = false;` in a
+  `[Before(HookType.TestDiscovery)]` hook.
+
 ## MockRepository
 
 Manage multiple mocks with shared behavior and batch operations:

src/TUnit.Mocks/MockEngine.Typed.cs

@@ -119,15 +119,9 @@ public TReturn HandleCallWithReturn<TReturn, T1>(int memberId, string memberName
         }
 #pragma warning restore IL3050, IL2026
 
-        if (Behavior == MockBehavior.Loose && typeof(TReturn).IsInterface)
+        if (TryGetLooseAutoMockResult(memberName, autoMockFactory: null, out TReturn autoMockResult))
         {
-            var cacheKey = memberName + "|" + typeof(TReturn).FullName;
-            var autoMock = AutoMockCache.GetOrAdd(cacheKey, _ =>
-            {
-                MockRegistry.TryCreateAutoMock(typeof(TReturn), Behavior, out var m);
-                return m;
-            });
-            if (autoMock is not null) return (TReturn)autoMock.ObjectInstance;
+            return autoMockResult;
         }
 
         if (Behavior == MockBehavior.Strict)
@@ -277,15 +271,9 @@ public TReturn HandleCallWithReturn<TReturn, T1, T2>(int memberId, string member
         }
 #pragma warning restore IL3050, IL2026
 
-        if (Behavior == MockBehavior.Loose && typeof(TReturn).IsInterface)
+        if (TryGetLooseAutoMockResult(memberName, autoMockFactory: null, out TReturn autoMockResult))
         {
-            var cacheKey = memberName + "|" + typeof(TReturn).FullName;
-            var autoMock = AutoMockCache.GetOrAdd(cacheKey, _ =>
-            {
-                MockRegistry.TryCreateAutoMock(typeof(TReturn), Behavior, out var m);
-                return m;
-            });
-            if (autoMock is not null) return (TReturn)autoMock.ObjectInstance;
+            return autoMockResult;
         }
 
         if (Behavior == MockBehavior.Strict)
@@ -435,15 +423,9 @@ public TReturn HandleCallWithReturn<TReturn, T1, T2, T3>(int memberId, string me
         }
 #pragma warning restore IL3050, IL2026
 
-        if (Behavior == MockBehavior.Loose && typeof(TReturn).IsInterface)
+        if (TryGetLooseAutoMockResult(memberName, autoMockFactory: null, out TReturn autoMockResult))
         {
-            var cacheKey = memberName + "|" + typeof(TReturn).FullName;
-            var autoMock = AutoMockCache.GetOrAdd(cacheKey, _ =>
-            {
-                MockRegistry.TryCreateAutoMock(typeof(TReturn), Behavior, out var m);
-                return m;
-            });
-            if (autoMock is not null) return (TReturn)autoMock.ObjectInstance;
+            return autoMockResult;
         }
 
         if (Behavior == MockBehavior.Strict)
@@ -593,15 +575,9 @@ public TReturn HandleCallWithReturn<TReturn, T1, T2, T3, T4>(int memberId, strin
         }
 #pragma warning restore IL3050, IL2026
 
-        if (Behavior == MockBehavior.Loose && typeof(TReturn).IsInterface)
+        if (TryGetLooseAutoMockResult(memberName, autoMockFactory: null, out TReturn autoMockResult))
         {
-            var cacheKey = memberName + "|" + typeof(TReturn).FullName;
-            var autoMock = AutoMockCache.GetOrAdd(cacheKey, _ =>
-            {
-                MockRegistry.TryCreateAutoMock(typeof(TReturn), Behavior, out var m);
-                return m;
-            });
-            if (autoMock is not null) return (TReturn)autoMock.ObjectInstance;
+            return autoMockResult;
         }
 
         if (Behavior == MockBehavior.Strict)
@@ -751,15 +727,9 @@ public TReturn HandleCallWithReturn<TReturn, T1, T2, T3, T4, T5>(int memberId, s
         }
 #pragma warning restore IL3050, IL2026
 
