Feature: Enable call forwarding and substitution for non virtual methods or sealed classes implementing an interface. (#700)

How to use:
var substitute = Substitute.ForTypeForwardingTo <ISomeInterface,SomeImplementation>(argsList);

In this case, it doesn't matter if methods are virtual or not; it will intercept all calls since we will be working with an interface all the time.
For
Limitations:

Overriding virtual methods effectively replaces its implementation both for internal and external calls. With this implementation NSubstitute will only intercept calls made by client classes using the interface. Calls made from inside the object itself to its own method, will hit the actual implementation.

Author: marcoregueira

src/NSubstitute/Core/IProxyFactory.cs

@@ -2,5 +2,5 @@ namespace NSubstitute.Core;
 
 public interface IProxyFactory
 {
-    object GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, object?[]? constructorArguments);
+    object GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, bool isPartial, object?[]? constructorArguments);
 }

src/NSubstitute/Core/SubstituteFactory.cs

@@ -14,7 +14,7 @@ public class SubstituteFactory(ISubstituteStateFactory substituteStateFactory, I
     /// <returns></returns>
     public object Create(Type[] typesToProxy, object?[] constructorArguments)
     {
-        return Create(typesToProxy, constructorArguments, callBaseByDefault: false);
+        return Create(typesToProxy, constructorArguments, callBaseByDefault: false, isPartial: false);
     }
 
     /// <summary>
@@ -33,10 +33,10 @@ public object CreatePartial(Type[] typesToProxy, object?[] constructorArguments)
             throw new CanNotPartiallySubForInterfaceOrDelegateException(primaryProxyType);
         }
 
-        return Create(typesToProxy, constructorArguments, callBaseByDefault: true);
+        return Create(typesToProxy, constructorArguments, callBaseByDefault: true, isPartial: true);
     }
 
-    private object Create(Type[] typesToProxy, object?[] constructorArguments, bool callBaseByDefault)
+    private object Create(Type[] typesToProxy, object?[] constructorArguments, bool callBaseByDefault, bool isPartial)
     {
         var substituteState = substituteStateFactory.Create(this);
         substituteState.CallBaseConfiguration.CallBaseByDefault = callBaseByDefault;
@@ -46,7 +46,7 @@ private object Create(Type[] typesToProxy, object?[] constructorArguments, bool
 
         var callRouter = callRouterFactory.Create(substituteState, canConfigureBaseCalls);
         var additionalTypes = typesToProxy.Where(x => x != primaryProxyType).ToArray();
-        var proxy = proxyFactory.GenerateProxy(callRouter, primaryProxyType, additionalTypes, constructorArguments);
+        var proxy = proxyFactory.GenerateProxy(callRouter, primaryProxyType, additionalTypes, isPartial, constructorArguments);
         return proxy;
     }
 

src/NSubstitute/Exceptions/TypeForwardingException.cs

@@ -0,0 +1,21 @@
+namespace NSubstitute.Exceptions;
+
+public abstract class TypeForwardingException(string message) : SubstituteException(message)
+{
+}
+
+public sealed class CanNotForwardCallsToClassNotImplementingInterfaceException(Type type) : TypeForwardingException(DescribeProblem(type))
+{
+    private static string DescribeProblem(Type type)
+    {
+        return string.Format("The provided class '{0}' doesn't implement all requested interfaces. ", type.Name);
+    }
+}
+
+public sealed class CanNotForwardCallsToAbstractClassException(Type type) : TypeForwardingException(DescribeProblem(type))
+{
+    private static string DescribeProblem(Type type)
+    {
+        return string.Format("The provided class '{0}' is abstract. ", type.Name);
+    }
+}

src/NSubstitute/Proxies/CastleDynamicProxy/CastleDynamicProxyFactory.cs

@@ -10,14 +10,14 @@ public class CastleDynamicProxyFactory(ICallFactory callFactory, IArgumentSpecif
     private readonly ProxyGenerator _proxyGenerator = new ProxyGenerator();
     private readonly AllMethodsExceptCallRouterCallsHook _allMethodsExceptCallRouterCallsHook = new AllMethodsExceptCallRouterCallsHook();
 
