How to: Migrate to /clr


For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

This topic discusses issues that arise when compiling native code with /clr (see /clr (Common Language Runtime Compilation) for more information). /clr allows Visual C++ modules to invoke and be invoked from .NET assemblies while retaining compatibility with unmanaged modules. See Mixed (Native and Managed) Assemblies and Native and .NET Interoperability for more information on the advantages of compiling with /clr.

Visual Studio contains some known issues when compiling library projects with /clr:

  • Your code may query types at runtime with CRuntimeClass::FromName. However, if a type is in an MSIL .dll (compiled with /clr), the call to CRuntimeClass::FromName may fail if it occurs before the static constructors run in the managed .dll (you will not see this problem if the FromName call happens after code has executed in the managed .dll). To work around this problem, you can force the construction of the managed static constructor by defining a function in the managed .dll, exporting it, and invoking it from the native MFC application. For example:

    // Extension DLL Header file:  
    __declspec( dllexport ) void EnsureManagedInitialization () {  
       // managed code that won't be optimized away  

Before using /clr on any module in your project, first compile and link your native project with Visual Studio 2010.

The following steps, followed in order, provide the easiest path to a /clr compilation. It is important to compile and run your project after each of these steps.

Versions Prior to Visual C++ 2003

If you are upgrading to Visual Studio 2010 from a version prior to Visual C++ 2003, you may see compiler errors related to the enhanced C++ standard conformance in Visual C++ 2003

Upgrading from Visual C++ 2003

Projects previous built with Visual C++ 2003 should also first be compiled without /clr as Visual Studio now has increased ANSI/ISO compliance and some breaking changes. The change that is likely to require the most attention is Security Features in the CRT. Code that uses the CRT is very likely to produce deprecation warnings. These warnings can be suppressed, but migrating to the new Security-Enhanced Versions of CRT Functions is preferred, as they provide better security and may reveal security issues in your code.

Upgrading from Managed Extensions for C++

Projects built with Visual C++ .NET or Visual C++ 2003 that used Managed Extensions for C++ will require at least one change to project settings, as these extensions are now deprecated. As a result, code written with Managed Extensions for C++ won't compile under /clr. Use /clr:oldSyntax instead.

Although Visual Studio will compile C files, it is necessary to convert them to C++ for a /clr compilation. The actual filename doesn't have to be changed; you can use /Tp (see /Tc, /Tp, /TC, /TP (Specify Source File Type).) Note that although C++ source code files are required for /clr, it is not necessary to re-factor your code to use object-oriented paradigms.

C code is very likely to require changes when compiled as a C++ file. The C++ type-safety rules are strict, so type conversions must be made explicit with casts. For example, malloc returns a void pointer, but can be assigned to a pointer to any type in C with a cast:

int* a = malloc(sizeof(int));   // C code  
int* b = (int*)malloc(sizeof(int));   // C++ equivalent  

Function pointers are also strictly type-safe in C++, so the following C code requires modification. In C++ it's best to create a typedef that defines the function pointer type, and then use that type to cast function pointers:

NewFunc1 = GetProcAddress( hLib, "Func1" );   // C code  
typedef int(*MYPROC)(int);   // C++ equivalent  
NewFunc2 = (MYPROC)GetProcAddress( hLib, "Func2" );  

C++ also requires that functions either be prototyped or fully defined before they can be referenced or invoked.

Identifiers used in C code that happen to be keywords in C++ (such as virtual, new, delete, bool, true, false, etc.) must be renamed. This can generally be done with simple search-and-replace operations.

Finally, whereas C-style COM calls require explicit use of the v-table and this pointer, C++ does not:

COMObj1->lpVtbl->Method(COMObj, args);  // C code  
COMObj2->Method(args);  // C++ equivalent  

After your project compiles and runs in Visual Studio 2010 you should create new project configurations for /clr rather than modifying the default configurations. /clr is incompatible with some compiler options and creating separate configurations lets you build your project as native or managed. When /clr is selected in the property pages dialog box, project settings not compatible with /clr are disabled (and disabled options are not automatically restored if /clr is subsequently unselected).

Create New Project Configurations

You can use Copy Settings From option in the New Project Configuration Dialog Box to create a project configuration based on your existing project settings. Do this once for the Debug configuration and once for Release configuration. Subsequent changes can then be applied to the /clr -specific configurations only, leaving the original project configurations intact.

Projects that use custom build rules may require extra attention.

This step has different implications for projects that use makefiles. In this case a separate build-target can be configured, or version specific to /clr compilation can be created from a copy of the original.

Change Project Settings

/clr can be selected in the development environment by following the instructions in /clr (Common Language Runtime Compilation). As mentioned previously, this step will automatically disable conflicting project settings.

