Optional arguments in constructor

[candychiu]

I can't seem to create sub's when the constructor contains optional arguments.

[Test]
public void Test()
{
var a = Substitute.For(1);
}

////// scenario 1: default constructor not explicitly defined
public class A {
internal A(int arg1 = 0, string arg2 = "") {}
}
Result Message: System.NotSupportedException : Parent does not have a default constructor. The default constructor must be explicitly defined.
Result StackTrace:
at System.Reflection.Emit.TypeBuilder.DefineDefaultConstructorNoLock(MethodAttributes attributes)
at System.Reflection.Emit.TypeBuilder.DefineDefaultConstructor(MethodAttributes attributes)
at System.Reflection.Emit.TypeBuilder.CreateTypeNoLock()
at System.Reflection.Emit.TypeBuilder.CreateType()
at Castle.DynamicProxy.Generators.Emitters.AbstractTypeEmitter.CreateType(TypeBuilder type)
at Castle.DynamicProxy.Generators.Emitters.AbstractTypeEmitter.BuildType()
at Castle.DynamicProxy.Generators.ClassProxyGenerator.GenerateType(String name, Type[] interfaces, INamingScope namingScope)
at Castle.DynamicProxy.Generators.ClassProxyGenerator.<>c__DisplayClass1.b__0(String n, INamingScope s)
at Castle.DynamicProxy.Generators.BaseProxyGenerator.ObtainProxyType(CacheKey cacheKey, Func`3 factory)
at Castle.DynamicProxy.Generators.ClassProxyGenerator.GenerateCode(Type[] interfaces, ProxyGenerationOptions options)
at Castle.DynamicProxy.DefaultProxyBuilder.CreateClassProxyType(Type classToProxy, Type[] additionalInterfacesToProxy, ProxyGenerationOptions options)
at Castle.DynamicProxy.ProxyGenerator.CreateClassProxy(Type classToProxy, Type[] additionalInterfacesToProxy, ProxyGenerationOptions options, Object[] constructorArguments, IInterceptor[] interceptors)
at NSubstitute.Proxies.CastleDynamicProxy.CastleDynamicProxyFactory.CreateProxyUsingCastleProxyGenerator(Type typeToProxy, Type[] additionalInterfaces, Object[] constructorArguments, IInterceptor interceptor, ProxyGenerationOptions proxyGenerationOptions)
at NSubstitute.Proxies.CastleDynamicProxy.CastleDynamicProxyFactory.GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[] additionalInterfaces, Object[] constructorArguments)
at NSubstitute.Proxies.ProxyFactory.GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[] additionalInterfaces, Object[] constructorArguments)
at NSubstitute.Core.SubstituteFactory.Create(Type[] typesToProxy, Object[] constructorArguments, SubstituteConfig config)
at NSubstitute.Core.SubstituteFactory.Create(Type[] typesToProxy, Object[] constructorArguments)
at NSubstitute.Substitute.For(Type[] typesToProxy, Object[] constructorArguments)
at NSubstitute.Substitute.For[T](Object[] constructorArguments)

////// scenario 2: default constructor explicitly defined
public class A {
internal A() {}
internal A(int arg1 = 0, string arg2 = "") {}
}
Result Message:
Castle.DynamicProxy.InvalidProxyConstructorArgumentsException : Can not instantiate proxy of class: Testing.NSubOptionalArg+A.
Could not find a constructor that would match given arguments:
System.Int32
Result StackTrace:
at Castle.DynamicProxy.ProxyGenerator.CreateClassProxyInstance(Type proxyType, List`1 proxyArguments, Type classToProxy, Object[] constructorArguments)
at Castle.DynamicProxy.ProxyGenerator.CreateClassProxy(Type classToProxy, Type[] additionalInterfacesToProxy, ProxyGenerationOptions options, Object[] constructorArguments, IInterceptor[] interceptors)
at NSubstitute.Proxies.CastleDynamicProxy.CastleDynamicProxyFactory.CreateProxyUsingCastleProxyGenerator(Type typeToProxy, Type[] additionalInterfaces, Object[] constructorArguments, IInterceptor interceptor, ProxyGenerationOptions proxyGenerationOptions)
at NSubstitute.Proxies.CastleDynamicProxy.CastleDynamicProxyFactory.GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[] additionalInterfaces, Object[] constructorArguments)
at NSubstitute.Proxies.ProxyFactory.GenerateProxy(ICallRouter callRouter, Type typeToProxy, Type[] additionalInterfaces, Object[] constructorArguments)
at NSubstitute.Core.SubstituteFactory.Create(Type[] typesToProxy, Object[] constructorArguments, SubstituteConfig config)
at NSubstitute.Core.SubstituteFactory.Create(Type[] typesToProxy, Object[] constructorArguments)
at NSubstitute.Substitute.For(Type[] typesToProxy, Object[] constructorArguments)
at NSubstitute.Substitute.For[T](Object[] constructorArguments)

[dtchepak]

Hi @candychiu,
It seems to work ok with public constructors if the arguments are explicitly passed or if there is a default, public constructor. It does not work with default args, I think because the compiler doesn't know to insert them in this case?

        public class A { public A(int arg1 = 0, string arg2 = "") { } }
        [Test]
        public void Sample() { var a = Substitute.For<A>(0, ""); }

If you have to keep the constructors internal, maybe try using InternalsVisibleTo?

[candychiu]

That works, thanks. Is this a limitation of the compiler or dynamic proxy library?

[dtchepak]

I'm not sure on the details sorry. There's an explanation of the internal thing on SO. My very rudimentary (read: probably wrong) knowledge of optional args in C# is that default values are inserted at compile time. If we are dynamically calling a constructor then those argument values will not have been compiled in with that call, so we need to pass them explicitly.