C# / C++ Interop, Odd behaviour from marshalled strings

Mikey_H

Hello, I am trying some Interop in my application. This is the first time I have had to pass arguments with function calls. I am experiencing some odd results.

// C# Function Call
RenderEngine.LoadNewMesh("BasicMesh", "Pyramid3", 1);

// Managed C++ Wrapper Function
void RenderEngine::LoadNewMesh(String^ meshType, String^ meshName, int nSubsets)
{
char* cMeshType = (char*)(void*)Marshal::StringToHGlobalAnsi(meshType);
char* cMeshName = (char*)(void*)Marshal::StringToHGlobalAnsi(meshName);

m\_engineRoot->LoadMesh.LoadNewMesh(cMeshType, cMeshName, nSubsets);

}

// Native C++ Function
void MeshLoader::LoadNewMesh(char* meshType, char* meshName, UINT nSubsets)
{
//char*
m_meshType = meshType;
//char*
m_meshName = meshName;
m_nSubsets = 0;
m_nTotalSubsets = nSubsets;

m\_pMesh							= new BasicMesh();

    // This message box will display Pyramid3
    MessageBox(0, meshName, 0, 0);

    // This code will not trigger the message box
    if (meshName == "Pyramid3")
            MessageBox(0, (char\*)"meshName is Pyramid3", 0, 0);

    // This code will trigger and m\_meshName is set directly as Pyramid3
    // if it is not set directly here the call to GetMeshByName later on returns NULL
if (strcmp(meshName,"Pyramid3") == 0)
	m\_meshName = "Pyramid3";

}

// Object is referenced by name here
MeshManager::CreateMesh(m_pMesh, m_meshName);

// Will return NULL if char* is not set directly
MeshManager::GetMeshByName("Pyramid3");

Chris Losinger

Mikey_H wrote:

if (meshName == "Pyramid3")

the value of meshName is the address of the first character of the string. it is not a string itself. in other words, meshName is a pointer, not a string.

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Mikey_H

The third condition here will result true, as does the second

if (meshName == "Pyramid3")
MessageBox(0, (char*)"meshName is Pyramid3", 0, 0);

if (strcmp(meshName,"Pyramid3") == 0)
m_meshName = "Pyramid3";

if (m_meshName == "Pyramid3")
MessageBox(0, (char*)"m_meshName is Pyramid3", 0, 0);

modified on Tuesday, April 14, 2009 12:35 PM

Chris Losinger · modified on Tuesday, April 14, 2009 12:35 PM

right, because you've assigned meshName to m_meshName, and i assume m_meshName is a string object of some kind, one with an overloaded "==" operator which knows how to compare a C-style (char*) string to its contents. (CString or std::string?) the problem with the first remains: you cannot compare a pointer to a static string. that's not how C-style strings work. and that's why strcmp exists.

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modified on Tuesday, April 14, 2009 11:39 AM

Iain Clarke Warrior Programmer · modified on Tuesday, April 14, 2009 12:35 PM

The second comparison is the correct one. Two facts you need to know. 1/ m_meshName is a pointer to character. Underneath, it's an integer holding an address in RAM. 2/ The compiler has seen two constant strings in your code, but only bothers to store one copy of the string "Pyramid3" in it's data segment. So, the strcmp works as intended, and makes m_meshName point to the constant string in your code. The 3rd comparison works by luck, as m_meshName is still pointing to that string. This is because your compiler is being efficient, it is not because your code is correct. You are lucky. If this doesn't sink in (it's hard until you have a lightbulb moment), I can recommend reading The Complete Guide to C++ Strings, Part I - Win32 Character Encodings[^] Iain.

In the process of moving to Sweden for love (awwww). If you're in Scandinavia and want an MVP on the payroll (or happy with a remote worker), give me a job! http://cv.imcsoft.co.uk/[^]

Iain Clarke Warrior Programmer · modified on Tuesday, April 14, 2009 11:39 AM

There's no overloading if his comments are right - they're char *'s. So, it's simpler than you thought... See http://www.codeproject.com/script/Forums/View.aspx?fid=1647&msg=3003509[^] for more. Iain.

In the process of moving to Sweden for love (awwww). If you're in Scandinavia and want an MVP on the payroll (or happy with a remote worker), give me a job! http://cv.imcsoft.co.uk/[^]

Chris Losinger

ah. you're right. i missed that comment.

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Mikey_H

Iain Clarke wrote:

If this doesn't sink in (it's hard until you have a lightbulb moment)

My lightbulb is on a dimmer switch... I think I understand what you are saying in your reply. I have just read Part 1, will follow up with Part 2 tomorrow, as it is 4.26am. If my understanding is correct I will be making a lot of changes to my application once I figure this out. Thank you both for your replies. :thumbsup: