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Mscorlib and Calling Into the Runtime

Author: Brian Grunkemeyer (@briangru) - 2006


Mscorlib is the assembly for defining the core parts of the type system, and a good portion of the Base Class Library in .NET Framework. It has been renamed to System.Private.CoreLib in .NET Core, though many places in the code and documentation still refer to it as mscorlib. Base data types live in this assembly, and it has a tight coupling with the CLR. Here you will learn exactly how & why mscorlib.dll is special, and the basics about calling into the CLR from managed code via QCall and FCall methods. It also discusses calling from within the CLR into managed code.


Since mscorlib defines base data types like Object, Int32, and String, mscorlib cannot depend on other managed assemblies. However, there is a strong dependency between mscorlib and the CLR. Many of the types in mscorlib need to be accessed from native code, so the layout of many managed types is defined both in managed code and in native code inside the CLR. Additionally, some fields may be defined only in debug or checked builds, so typically mscorlib must be compiled separately for checked vs. retail builds.

For 64 bit platforms, some constants are also defined at compile time. So a 64 bit mscorlib.dll is slightly different from a 32 bit mscorlib.dll. Due to these constants, such as IntPtr.Size, most libraries above mscorlib should not need to build separately for 32 bit vs. 64 bit.

What Makes Mscorlib Special?

Mscorlib has several unique properties, many of which are due to its tight coupling to the CLR.

  • Mscorlib defines the core types necessary to implement the CLR's Virtual Object System, such as the base data types (Object, Int32, String, etc).
  • The CLR must load mscorlib on startup to load certain system types.
  • Can only have one mscorlib loaded in the process at a time, due to layout issues. Loading multiple mscorlibs would require formalizing a contract of behavior, FCall methods, and datatype layout between CLR & mscorlib, and keeping that contract relatively stable across versions.
  • Mscorlib's types will be used heavily for native interop, and managed exceptions should map correctly to native error codes/formats.
  • The CLR's multiple JIT compilers may special case a small group of certain methods in mscorlib for performance reasons, both in terms of optimizing away the method (such as Math.Cos(double)), or calling a method in peculiar ways (such as Array.Length, or some implementation details on StringBuilder for getting the current thread).
  • Mscorlib will need to call into native code, via P/Invoke where appropriate, primarily into the underlying operating system or occasionally a platform adaptation layer.
  • Mscorlib will require calling into the CLR to expose some CLR-specific functionality, such as triggering a garbage collection, to load classes, or to interact with the type system in a non-trivial way. This requires a bridge between managed code and native, "manually managed" code within the CLR.
  • The CLR will need to call into managed code to call managed methods, and to get at certain functionality that is only implemented in managed code.

Interface between managed & CLR code

To reiterate, the needs of managed code in mscorlib include:

  • The ability to access fields of some managed data structures in both managed code and "manually managed" code within the CLR
  • Managed code must be able to call into the CLR
  • The CLR must be able to call managed code.

To implement these, we need a way for the CLR to specify and optionally verify the layout of a managed object in native code, a managed mechanism for calling into native code, and a native mechanism for calling into managed code.

The managed mechanism for calling into native code must also support the special managed calling convention used by String's constructors, where the constructor allocates the memory used by the object (instead of the typical convention where the constructor is called after the GC allocates memory).

The CLR provides a mscorlib binder internally, providing a mapping between unmanaged types and fields to managed types & fields. The binder will look up & load classes, allow you to call managed methods. It also does some simple verification to ensure the correctness of any layout information specified in both managed & native code. The binder ensures that the managed class you're attempting to use exists in mscorlib, has been loaded, and the field offsets are correct. It also needs the ability to differentiate between method overloads with different signatures.

Calling from managed to native code

We have two techniques for calling into the CLR from managed code. FCall allows you to call directly into the CLR code, and provides a lot of flexibility in terms of manipulating objects, though it is easy to cause GC holes by not tracking object references correctly. QCall allows you to call into the CLR via the P/Invoke, and is much harder to accidentally mis-use than FCall. FCalls are identified in managed code as extern methods with the MethodImplOptions.InternalCall bit set. QCalls are static extern methods that look like regular P/Invokes, but to a library called "QCall".