-    public object GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, object?[]? constructorArguments)
+    public object GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, bool isPartial, object?[]? constructorArguments)
     {
         return typeToProxy.IsDelegate()
             ? GenerateDelegateProxy(callRouter, typeToProxy, additionalInterfaces, constructorArguments)
-            : GenerateTypeProxy(callRouter, typeToProxy, additionalInterfaces, constructorArguments);
+            : GenerateTypeProxy(callRouter, typeToProxy, additionalInterfaces, isPartial, constructorArguments);
     }
 
-    private object GenerateTypeProxy(ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, object?[]? constructorArguments)
+    private object GenerateTypeProxy(ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, bool isPartial, object?[]? constructorArguments)
     {
         VerifyClassHasNotBeenPassedAsAnAdditionalInterface(additionalInterfaces);
 
@@ -31,7 +31,8 @@ private object GenerateTypeProxy(ICallRouter callRouter, Type typeToProxy, Type[
             additionalInterfaces,
             constructorArguments,
             [proxyIdInterceptor, forwardingInterceptor],
-            proxyGenerationOptions);
+            proxyGenerationOptions,
+            isPartial);
 
         forwardingInterceptor.SwitchToFullDispatchMode();
         return proxy;
@@ -54,7 +55,8 @@ private object GenerateDelegateProxy(ICallRouter callRouter, Type delegateType,
             additionalInterfaces: null,
             constructorArguments: null,
             interceptors: [proxyIdInterceptor, forwardingInterceptor],
-            proxyGenerationOptions);
+            proxyGenerationOptions,
+            isPartial: false);
 
         forwardingInterceptor.SwitchToFullDispatchMode();
 
@@ -75,8 +77,13 @@ private CastleForwardingInterceptor CreateForwardingInterceptor(ICallRouter call
     private object CreateProxyUsingCastleProxyGenerator(Type typeToProxy, Type[]? additionalInterfaces,
                                                         object?[]? constructorArguments,
                                                         IInterceptor[] interceptors,
-                                                        ProxyGenerationOptions proxyGenerationOptions)
+                                                        ProxyGenerationOptions proxyGenerationOptions,
+                                                        bool isPartial)
     {
+        if (isPartial)
+            return CreatePartialProxy(typeToProxy, additionalInterfaces, constructorArguments, interceptors, proxyGenerationOptions, isPartial);
+
+
         if (typeToProxy.GetTypeInfo().IsInterface)
         {
             VerifyNoConstructorArgumentsGivenForInterface(constructorArguments);
@@ -96,13 +103,40 @@ private object CreateProxyUsingCastleProxyGenerator(Type typeToProxy, Type[]? ad
             additionalInterfaces = interfaces;
         }
 
+
         return _proxyGenerator.CreateClassProxy(typeToProxy,
             additionalInterfaces,
             proxyGenerationOptions,
             constructorArguments,
             interceptors);
     }
 
+    private object CreatePartialProxy(Type typeToProxy, Type[]? additionalInterfaces, object?[]? constructorArguments, IInterceptor[] interceptors, ProxyGenerationOptions proxyGenerationOptions, bool isPartial)
+    {
+        if (typeToProxy.GetTypeInfo().IsClass &&
+            additionalInterfaces != null &&
+            additionalInterfaces.Any())
+        {
+            VerifyClassIsNotAbstract(typeToProxy);
+            VerifyClassImplementsAllInterfaces(typeToProxy, additionalInterfaces);
+
+            var targetObject = Activator.CreateInstance(typeToProxy, constructorArguments);
+            typeToProxy = additionalInterfaces.First();
+
+            return _proxyGenerator.CreateInterfaceProxyWithTarget(typeToProxy,
+                 additionalInterfaces,
+                 target: targetObject,
+                 options: proxyGenerationOptions,
+                 interceptors: interceptors);
+        }
+
+        return _proxyGenerator.CreateClassProxy(typeToProxy,
+               additionalInterfaces,
+               proxyGenerationOptions,
+               constructorArguments,
+               interceptors);
+    }
+
     private ProxyGenerationOptions GetOptionsToMixinCallRouterProvider(ICallRouter callRouter)
     {
         var options = new ProxyGenerationOptions(_allMethodsExceptCallRouterCallsHook);
@@ -116,6 +150,22 @@ private ProxyGenerationOptions GetOptionsToMixinCallRouterProvider(ICallRouter c
         return options;
     }
 
