Optional parameters in managed C++/CLI methods

Question

How can I declare a managed method in C++/CLI that has an optional parameter when used from C#?

I've decorated the parameter with both an Optional and a DefaultParameterValue attribute (see: How default parameter values are encoded), but only the Optional attribute seems to be honored.

---

C++/CLI:

public ref class MyClass1
{
 public:
  MyClass1([System::Runtime::InteropServices::Optional]
           [System::Runtime::InteropServices::DefaultParameterValue(2)]
           int myParam1)                                            ↑
  {
    System::Console::WriteLine(myParam1);
  }
};

C#:

var myInstance1 = new MyClass1();  // compiles and runs

Output:

0

Expected Output:

2

Visual C# IntelliSense:

MyClass1.MyClass1([int myParam1 = 0]);  // wrong default value
                                  ↑

---

Edit: A closer look with a disassembler reveals that the C++/CLI compiler does indeed not generate the required .param [1] = int32(2) directive. The IL code shown by Reflector is wrong.

Reflector:

.method public hidebysig specialname rtspecialname instance void .ctor([opt] int32 myParam1) cil managed
{
    .param [1] = int32(2)  // bug
    ...

ILDASM:

.method public hidebysig specialname rtspecialname instance void .ctor([opt] int32 myParam1) cil managed
{
    .param [1]
    .custom instance void [System]System.Runtime.InteropServices.DefaultParameterValueAttribute::.ctor(object) = ( 01 00 08 02 00 00 00 00 00 ) 
    ...

Solution 1

The C# compiler doesn't use the [DefaultParameterValue] attribute to set the default value, it uses the .param directive to get the value embedded in the metadata. Barely documented in the CLI spec btw, only Partition II, chapter 15.4.1 mentions that it can have a FieldInit value, 15.4.1.4 is silent about it.

That's where the buck stops, the C++/CLI compiler doesn't know how to generate the directive. You cannot make this work.

Solution 2

There is a trick to make this working (workaround). the magic word is nullable, as for nullable types the default is always "null" (.HasValue == false).

Example:

C++ CLI in header:

String^ test([Optional] Nullable<bool> boolTest);

C++ CLI in .cpp file:

String^ YourClass::test(Nullable<bool> boolTest)
{
    if (!boolTest.HasValue) { boolTest = true; }
    return (boolTest ? gcnew String("True") : gcnew String("False"));
}

to Test it in C#:

MessageBox.Show(YourClass.test());

Note that [Optional] in the example above is located in the namespace System::Runtime::InteropServices. To access it, add the following line:

using namespace System::Runtime::InteropServices;

Solution 3

As a workaround, you can just overload the constructor, and use delegation. It will be inlined by the JIT and should end up with the same final result as a default parameter value.