Skip to main content

out, ref and InvokeMember !!!

When I was working on the .NET reflection extravaganza thing that I explained in my previous column, i learnt one another interesting thing, that is about the Type.InvokeMember. How will pass out or ref parameters for the method invoked using Type.InvokeMember ? If you are going to invoke a method with the prototype

int DoSomething(string someString, int someInt);
then you would use InvokeMember like this:-
object obj = someType.InvokeMember("DoSomething", 
BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance,
null,
this,
new object[] {"Largest Integer", 1});
or use some variables in the new object[] {...}. But what do you with the args if DoSomething takes out or ref parameters ?

int DoSomething(out string someString, ref int someInt);
Something like this will not work
string someText = string.Empty;
int someInt = 0;
object obj = someType.InvokeMember("DoSomething",
BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance,
null,
this,
new object[] {someText, someInt});

It is tricky.

object[] args = new object[] { someText, someInt };
object obj = someType.InvokeMember("DoSomething",
BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance,
null,
this,
args);

or even suprisingly this works:-

object[] args = new object[2];
// or object[] args = new object[] { null, null };

object obj = someType.InvokeMember("DoSomething",
BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance,
null,
this,
args);

Access the values by indexing args. So declaring the argument object[] as a local variable solves the problem, but I do not understand why this behaviour. May be somebody can explain !!!

Comments

Anonymous said…
Actually, I don't think what you have proposed works. At least it didn't during my testing.
Anonymous said…
Amazing!!!

Thx!!!

Popular posts from this blog

Extension Methods - A Polished C++ Feature !!!

Extension Method is an excellent feature in C# 3.0. It is a mechanism by which new methods can be exposed from an existing type (interface or class) without directly adding the method to the type. Why do we need extension methods anyway ? Ok, that is the big story of lamba and LINQ. But from a conceptual standpoint, the extension methods establish a mechanism to extend the public interface of a type. The compiler is smart enough to make the method a part of the public interface of the type. Yeah, that is what it does, and the intellisense is very cool in making us believe that. It is cleaner and easier (for the library developers and for us programmers even) to add extra functionality (methods) not provided in the type. That is the intent. And we know that was exercised extravagantly in LINQ. The IEnumerable was extended with a whole lot set of methods to aid the LINQ design. Remember the Where, Select etc methods on IEnumerable. An example code snippet is worth a thousand ...

Implementing COM OutOfProc Servers in C# .NET !!!

Had to implement our COM OOP Server project in .NET, and I found this solution from the internet after a great deal of search, but unfortunately the whole idea was ruled out, and we wrapped it as a .NET assembly. This is worth knowing. Step 1: Implement IClassFactory in a class in .NET. Use the following definition for IClassFactory. namespace COM { static class Guids { public const string IClassFactory = "00000001-0000-0000-C000-000000000046"; public const string IUnknown = "00000000-0000-0000-C000-000000000046"; } /// /// IClassFactory declaration /// [ComImport(), InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid(COM.Guids.IClassFactory)] internal interface IClassFactory { [PreserveSig] int CreateInstance(IntPtr pUnkOuter, ref Guid riid, out IntPtr ppvObject); [PreserveSig] int LockServer(bool fLock); } } Step 2: [DllImport("ole32.dll")] private static extern int CoR...

Passing CComPtr By Value !!!

This is about a killer bug identified by our chief software engineer in our software. What was devised for ease of use and write smart code ended up in this killer defect due to improper perception. Ok, let us go! CComPtr is a template class in ATL designed to wrap the discrete functionality of COM object management - AddRef and Release. Technically it is a smart pointer for a COM object. void SomeMethod() { CComPtr siPtr; HRESULT hr = siPtr.CoCreateInstance(CLSID_SomeComponent); siPtr->MethodOne(20, L"Hello"); } Without CComPtr, the code wouldn't be as elegant as above. The code would be spilled with AddRef and Release. Besides, writing code to Release after use under any circumstance is either hard or ugly. CComPtr automatically takes care of releasing in its destructor just like std::auto_ptr . As a C++ programmer, we must be able to appreciate the inevitability of the destructor and its immense use in writing smart code. However there is a difference...