Add protected BaseResult() method to CallInfo.

.gitignore

@@ -307,3 +307,6 @@ docs/_site/*
 
 # Ignore Ionide files (https://ionide.io/)
 .ionide
+
+# kdiff/merge files
+*.orig

src/NSubstitute/Callback.cs

@@ -18,7 +18,7 @@ public class Callback
         /// </summary>
         /// <param name="doThis"></param>
         /// <returns></returns>
-        public static ConfiguredCallback First(Action<CallInfo> doThis)
+        public static ConfiguredCallback First(Action<ICallInfo> doThis)
         {
             return new ConfiguredCallback().Then(doThis);
         }
@@ -28,7 +28,7 @@ public static ConfiguredCallback First(Action<CallInfo> doThis)
         /// </summary>
         /// <param name="doThis"></param>
         /// <returns></returns>
-        public static Callback Always(Action<CallInfo> doThis)
+        public static Callback Always(Action<ICallInfo> doThis)
         {
             return new ConfiguredCallback().AndAlways(doThis);
         }
@@ -38,7 +38,7 @@ public static Callback Always(Action<CallInfo> doThis)
         /// </summary>
         /// <param name="throwThis"></param>
         /// <returns></returns>
-        public static ConfiguredCallback FirstThrow<TException>(Func<CallInfo, TException> throwThis) where TException : Exception
+        public static ConfiguredCallback FirstThrow<TException>(Func<ICallInfo, TException> throwThis) where TException : Exception
         {
             return new ConfiguredCallback().ThenThrow(throwThis);
         }
@@ -59,7 +59,7 @@ public static Callback Always(Action<CallInfo> doThis)
         /// <typeparam name="TException">The type of the exception.</typeparam>
         /// <param name="throwThis">The throw this.</param>
         /// <returns></returns>
-        public static Callback AlwaysThrow<TException>(Func<CallInfo, TException> throwThis) where TException : Exception
+        public static Callback AlwaysThrow<TException>(Func<ICallInfo, TException> throwThis) where TException : Exception
         {
             return new ConfiguredCallback().AndAlways(ToCallback(throwThis));
         }
@@ -75,33 +75,33 @@ public static Callback Always(Action<CallInfo> doThis)
             return AlwaysThrow(_ => exception);
         }
 
-        protected static Action<CallInfo> ToCallback<TException>(Func<CallInfo, TException> throwThis)
+        protected static Action<ICallInfo> ToCallback<TException>(Func<ICallInfo, TException> throwThis)
             where TException : notnull, Exception
         {
             return ci => { if (throwThis != null) throw throwThis(ci); };
         }
 
         internal Callback() { }
-        private readonly ConcurrentQueue<Action<CallInfo>> callbackQueue = new ConcurrentQueue<Action<CallInfo>>();
-        private Action<CallInfo> alwaysDo = x => { };
-        private Action<CallInfo> keepDoing = x => { };
+        private readonly ConcurrentQueue<Action<ICallInfo>> callbackQueue = new ConcurrentQueue<Action<ICallInfo>>();
+        private Action<ICallInfo> alwaysDo = x => { };
+        private Action<ICallInfo> keepDoing = x => { };
 
-        protected void AddCallback(Action<CallInfo> doThis)
+        protected void AddCallback(Action<ICallInfo> doThis)
         {
             callbackQueue.Enqueue(doThis);
         }
 
-        protected void SetAlwaysDo(Action<CallInfo> always)
+        protected void SetAlwaysDo(Action<ICallInfo> always)
         {
             alwaysDo = always ?? (_ => { });
         }
 
-        protected void SetKeepDoing(Action<CallInfo> keep)
+        protected void SetKeepDoing(Action<ICallInfo> keep)
         {
             keepDoing = keep ?? (_ => { });
         }
 
-        public void Call(CallInfo callInfo)
+        public void Call(ICallInfo callInfo)
         {
             try
             {
@@ -113,7 +113,7 @@ public void Call(CallInfo callInfo)
             }
         }
 
