'System' : a namespace with this name does not exist

Question

I had a C++ project which was running perfectly. Now I copied the project to another folder and added the project to a WPF application solution.

Now the C++ project is giving me lot of build errors. One of them is "System' : a namespace with this name does not exist".

Below are the lines which creates this error.

#include "stdafx.h"

using namespace System;
using namespace System::Reflection;
using namespace System::Runtime::CompilerServices;
using namespace System::Runtime::InteropServices;
using namespace System::Security::Permissions;

The above lines are in AssemblyInfo.cpp file. I don't have any experience of C++, so finding it difficult to remove the errors. Any help is appreciated. Below is another error which might be helpful for you to resolve the issue.

managed targeted code requires a '/clr' option

Thanks in advance.

Do you intend to write in C++ for .NET or natively? Existence of `System::*` shows clearly, that its a C++/CLI project (not C++/Win32). — Spook, May 24 '13 at 10:24
Actually C++ is has some native code and wrapper and WPF does the UI. I know it is C++/CLI project. But why is it giving errors when added to another solution as this does not depend upon any other project. — Narendra, May 24 '13 at 10:25
Check in project's preferences, whether the compiler knows, that it's a C++/CLI project, because it seems, that it doesn't :) — Spook, May 24 '13 at 10:31
Project properties | Configuration properties | General | Common Language Runtime Support. If the project is a C++/CLI one, there should be `Common Language Runtime Support (/clr)`. — Spook, May 24 '13 at 10:39

score 59 · Accepted Answer · edited Sep 23 '20 at 22:01

59

Set Common Language RunTime Support to be "Common Language RunTime Support (/clr)" in 2 places in your project properties :

For Visual Studio before 2019:

Configuration Properties -> General
Configuration Properties -> C/C++ -> General

For Visual Studio 2019:

Configuration Properties -> Advanced -> C++/CLI Properties
Configuration Properties -> C/C++ -> General

edited Sep 23 '20 at 22:01

remram

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answered Dec 15 '13 at 14:44

Mif

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Had the same issue and this worked for me, should be marked as the answer, – i know nothing Jul 04 '14 at 08:41
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This worked for me on Visual Studio 2015, but I only changed the property in C/C++ -> General. I didn't see the property in just General. – Luis B Feb 29 '16 at 19:59
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For Visual Studio 2019, go to the Configuration Properties -> Advanced -> C++/CLI Properties and do it. – eliasetm Sep 23 '20 at 08:59
What about projects that are made without Visual Studio? – Rafaelplayerxd YT Sep 24 '21 at 22:40

Markus · Answer 2 · 2022-02-02T10:13:01.110

Mif's answer was a little incomplete for me. If you still have the issue, you'd might want to check if you set the correct C++-version. For me it specifically needed ISO C++17 because the VS default preset ISO C++14 (at least a preset in my version) does not support the required CLI-packages. Thus, still set the Runtime support to Common Language RunTime Support (/clr) and set the correct C++ Language Standard in the Configuration Properties -> General tab.

Note: As of now (October 2021) ISO C++20 is currently not supported in CLI and will also fail to find System - I only found the 17-version to be compatible.

Since its a little easer the visual way, here the screenshots. And also don't forget Yinon_90's hint to have the version as well as dependencies installed!