First (failed) attempt

src/NSubstitute/Core/IProxyFactory.cs

@@ -3,4 +3,5 @@ namespace NSubstitute.Core;
 public interface IProxyFactory
 {
     object GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, bool isPartial, object?[]? constructorArguments);
+    object GenerateProxy(object targetObject, ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, bool isPartial, object?[]? constructorArguments);
 }

src/NSubstitute/Core/ISubstituteFactory.cs

@@ -4,4 +4,5 @@ public interface ISubstituteFactory
 {
     object Create(Type[] typesToProxy, object[] constructorArguments);
     object CreatePartial(Type[] typesToProxy, object[] constructorArguments);
+    object Create(object targetObject, Type[] typesToProxy, object?[] constructorArguments);
 }

src/NSubstitute/Core/SubstituteFactory.cs

@@ -36,6 +36,34 @@ public object CreatePartial(Type[] typesToProxy, object?[] constructorArguments)
         return Create(typesToProxy, constructorArguments, callBaseByDefault: true, isPartial: true);
     }
 
+    /// <summary>
+    /// Create a substitute for the given types, with calls configured to call the implementation on <paramref name="targetObject"/>
+    /// where possible. (virtual) Parts of the instance can be substituted using
+    /// <see cref="SubstituteExtensions.Returns{T}(T,T,T[])">Returns()</see>.
+    /// </summary>
+    /// <param name="targetObject">The instance whose implementation will be called if a corresponding member from <paramref name="typesToProxy"/> is called.</param>
+    /// <param name="typesToProxy"></param>
+    /// <param name="constructorArguments"></param>
+    /// <returns></returns>
+    public object Create(object targetObject, Type[] typesToProxy, object?[] constructorArguments)
+    {
+        return Create(targetObject, typesToProxy, constructorArguments, callBaseByDefault: false, isPartial: false);
+    }
+
+    private object Create(object targetObject, Type[] typesToProxy, object?[] constructorArguments, bool callBaseByDefault, bool isPartial)
+    {
+        var substituteState = substituteStateFactory.Create(this);
+        substituteState.CallBaseConfiguration.CallBaseByDefault = callBaseByDefault;
+
+        var primaryProxyType = GetPrimaryProxyType(typesToProxy);
+        var canConfigureBaseCalls = callBaseByDefault || CanCallBaseImplementation(primaryProxyType);
+
+        var callRouter = callRouterFactory.Create(substituteState, canConfigureBaseCalls);
+        var additionalTypes = typesToProxy.Where(x => x != primaryProxyType).ToArray();
+        var proxy = proxyFactory.GenerateProxy(targetObject, callRouter, primaryProxyType, additionalTypes, isPartial, constructorArguments);
+        return proxy;
+    }
+
     private object Create(Type[] typesToProxy, object?[] constructorArguments, bool callBaseByDefault, bool isPartial)
     {
         var substituteState = substituteStateFactory.Create(this);

src/NSubstitute/Proxies/CastleDynamicProxy/CastleDynamicProxyFactory.cs

@@ -17,6 +17,15 @@ public object GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[]? ad
             : GenerateTypeProxy(callRouter, typeToProxy, additionalInterfaces, isPartial, constructorArguments);
     }
 
+    public object GenerateProxy(object targetObject, ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, bool isPartial, object?[]? constructorArguments)
+    {
+        return typeToProxy.IsDelegate()
+            ? !targetObject.GetType().IsDelegate()
+                ? throw new NotSupportedException()
+                : throw new NotImplementedException() // TODO: Technically, there could be a use case for this. Implement if needed.
+            : GenerateTypeProxy(targetObject, callRouter, typeToProxy, additionalInterfaces, isPartial, constructorArguments);
+    }
+
     private object GenerateTypeProxy(ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, bool isPartial, object?[]? constructorArguments)
     {
         VerifyClassHasNotBeenPassedAsAnAdditionalInterface(additionalInterfaces);
@@ -38,6 +47,28 @@ private object GenerateTypeProxy(ICallRouter callRouter, Type typeToProxy, Type[
         return proxy;
     }
 
+    private object GenerateTypeProxy(object targetObject, ICallRouter callRouter, Type typeToProxy, Type[]? additionalInterfaces, bool isPartial, object?[]? constructorArguments)
+    {
+        VerifyClassHasNotBeenPassedAsAnAdditionalInterface(additionalInterfaces);
+
+        var proxyIdInterceptor = new ProxyIdInterceptor(typeToProxy);
+        var forwardingInterceptor = CreateForwardingInterceptor(callRouter);
+
+        var proxyGenerationOptions = GetOptionsToMixinCallRouterProvider(callRouter);
+
+        var proxy = CreateProxyUsingCastleProxyGenerator(
+            targetObject,
+            typeToProxy,
+            additionalInterfaces,
+            constructorArguments,
+            [proxyIdInterceptor, forwardingInterceptor],
+            proxyGenerationOptions,
+            isPartial);
+
+        forwardingInterceptor.SwitchToFullDispatchMode();
+        return proxy;
+    }
+
     private object GenerateDelegateProxy(ICallRouter callRouter, Type delegateType, Type[]? additionalInterfaces, object?[]? constructorArguments)
     {
         VerifyNoAdditionalInterfacesGivenForDelegate(additionalInterfaces);
@@ -111,6 +142,45 @@ private object CreateProxyUsingCastleProxyGenerator(Type typeToProxy, Type[]? ad
             interceptors);
     }
 