There is a small variant of FCall called HCall (for Helper call) for implementing JIT helpers, for doing things like accessing multi-dimensional array elements, range checks, etc. The only difference between HCall and FCall is that HCall methods won't show up in an exception stack trace.

Choosing between FCall, QCall, P/Invoke, and writing in managed code

First, remember that you should be writing as much as possible in managed code. You avoid a raft of potential GC hole issues, you get a good debugging experience, and the code is often simpler. It also is preparation for ongoing refactoring of mscorlib into smaller layered fully managed libraries in corefx.

Reasons to write FCalls in the past generally fell into three camps: missing language features, better performance, or implementing unique interactions with the runtime. C# now has almost every useful language feature that you could get from C++, including unsafe code & stack-allocated buffers, and this eliminates the first two reasons for FCalls. We have ported some parts of the CLR that were heavily reliant on FCalls to managed code in the past (such as Reflection and some Encoding & String operations), and we want to continue this momentum. We may port our number formatting & String comparison code to managed in the future.

If the only reason you're defining a FCall method is to call a native Win32 method, you should be using P/Invoke to call Win32 directly. P/Invoke is the public native method interface, and should be doing everything you need in a correct manner.

If you still need to implement a feature inside the runtime, now consider if there is a way to reduce the frequency of transitioning to native code. Can you write the common case in managed, and only call into native for some rare corner cases? You're usually best off keeping as much as possible in managed code.

QCalls are the preferred mechanism going forward. You should only use FCalls when you are "forced" to. This happens when there is common "short path" through the code that is important to optimize. This short path should not be more than a few hundred instructions, cannot allocate GC memory, take locks or throw exceptions (GC_NOTRIGGER, NOTHROWS). In all other circumstances (and especially when you enter a FCall and then simply erect HelperMethodFrame), you should be using QCall.

FCalls were specifically designed for short paths of code that must be optimized. They allowed you to take explicit control over when erecting a frame was done. However it is error prone and is not worth it for many APIs. QCalls are essentially P/Invokes into CLR.

As a result, QCalls give you some advantageous marshaling for SafeHandles automatically – your native method just takes a HANDLE type, and can use it without worrying whether someone will free the handle while you are in that method body. The resulting FCall method would need to use a SafeHandleHolder, and may need to protect the SafeHandle, etc. Leveraging the P/Invoke marshaler can avoid this additional plumbing code.

QCall Functional Behavior

QCalls are very much like a normal P/Invoke from mscorlib.dll to CLR. Unlike FCalls, QCalls will marshal all arguments as unmanaged types like a normal P/Invoke. QCall also switch to preemptive GC mode like a normal P/Invoke. These two features should make QCalls easier to write reliably compared to FCalls. QCalls are not prone to GC holes and GC starvation bugs that are common with FCalls.

QCalls perform better than FCalls that erect a HelperMethodFrame. The overhead is about 1.4x less compared to FCall w/ HelperMethodFrame overhead on x86 and x64.

The preferred types for QCall arguments are primitive types that are efficiently handled by the P/Invoke marshaler (INT32, LPCWSTR, BOOL). Notice that BOOL is the correct boolean flavor for QCall arguments. On the other hand, CLR_BOOL is the correct boolean flavor for FCall arguments.

The pointers to common unmanaged EE structures should be wrapped into handle types. This is to make the managed implementation type safe and avoid falling into unsafe C# everywhere. See AssemblyHandle in vm\qcall.h for an example.

There is a way to pass a raw object references in and out of QCalls. It is done by wrapping a pointer to a local variable in a handle. It is intentionally cumbersome and should be avoided if reasonably possible. See the StringHandleOnStack in the example below. Returning objects, especially strings, from QCalls is the only common pattern where passing the raw objects is widely acceptable. (For reasoning on why this set of restrictions helps make QCalls less prone to GC holes, read the "GC Holes, FCall, and QCall" section below.)