-        if (Behavior == MockBehavior.Loose && typeof(TReturn).IsInterface)
+        if (TryGetLooseAutoMockResult(memberName, autoMockFactory: null, out TReturn autoMockResult))
         {
-            var cacheKey = memberName + "|" + typeof(TReturn).FullName;
-            var autoMock = AutoMockCache.GetOrAdd(cacheKey, _ =>
-            {
-                MockRegistry.TryCreateAutoMock(typeof(TReturn), Behavior, out var m);
-                return m;
-            });
-            if (autoMock is not null) return (TReturn)autoMock.ObjectInstance;
+            return autoMockResult;
         }
 
         if (Behavior == MockBehavior.Strict)
@@ -909,15 +879,9 @@ public TReturn HandleCallWithReturn<TReturn, T1, T2, T3, T4, T5, T6>(int memberI
         }
 #pragma warning restore IL3050, IL2026
 
-        if (Behavior == MockBehavior.Loose && typeof(TReturn).IsInterface)
+        if (TryGetLooseAutoMockResult(memberName, autoMockFactory: null, out TReturn autoMockResult))
         {
-            var cacheKey = memberName + "|" + typeof(TReturn).FullName;
-            var autoMock = AutoMockCache.GetOrAdd(cacheKey, _ =>
-            {
-                MockRegistry.TryCreateAutoMock(typeof(TReturn), Behavior, out var m);
-                return m;
-            });
-            if (autoMock is not null) return (TReturn)autoMock.ObjectInstance;
+            return autoMockResult;
         }
 
         if (Behavior == MockBehavior.Strict)
@@ -1067,15 +1031,9 @@ public TReturn HandleCallWithReturn<TReturn, T1, T2, T3, T4, T5, T6, T7>(int mem
         }
 #pragma warning restore IL3050, IL2026
 
-        if (Behavior == MockBehavior.Loose && typeof(TReturn).IsInterface)
+        if (TryGetLooseAutoMockResult(memberName, autoMockFactory: null, out TReturn autoMockResult))
         {
-            var cacheKey = memberName + "|" + typeof(TReturn).FullName;
-            var autoMock = AutoMockCache.GetOrAdd(cacheKey, _ =>
-            {
-                MockRegistry.TryCreateAutoMock(typeof(TReturn), Behavior, out var m);
-                return m;
-            });
-            if (autoMock is not null) return (TReturn)autoMock.ObjectInstance;
+            return autoMockResult;
         }
 
         if (Behavior == MockBehavior.Strict)
@@ -1225,15 +1183,9 @@ public TReturn HandleCallWithReturn<TReturn, T1, T2, T3, T4, T5, T6, T7, T8>(int
         }
 #pragma warning restore IL3050, IL2026
 
-        if (Behavior == MockBehavior.Loose && typeof(TReturn).IsInterface)
+        if (TryGetLooseAutoMockResult(memberName, autoMockFactory: null, out TReturn autoMockResult))
         {
-            var cacheKey = memberName + "|" + typeof(TReturn).FullName;
-            var autoMock = AutoMockCache.GetOrAdd(cacheKey, _ =>
-            {
-                MockRegistry.TryCreateAutoMock(typeof(TReturn), Behavior, out var m);
-                return m;
-            });
-            if (autoMock is not null) return (TReturn)autoMock.ObjectInstance;
+            return autoMockResult;
         }
 
         if (Behavior == MockBehavior.Strict)

src/TUnit.Mocks/MockEngine.cs

@@ -55,7 +55,9 @@ private sealed class CallArrays(CallRecordBuffer?[] buffers, int[] counts)
     private ConcurrentQueue<(string EventName, bool IsSubscribe)>? _eventSubscriptions;
     private ConcurrentDictionary<string, Action>? _onSubscribeCallbacks;
     private ConcurrentDictionary<string, Action>? _onUnsubscribeCallbacks;
-    private ConcurrentDictionary<string, IMock?>? _autoMockCache;
+    // Keyed by member AND the actual Type identity (never a name string): two same-full-named
+    // types from different assemblies must not share a cached auto-mock/stub.
+    private ConcurrentDictionary<(string MemberName, Type ReturnType), IMock?>? _autoMockCache;
 