-        private void CallFromStack(CallInfo callInfo)
+        private void CallFromStack(ICallInfo callInfo)
         {
             if (callbackQueue.TryDequeue(out var callback))
             {

src/NSubstitute/Callbacks/ConfiguredCallback.cs

@@ -13,7 +13,7 @@ public class ConfiguredCallback : EndCallbackChain
         /// <summary>
         /// Perform this action once in chain of called callbacks.
         /// </summary>
-        public ConfiguredCallback Then(Action<CallInfo> doThis)
+        public ConfiguredCallback Then(Action<ICallInfo> doThis)
         {
             AddCallback(doThis);
             return this;
@@ -22,7 +22,7 @@ public ConfiguredCallback Then(Action<CallInfo> doThis)
         /// <summary>
         /// Keep doing this action after the other callbacks have run.
         /// </summary>
-        public EndCallbackChain ThenKeepDoing(Action<CallInfo> doThis)
+        public EndCallbackChain ThenKeepDoing(Action<ICallInfo> doThis)
         {
             SetKeepDoing(doThis);
             return this;
@@ -31,7 +31,7 @@ public EndCallbackChain ThenKeepDoing(Action<CallInfo> doThis)
         /// <summary>
         /// Keep throwing this exception after the other callbacks have run.
         /// </summary>
-        public EndCallbackChain ThenKeepThrowing<TException>(Func<CallInfo, TException> throwThis) where TException : Exception =>
+        public EndCallbackChain ThenKeepThrowing<TException>(Func<ICallInfo, TException> throwThis) where TException : Exception =>
             ThenKeepDoing(ToCallback(throwThis));
 
         /// <summary>
@@ -45,7 +45,7 @@ public EndCallbackChain ThenKeepDoing(Action<CallInfo> doThis)
         /// </summary>
         /// <typeparam name="TException">The type of the exception</typeparam>
         /// <param name="throwThis">Produce the exception to throw for a CallInfo</param>
-        public ConfiguredCallback ThenThrow<TException>(Func<CallInfo, TException> throwThis) where TException : Exception
+        public ConfiguredCallback ThenThrow<TException>(Func<ICallInfo, TException> throwThis) where TException : Exception
         {
             AddCallback(ToCallback(throwThis));
             return this;
@@ -68,7 +68,7 @@ public class EndCallbackChain : Callback
         /// Perform the given action for every call.
         /// </summary>
         /// <param name="doThis">The action to perform for every call</param>
-        public Callback AndAlways(Action<CallInfo> doThis)
+        public Callback AndAlways(Action<ICallInfo> doThis)
         {
             SetAlwaysDo(doThis);
             return this;

src/NSubstitute/Core/CallInfo.cs

@@ -9,72 +9,64 @@
 
 namespace NSubstitute.Core
 {
-    public class CallInfo
+    public class CallInfo : ICallInfo
     {
         private readonly Argument[] _callArguments;
+        private readonly Func<Maybe<object>> _baseResult;
 
-        public CallInfo(Argument[] callArguments)
-        {
+        public CallInfo(Argument[] callArguments, Func<Maybe<object>> baseResult) {
             _callArguments = callArguments;
+            _baseResult = baseResult;
+        }
+
+        protected CallInfo(CallInfo info) : this(info._callArguments, info._baseResult) {
         }
 
         /// <summary>
-        /// Gets the nth argument to this call.
+        /// Call and returns the result from the base implementation of a substitute for a class.
+        /// Will throw an exception if no base implementation exists.
         /// </summary>
-        /// <param name="index">Index of argument</param>
-        /// <returns>The value of the argument at the given index</returns>
-        public object this[int index]
-        {
+        /// <returns>Result from base implementation</returns>
+        /// <exception cref="NoBaseImplementationException">Throws in no base implementation exists</exception>
+        protected object GetBaseResult() {
+            return _baseResult().ValueOr(() => throw new NoBaseImplementationException());
+        }
+
+        /// <inheritdoc/>
+        public object this[int index] {
             get => _callArguments[index].Value;
-            set
-            {
+            set {
                 var argument = _callArguments[index];
                 EnsureArgIsSettable(argument, index, value);
                 argument.Value = value;
             }
         }
 
-        private void EnsureArgIsSettable(Argument argument, int index, object value)
-        {
-            if (!argument.IsByRef)
-            {
+        private void EnsureArgIsSettable(Argument argument, int index, object value) {
+            if (!argument.IsByRef) {
                 throw new ArgumentIsNotOutOrRefException(index, argument.DeclaredType);
             }
 
-            if (value != null && !argument.CanSetValueWithInstanceOf(value.GetType()))
-            {
+            if (value != null && !argument.CanSetValueWithInstanceOf(value.GetType())) {
                 throw new ArgumentSetWithIncompatibleValueException(index, argument.DeclaredType, value.GetType());
             }
         }
 
-        /// <summary>
-        /// Get the arguments passed to this call.
-        /// </summary>
-        /// <returns>Array of all arguments passed to this call</returns>
+        /// <inheritdoc/>
         public object[] Args() => _callArguments.Select(x => x.Value).ToArray();
 
-        /// <summary>
-        /// Gets the types of all the arguments passed to this call.
-        /// </summary>
-        /// <returns>Array of types of all arguments passed to this call</returns>
+        /// <inheritdoc/>
         public Type[] ArgTypes() => _callArguments.Select(x => x.DeclaredType).ToArray();
 