QCall Example - Managed Part

Do not replicate the comments into your actual QCall implementation. This is for illustrative purposes.

class Foo
    // All QCalls should have the following DllImport attribute
    [DllImport(RuntimeHelpers.QCall, CharSet = CharSet.Unicode)]
    // QCalls should always be static extern.
    private static extern bool Bar(int flags, string inString, StringHandleOnStack retString);

    // Many QCalls have a thin managed wrapper around them to expose them to
    // the world in more meaningful way.
    public string Bar(int flags)
        string retString = null;

        // The strings are returned from QCalls by taking address
        // of a local variable using StringHandleOnStack
        if (!Bar(flags, this.Id, new StringHandleOnStack(ref retString)))

        return retString;

QCall Example - Unmanaged Part

Do not replicate the comments into your actual QCall implementation.

The QCall entrypoint has to be registered in tables in vm\ecalllist.h using QCFuncEntry macro. See "Registering your QCall or FCall Method" below.

class FooNative
    // All QCalls should be static and should be tagged with QCALLTYPE
    BOOL QCALLTYPE Bar(int flags, LPCWSTR wszString, QCall::StringHandleOnStack retString);

BOOL QCALLTYPE FooNative::Bar(int flags, LPCWSTR wszString, QCall::StringHandleOnStack retString)
    // All QCalls should have QCALL_CONTRACT.

    // Optionally, use QCALL_CHECK instead and the expanded form of the contract
    // if you want to specify preconditions:
    // CONTRACTL {
    //     QCALL_CHECK;
    //     PRECONDITION(wszString != NULL);

    // The only line between QCALL_CONTRACT and BEGIN_QCALL
    // should be the return value declaration if there is one.
    BOOL retVal = FALSE;

    // The body has to be enclosed in BEGIN_QCALL/END_QCALL macro. It is necessary
    // to make the exception handling work.

    // Validate arguments if necessary and throw exceptions.
    // There is no convention currently on whether the argument validation should be
    // done in managed or unmanaged code.
    if (flags != 0)
        COMPlusThrow(kArgumentException, L"InvalidFlags");

    // No need to worry about GC moving strings passed into QCall.
    // Marshalling pins them for us.
    printf("%S", wszString);

    // This is most the efficient way to return strings back
    // to managed code. No need to use StringBuilder.

    // You can not return from inside of BEGIN_QCALL/END_QCALL.
    // The return value has to be passed out in helper variable.
    retVal = TRUE;


    return retVal;

FCall Functional Behavior

FCalls allow more flexibility in terms of passing object references around, with a higher code complexity and more opportunities to hang yourself. Additionally, FCall methods must either erect a helper method frame along their common code paths, or for any FCall of non-trivial length, explicitly poll for whether a garbage collection must occur. Failing to do so will lead to starvation issues if managed code repeatedly calls the FCall method in a tight loop, because FCalls execute while the thread only allows the GC to run in a cooperative manner.

FCalls require a lot of glue, too much to describe here. Look at fcall.h for details.

GC Holes, FCall, and QCall

A much more complete discussion on GC holes can be found in the CLR Code Guide. Look for "Is your code GC-safe?". This tailored discussion motivates some of the reasons why FCall and QCall have some of their strange conventions.

Object references passed as parameters to FCall methods are not GC-protected, meaning that if a GC occurs, those references will point to the old location in memory of an object, not the new location. For this reason, FCalls usually follow the discipline of accepting something like "StringObject*" as their parameter type, then explicitly converting that to a STRINGREF before doing operations that may trigger a GC. You must GC protect object references before triggering a GC, if you expect to be able to use that object reference later.

All GC heap allocations within an FCall method must happen within a helper method frame. If you allocate memory on the GC's heap, the GC may collect dead objects & move objects around in unpredictable ways, with some low probability. For this reason, you must manually report any object references in your method to the GC, so that if a garbage collection occurs, your object reference will be updated to refer to the new location in memory. Any pointers into managed objects (like arrays or Strings) within your code will not be updated automatically, and must be re-fetched after any operation that may allocate memory and before your first usage. Reporting a reference can be done via the GCPROTECT macros, or as parameters when you erect a helper method frame.