+    private static void VerifyClassImplementsAllInterfaces(Type classType, IEnumerable<Type> additionalInterfaces)
+    {
+        if (!additionalInterfaces.All(x => x.GetTypeInfo().IsAssignableFrom(classType.GetTypeInfo())))
+        {
+            throw new CanNotForwardCallsToClassNotImplementingInterfaceException(classType);
+        }
+    }
+
+    private static void VerifyClassIsNotAbstract(Type classType)
+    {
+        if (classType.GetTypeInfo().IsAbstract)
+        {
+            throw new CanNotForwardCallsToAbstractClassException(classType);
+        }
+    }
+
     private static void VerifyNoConstructorArgumentsGivenForInterface(object?[]? constructorArguments)
     {
         if (HasItems(constructorArguments))

src/NSubstitute/Proxies/CastleDynamicProxy/CastleInvocationMapper.cs

@@ -10,8 +10,7 @@ public virtual ICall Map(IInvocation castleInvocation)
         Func<object>? baseMethod = null;
         if (castleInvocation.InvocationTarget != null &&
             castleInvocation.MethodInvocationTarget.IsVirtual &&
-            !castleInvocation.MethodInvocationTarget.IsAbstract &&
-            !castleInvocation.MethodInvocationTarget.IsFinal)
+            !castleInvocation.MethodInvocationTarget.IsAbstract)
         {
             baseMethod = CreateBaseResultInvocation(castleInvocation);
         }

src/NSubstitute/Proxies/DelegateProxy/DelegateProxyFactory.cs

@@ -8,9 +8,9 @@ public class DelegateProxyFactory(CastleDynamicProxyFactory objectProxyFactory)
 {
     private readonly CastleDynamicProxyFactory _castleObjectProxyFactory = objectProxyFactory ?? throw new ArgumentNullException(nameof(objectProxyFactory));
 
-    public object GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, object?[]? constructorArguments)
+    public object GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, bool isPartial, object?[]? constructorArguments)
     {
         // Castle factory can now resolve delegate proxies as well.
-        return _castleObjectProxyFactory.GenerateProxy(callRouter, typeToProxy, additionalInterfaces, constructorArguments);
+        return _castleObjectProxyFactory.GenerateProxy(callRouter, typeToProxy, additionalInterfaces, isPartial, constructorArguments);
     }
 }

src/NSubstitute/Proxies/ProxyFactory.cs

@@ -5,11 +5,11 @@ namespace NSubstitute.Proxies;
 [Obsolete("This class is deprecated and will be removed in future versions of the product.")]
 public class ProxyFactory(IProxyFactory delegateFactory, IProxyFactory dynamicProxyFactory) : IProxyFactory
 {
-    public object GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, object?[]? constructorArguments)
+    public object GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, bool isPartial, object?[]? constructorArguments)
     {
         var isDelegate = typeToProxy.IsDelegate();
         return isDelegate
-            ? delegateFactory.GenerateProxy(callRouter, typeToProxy, additionalInterfaces, constructorArguments)
-            : dynamicProxyFactory.GenerateProxy(callRouter, typeToProxy, additionalInterfaces, constructorArguments);
+            ? delegateFactory.GenerateProxy(callRouter, typeToProxy, additionalInterfaces, isPartial, constructorArguments)
+            : dynamicProxyFactory.GenerateProxy(callRouter, typeToProxy, additionalInterfaces, isPartial, constructorArguments);
     }
 }