-        /// <summary>
-        /// Gets the argument of type `T` passed to this call. This will throw if there are no arguments
-        /// of this type, or if there is more than one matching argument.
-        /// </summary>
-        /// <typeparam name="T">The type of the argument to retrieve</typeparam>
-        /// <returns>The argument passed to the call, or throws if there is not exactly one argument of this type</returns>
-        public T Arg<T>()
-        {
+        /// <inheritdoc/>
+        public T Arg<T>() {
             T arg;
             if (TryGetArg(x => x.IsDeclaredTypeEqualToOrByRefVersionOf(typeof(T)), out arg)) return arg;
             if (TryGetArg(x => x.IsValueAssignableTo(typeof(T)), out arg)) return arg;
             throw new ArgumentNotFoundException("Can not find an argument of type " + typeof(T).FullName + " to this call.");
         }
 
-        private bool TryGetArg<T>(Func<Argument, bool> condition, [MaybeNullWhen(false)] out T value)
-        {
+        private bool TryGetArg<T>(Func<Argument, bool> condition, [MaybeNullWhen(false)] out T value) {
             value = default;
 
             var matchingArgs = _callArguments.Where(condition);
@@ -85,10 +77,8 @@ private bool TryGetArg<T>(Func<Argument, bool> condition, [MaybeNullWhen(false)]
             return true;
         }
 
-        private void ThrowIfMoreThanOne<T>(IEnumerable<Argument> arguments)
-        {
-            if (arguments.Skip(1).Any())
-            {
+        private void ThrowIfMoreThanOne<T>(IEnumerable<Argument> arguments) {
+            if (arguments.Skip(1).Any()) {
                 throw new AmbiguousArgumentsException(
                     "There is more than one argument of type " + typeof(T).FullName + " to this call.\n" +
                     "The call signature is (" + DisplayTypes(ArgTypes()) + ")\n" +
@@ -97,33 +87,24 @@ private void ThrowIfMoreThanOne<T>(IEnumerable<Argument> arguments)
             }
         }
 
-        /// <summary>
-        /// Gets the argument passed to this call at the specified zero-based position, converted to type `T`.
-        /// This will throw if there are no arguments, if the argument is out of range or if it
-        /// cannot be converted to the specified type.
-        /// </summary>
-        /// <typeparam name="T">The type of the argument to retrieve</typeparam>
-        /// <param name="position">The zero-based position of the argument to retrieve</param>
-        /// <returns>The argument passed to the call, or throws if there is not exactly one argument of this type</returns>
-        public T ArgAt<T>(int position)
-        {
-            if (position >= _callArguments.Length)
-            {
+        /// <inheritdoc/>
+        public T ArgAt<T>(int position) {
+            if (position >= _callArguments.Length) {
                 throw new ArgumentOutOfRangeException(nameof(position), $"There is no argument at position {position}");
             }
 
-            try
-            {
-                return (T) _callArguments[position].Value!;
-            }
-            catch (InvalidCastException)
-            {
+            try {
+                return (T)_callArguments[position].Value!;
+            } catch (InvalidCastException) {
                 throw new InvalidCastException(
                     $"Couldn't convert parameter at position {position} to type {typeof(T).FullName}");
             }
         }
 
         private static string DisplayTypes(IEnumerable<Type> types) =>
             string.Join(", ", types.Select(x => x.Name).ToArray());
+
+        /// <inheritdoc/>
+        public ICallInfo<T> ForCallReturning<T>() => new CallInfo<T>(this);
     }
 }

src/NSubstitute/Core/CallInfoFactory.cs

@@ -5,7 +5,7 @@ public class CallInfoFactory : ICallInfoFactory
         public CallInfo Create(ICall call)
         {
             var arguments = GetArgumentsFromCall(call);
-            return new CallInfo(arguments);
+            return new CallInfo(arguments, () => call.TryCallBase());
         }
 
         private static Argument[] GetArgumentsFromCall(ICall call)

src/NSubstitute/Core/CallInfoWithReturns.cs

@@ -0,0 +1,16 @@
+namespace NSubstitute.Core
+{
+    /// <summary>
+    /// Information for a call that returns a value of type <c>T</c>.
+    /// </summary>
+    /// <typeparam name="T"></typeparam>
+    public class CallInfo<T> : CallInfo, ICallInfo<T>
+    {
+        internal CallInfo(CallInfo info) : base(info) {
+        }
+
+        public T BaseResult() {
+            return (T)GetBaseResult();
+        }
+    }
+}