System_CAPS_ICON_note.jpg Note

When upgrading a managed library or web service project from Visual C++ 2003, the /Zl compiler option will added to the Command Line property page. This will cause LNK2001. Remove /Zl from the Command Line property page to resolve. See /Zl (Omit Default Library Name) and How to: Open Project Property Pages for more information. Or, add msvcrt.lib and msvcmrt.lib to the linker's Additional Dependencies property.

For projects built with makefiles, incompatible compiler options must be disabled manually once /clr is added. See //clr Restrictions for information on compiler options that are not compatible with /clr.

Precompiled Headers

Precompiled headers are supported under /clr. However, if you only compile some of your CPP files with /clr (compiling the rest as native) some changes will be required because precompiled headers generated with /clr are not compatible with those generated without /clr. This incompatibility is due to the fact that /clr generates and requires metadata. Modules compiled /clr can therefore not use precompiled headers that don't include metadata, and non /clr modules can't use precompiled header files that do contain meta data.

The easiest way to compile a project where some modules are compiled /clr is to disable precompiled headers entirely. (In the project Property Pages dialog, open the C/C++ node, and select Precompiled Headers. Then change the Create/Use Precompiled Headers property to "Not Using Precompiled Headers".)

However, particularly for large projects, precompiled headers provide much better compilation speed, so disabling this feature is not desirable. In this case it's best to configure the /clr and non /clr files to use separate precompiled headers. This can be done in one step by multi-selecting the modules to be compiled /clr using the Solution Explorer, right-clicking on the group, and selecting Properties. Then change both the Create/Use PCH Through File and Precompiled Header File properties to use a different header file name and PCH file respectively.

Compiling with /clr may result in compiler, linker or runtime errors. This section discusses the most common problems.

Metadata Merge

Differing versions of data types can cause the linker to fail because the metadata generated for the two types doesn't match. (This is usually caused when members of a type are conditionally defined, but the conditions are not the same for all CPP files that use the type.) In this case the linker fails, reporting only the symbol name and the name of the second OBJ file where the type was defined. It is often useful to rotate the order that OBJ files are sent to the linker to discover the location of the other version of the data type.

Loader Lock Deadlock

In Visual C++ .NET and Visual C++ 2003, initialization under /clr was susceptible to non-deterministic deadlock. This issue is known as "loader lock deadlock". In Visual Studio 2010, this deadlock is easier to avoid, it is detected and reported at runtime, and is no longer non-deterministic. Encountering the loader lock problem is still possible, but now it's much easier to avoid and fix. See Initialization of Mixed Assemblies for detailed background, guidance, and solutions.

Data Exports

Exporting DLL data is error-prone, and not recommended. This is because the data section of a DLL is not guaranteed to be initialized until some managed portion of the DLL has been executed. Reference metadata with #using Directive.

Type Visibility

Native types are now private by default. In Visual C++ .NET 2002 and Visual C++ 2003, native types were public by default. This can result in a native type not being visible outside the DLL. Resolve this error by adding public to these types. See Type and Member Visibility for more information.

Floating Point and Alignment Issues

__controlfp is not supported on the common language runtime (see _control87, _controlfp, __control87_2 for more information). The CLR will also not respect align.

COM Initialization

The Common Language Runtime initializes COM automatically when a module is initialized (when COM is initialized automatically it’s done so as MTA). As a result, explicitly initializing COM yields return codes indicating that COM is already initialized. Attempting to explicitly initialize COM with one threading model when the CLR has already initialized COM to another threading model can cause your application to fail.

The common language runtime starts COM as MTA by default; use /CLRTHREADATTRIBUTE (Set CLR Thread Attribute) to modify this.

Performance Issues

You may see decreased performance when native C++ methods generated to MSIL are called indirectly (virtual function calls or using function pointers). To learn more about this, see Double Thunking.

When moving from native to MSIL, you will notice an increase in the size of your working set. This is because the common language runtime provides many features to ensure that programs run correctly. If your /clr application is not running correctly, you may want to enable C4793 (off by default), see Compiler Warning (level 1 and 3) C4793 for more information.

Program Crashes on Shutdown

In some cases, the CLR can shutdown before your managed code is finished running. Using std::set_terminate and SIGTERM can cause this. See signal Constants and set_terminate for more information.

After your application compiles, links, and runs, you can begin using .NET features in any module compiled with /clr. For more information, see Component Extensions for Runtime Platforms.

If you used Managed Extensions for C++, you can convert your code to use the new syntax. For a summary of syntactical differences, see the (NOTINBUILD)Managed Extensions for C++ Syntax Upgrade Checklist. For details on converting Managed Extensions for C++, see C++/CLI Migration Primer.

For information on .NET programming in Visual C++ see:

Mixed (Native and Managed) Assemblies