src/NSubstitute/Substitute.cs

@@ -89,4 +89,24 @@ public static T ForPartsOf<T>(params object[] constructorArguments)
         var substituteFactory = SubstitutionContext.Current.SubstituteFactory;
         return (T)substituteFactory.CreatePartial([typeof(T)], constructorArguments);
     }
+
+    /// <summary>
+    /// Creates a proxy for a class that implements an interface, forwarding methods and properties to an instance of the class, effectively mimicking a real instance.
+    /// Both the interface and the class must be provided as parameters.
+    /// The proxy will log calls made to the interface members and delegate them to an instance of the class. Specific members can be substituted 
+    /// by using <see cref="WhenCalled{T}.DoNotCallBase()">When(() => call).DoNotCallBase()</see> or by
+    /// <see cref="SubstituteExtensions.Returns{T}(T,T,T[])">setting a value to return value</see> for that member.
+    /// This extension supports sealed classes and non-virtual members, with some limitations. Since the substituted method is non-virtual, internal calls within the object will invoke the original implementation and will not be logged.
+    /// </summary>
+    /// <typeparam name="TInterface">The interface the substitute will implement.</typeparam>
+    /// <typeparam name="TClass">The class type implementing the interface. Must be a class; not a delegate or interface. </typeparam>
+    /// <param name="constructorArguments"></param>
+    /// <returns>An object implementing the selected interface. Calls will be forwarded to the actuall methods, but allows parts to be selectively 
+    /// overridden via `Returns` and `When..DoNotCallBase`.</returns>
+    public static TInterface ForTypeForwardingTo<TInterface, TClass>(params object[] constructorArguments)
+        where TInterface : class
+    {
+        var substituteFactory = SubstitutionContext.Current.SubstituteFactory;
+        return (TInterface)substituteFactory.CreatePartial([typeof(TInterface), typeof(TClass)], constructorArguments);
+    }
 }

tests/NSubstitute.Acceptance.Specs/SubbingForConcreteTypesAndMultipleInterfaces.cs

@@ -1,4 +1,5 @@
 using NUnit.Framework;
+using NUnit.Framework.Legacy;
 
 namespace NSubstitute.Acceptance.Specs;
 
@@ -31,6 +32,30 @@ public void Can_sub_for_concrete_type_and_implement_other_interfaces()
         subAsIFirst.Received().First();
     }
 
+    [Test]
+    public void Can_sub_for_abstract_type_and_implement_other_two_interfaces()
+    {
+        // test from docs
+        var substitute = Substitute.For([typeof(IFirst), typeof(ISecond), typeof(ClassWithCtorArgs)],
+            ["hello world", 5]);
+
+        ClassicAssert.IsInstanceOf<IFirst>(substitute);
+        ClassicAssert.IsInstanceOf<ISecond>(substitute);
+        ClassicAssert.IsInstanceOf<ClassWithCtorArgs>(substitute);
+    }
+
+    [Test]
+    public void Can_sub_for_concrete_type_and_implement_other_two_interfaces()
+    {
+        // test from docs
+        var substitute = Substitute.For([typeof(IFirst), typeof(ISecond), typeof(ConcreteClassWithCtorArgs)],
+            ["hello world", 5]);
+
+        ClassicAssert.IsInstanceOf<IFirst>(substitute);
+        ClassicAssert.IsInstanceOf<ISecond>(substitute);
+        ClassicAssert.IsInstanceOf<ConcreteClassWithCtorArgs>(substitute);
+    }
+
     [Test]
     public void Partial_sub()
     {
@@ -90,8 +115,13 @@ public class Partial
         public virtual int Number() { return -1; }
         public int GetNumberPlusOne() { return Number() + 1; }
     }
+
     public abstract class ClassWithCtorArgs(string s, int a)
     {
         public string StringFromCtorArg { get; set; } = s; public int IntFromCtorArg { get; set; } = a;
     }
+
+    public class ConcreteClassWithCtorArgs(string s, int a) : ClassWithCtorArgs(s, a)
+    {
+    }
 }