+    private object CreateProxyUsingCastleProxyGenerator(object targetObject, Type typeToProxy, Type[]? additionalInterfaces,
+        object?[]? constructorArguments,
+        IInterceptor[] interceptors,
+        ProxyGenerationOptions proxyGenerationOptions,
+        bool isPartial)
+    {
+        if (isPartial)
+            return CreatePartialProxy(targetObject, typeToProxy, additionalInterfaces, constructorArguments, interceptors, proxyGenerationOptions, isPartial);
+
+        // We make a proxy/wrapper for the target object type.
+        // We forward only implementation of the specified base type/interfaces to the target, so we don't want to use its type as typeToProxy.
+        if (typeToProxy.GetTypeInfo().IsInterface)
+        {
+            VerifyNoConstructorArgumentsGivenForInterface(constructorArguments);
+
+            var interfacesArrayLength = additionalInterfaces != null ? additionalInterfaces.Length + 1 : 1;
+            var interfaces = new Type[interfacesArrayLength];
+
+            interfaces[0] = typeToProxy;
+            if (additionalInterfaces != null)
+            {
+                Array.Copy(additionalInterfaces, 0, interfaces, 1, additionalInterfaces.Length);
+            }
+
+            // We need to create a proxy for the object type, so we can intercept the ToString() method.
+            // Therefore, we put the desired primary interface to the secondary list.
+            typeToProxy = typeof(object);
+            additionalInterfaces = interfaces;
+        }
+
+
+        return _proxyGenerator.CreateClassProxyWithTarget(typeToProxy,
+            additionalInterfaces,
+            targetObject,
+            proxyGenerationOptions,
+            constructorArguments,
+            interceptors);
+    }
+
     private object CreatePartialProxy(Type typeToProxy, Type[]? additionalInterfaces, object?[]? constructorArguments, IInterceptor[] interceptors, ProxyGenerationOptions proxyGenerationOptions, bool isPartial)
     {
         if (typeToProxy.GetTypeInfo().IsClass &&
@@ -137,6 +207,16 @@ private object CreatePartialProxy(Type typeToProxy, Type[]? additionalInterfaces
                interceptors);
     }
 
+    private object CreatePartialProxy(object targetObject, Type typeToProxy, Type[]? additionalInterfaces, object?[]? constructorArguments, IInterceptor[] interceptors, ProxyGenerationOptions proxyGenerationOptions, bool isPartial)
+    {
+        return _proxyGenerator.CreateClassProxyWithTarget(typeToProxy,
+            additionalInterfaces,
+            targetObject,
+            proxyGenerationOptions,
+            constructorArguments,
+            interceptors);
+    }
+
     private ProxyGenerationOptions GetOptionsToMixinCallRouterProvider(ICallRouter callRouter)
     {
         var options = new ProxyGenerationOptions(_allMethodsExceptCallRouterCallsHook);

src/NSubstitute/Substitute.cs

@@ -109,4 +109,23 @@ public static TInterface ForTypeForwardingTo<TInterface, TClass>(params object[]
         var substituteFactory = SubstitutionContext.Current.SubstituteFactory;
         return (TInterface)substituteFactory.CreatePartial([typeof(TInterface), typeof(TClass)], constructorArguments);
     }
+
+    /// <summary>
+    /// Creates a proxy for a class that implements an interface or class, forwarding methods and properties to an instance of the class, effectively mimicking a real instance.
+    /// The proxy will log calls made to the interface and/or virtual class members and delegate them to an instance of the target if it implements them. 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="T">The interface or class the substitute will implement.</typeparam>
+    /// <param name="target">The target instance providing implementation for (parts of) the interface</param>
+    /// <param name="constructorArguments"></param>
+    /// <returns>An object implementing the selected interface or class. Calls will be forwarded to the actual methods if possible, but allows parts to be selectively 
+    /// overridden via `Returns` and `When..DoNotCallBase`.</returns>
+    public static T ForTypeForwardingTo<T>(object target, params object[] constructorArguments)
+        where T : class
+    {
+        var substituteFactory = SubstitutionContext.Current.SubstituteFactory;
+        return (T)substituteFactory.Create(target, [typeof(T)], constructorArguments);
+    }
 }

tests/NSubstitute.Acceptance.Specs/TypeForwarding.cs

@@ -54,6 +54,17 @@ public void PartialSubstituteFailsIfClassDoesntImplementInterface()
             () => Substitute.ForTypeForwardingTo<ITestInterface, TestRandomConcreteClass>());
     }
 
+
+    [Test]
+    public void SubstitutePartialForwarding()
+    {
+        List<int> wrappedInstance = [2];
+        var sub = Substitute.ForTypeForwardingTo<IReadOnlyList<int>>(wrappedInstance);
+        using var _ = Assert.EnterMultipleScope();
+        Assert.That(sub.Count, Is.EqualTo(1));
+        Assert.That(sub[0], Is.EqualTo(2));
+        Assert.That(sub.FirstOrDefault(), Is.EqualTo(2));
+    }
     [Test]
     public void PartialSubstituteFailsIfClassIsAbstract()
     {