Question

Below is a simple C++/CLI example.

// TestCLR.cpp : main project file.

#include "stdafx.h"

using namespace System;

int main(array<System::String ^> ^args)
{
    System::Collections::Generic::List<String^> TestList;

    for(int i = 0; i < 10 ; i++)
    {
        TestList.Add(i.ToString());
    }

    for each(String^% st in TestList)
    {
        st += "TEST";
        Console::WriteLine(st);
    }

    for each(String^ st in TestList)
    {
        Console::WriteLine(st);
    }

    return 0;
}

I get the following output:

0TEST
1TEST
2TEST
3TEST
4TEST
5TEST
6TEST
7TEST
8TEST
9TEST
0
1
2
3
4
5
6
7
8
9

In short, the values inside TestList do not change even though I am using a tracking pointer to change its value to “TEST”.

What should I modify in the above snippet so that the value is permanently changed?

Answer 1

You’ve got a System::Collections::Generic::List, which uses a property to access items. You can’t bind a tracking reference to a property, you end up with a reference to a temporary copy of the value instead.

That code would work for an array, but foreach can’t be used to mutate elements in-place in a container. You’ll need a for loop, because you need the index to overwrite list elements:

for(int i = 0, cnt = TestList.Count; i < cnt; ++i)
{
    TestList[i] += "TEST";
    Console::WriteLine(TestList[i]);

    // doesn't work right: String^% st = TestList[i];
    // since TestList[i] is not an lvalue, it's a function call to a getter method
}

Didn’t you get a compiler warning telling you that you were binding a non-const reference to a temporary object?