     /// <summary>
     /// The current state name for state machine mocking. Null means no state (all setups match).
@@ -123,24 +125,32 @@ private ConcurrentDictionary<string, Action> OnSubscribeCallbacks
     private ConcurrentDictionary<string, Action> OnUnsubscribeCallbacks
         => LazyInitializer.EnsureInitialized(ref _onUnsubscribeCallbacks)!;
 
-    private ConcurrentDictionary<string, IMock?> AutoMockCache
+    private ConcurrentDictionary<(string MemberName, Type ReturnType), IMock?> AutoMockCache
         => LazyInitializer.EnsureInitialized(ref _autoMockCache)!;
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     private bool TryGetLooseAutoMockResult<TReturn>(string memberName, Func<MockBehavior, IMock>? autoMockFactory, out TReturn result)
     {
         if (Behavior == MockBehavior.Loose && typeof(TReturn).IsInterface)
         {
-            var cacheKey = memberName + "|" + typeof(TReturn).FullName;
+            var cacheKey = (memberName, typeof(TReturn));
             var autoMock = AutoMockCache.GetOrAdd(cacheKey, _ =>
             {
                 if (autoMockFactory is not null)
                 {
                     return autoMockFactory(Behavior);
                 }
 
-                MockRegistry.TryCreateAutoMock(typeof(TReturn), Behavior, out var mock);
-                return mock;
+                if (MockRegistry.TryCreateAutoMock(typeof(TReturn), Behavior, out var mock))
+                {
+                    return mock;
+                }
+
+                // No source-generated factory — the type is either internal to another assembly
+                // (unnameable at compile time, #6514) or was simply never mocked. Fall back to a
+                // runtime-emitted stub where the platform allows it.
+                RuntimeStubs.RuntimeStubGenerator.TryCreateStub(typeof(TReturn), out var stub);
+                return stub;
             });
 
             if (autoMock is not null)
@@ -656,7 +666,28 @@ public Diagnostics.MockDiagnostics GetDiagnostics()
     }
 
     /// <summary>
-    /// Tries to get a cached auto-mock by its cache key. Used by Mock&lt;T&gt;.GetAutoMock.
+    /// Tries to get a cached auto-mock for a member and return type.
+    /// </summary>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    public bool TryGetAutoMock(string memberName, Type returnType, [System.Diagnostics.CodeAnalysis.NotNullWhen(true)] out IMock? mock)
+    {
+        if (Volatile.Read(ref _autoMockCache) is not { } cache)
+        {
+            mock = null;
+            return false;
+        }
+
+        // A cached null records a definitive miss (stubs disabled/unavailable/failed) — not a
+        // usable mock, and NotNullWhen(true) promises non-null on true.
+        return cache.TryGetValue((memberName, returnType), out mock) && mock is not null;
+    }
+
+    /// <summary>
+    /// Tries to get a cached auto-mock by the legacy string cache key
+    /// (<c>memberName + "|" + returnType.FullName</c>). Kept for binary compatibility with
+    /// assemblies compiled against the previous shape of this helper; new code uses the
+    /// <see cref="TryGetAutoMock(string, Type, out IMock?)"/> overload, which keys by Type
+    /// identity instead of a name string.
     /// </summary>
     [EditorBrowsable(EditorBrowsableState.Never)]
     public bool TryGetAutoMock(string cacheKey, [System.Diagnostics.CodeAnalysis.NotNullWhen(true)] out IMock? mock)
@@ -666,7 +697,19 @@ public bool TryGetAutoMock(string cacheKey, [System.Diagnostics.CodeAnalysis.Not
             mock = null;
             return false;
         }
-        return cache.TryGetValue(cacheKey, out mock);
+
+        foreach (var entry in cache)
+        {
+            if (entry.Value is not null &&
+                string.Equals(entry.Key.MemberName + "|" + entry.Key.ReturnType.FullName, cacheKey, StringComparison.Ordinal))
+            {
+                mock = entry.Value;
+                return true;
+            }
+        }
+
+        mock = null;
+        return false;
     }
 
     /// <summary>