Failing to properly report an OBJECTREF or to update an interior pointer is commonly referred to as a "GC hole", because the OBJECTREF class will do some validation that it points to a valid object every time you dereference it in checked builds. When an OBJECTREF pointing to an invalid object is dereferenced, you'll get an assert saying something like "Detected an invalid object reference. Possible GC hole?". This assert is unfortunately easy to hit when writing "manually managed" code.

Note that QCall's programming model is restrictive to sidestep GC holes most of the time, by forcing you to pass in the address of an object reference on the stack. This guarantees that the object reference is GC protected by the JIT's reporting logic, and that the actual object reference will not move because it is not allocated in the GC heap. QCall is our recommended approach, precisely because it makes GC holes harder to write.

FCall Epilogue Walker for x86

The managed stack walker needs to be able to find its way from FCalls. It is relative easy on newer platforms that define conventions for stack unwinding as part of the ABI. The stack unwinding conventions are not defined by ABI for x86. The runtime works around it by implementing a epilog walker. The epilog walker computes the FCall return address and callee save registers by simulating the FCall execution. This imposes limits on what constructs are allowed in the FCall implementation.

Complex constructs like stack allocated objects with destructors or exception handling in the FCall implementation may confuse the epilog walker. It leads to GC holes or crashes during stack walking. There is no exact list of what constructs should be avoided to prevent this class of bugs. An FCall implementation that is fine one day may break with the next C++ compiler update. We depend on stress runs & code coverage to find bugs in this area.

Setting a breakpoint inside an FCall implementation may confuse the epilog walker. It leads to an "Invalid breakpoint in a helpermethod frame epilog" assert inside vm\i386\gmsx86.cpp.

FCall Example – Managed Part

Here's a real-world example from the String class:

public partial sealed class String
    // Replaces all instances of oldChar with newChar.
    public extern String Replace (char oldChar, char newChar);

FCall Example – Native Part

The FCall entrypoint has to be registered in tables in vm\ecalllist.h using FCFuncEntry macro. See "Registering your QCall or FCall Method".

Notice how oldBuffer and newBuffer (interior pointers into String instances) are re-fetched after allocating memory. Also, this method is an instance method in managed code, with the "this" parameter passed as the first argument. We use StringObject* as the argument type, then copy it into a STRINGREF so we get some error checking when we use it.

FCIMPL3(LPVOID, COMString::Replace, StringObject* thisRefUNSAFE, CLR_CHAR oldChar, CLR_CHAR newChar)

    int length = 0;
    int firstFoundIndex = -1;
    WCHAR *oldBuffer = NULL;
    WCHAR *newBuffer;

    STRINGREF   newString   = NULL;
    STRINGREF   thisRef     = (STRINGREF)thisRefUNSAFE;

    if (thisRef==NULL) {
        FCThrowRes(kNullReferenceException, L"NullReference_This");

    [... Removed some uninteresting code here for illustrative purposes...]


    //Get the length and allocate a new String
    //We will definitely do an allocation here.
    newString = NewString(length);

    //After allocation, thisRef may have moved
    oldBuffer = thisRef->GetBuffer();

    //Get the buffers in both of the Strings.
    newBuffer = newString->GetBuffer();

    //Copy the characters, doing the replacement as we go.
    for (int i=0; i<firstFoundIndex; i++) {
    for (int i=firstFoundIndex; i<length; i++) {


    return OBJECTREFToObject(newString);

Registering your QCall or FCall Method

The CLR must know the name of your QCall and FCall methods, both in terms of the managed class & method names, as well as which native methods to call. That is done in ecalllist.h, with two arrays. The first array maps namespace & class names to an array of function elements. That array of function elements then maps individual method names & signatures to function pointers.

Say we defined an FCall method for String.Replace(char, char), in the example above. First, we need to ensure that we have an array of function elements for the String class.

// Note these have to remain sorted by name:namespace pair (Assert will wack you if you
    FCClassElement("String", "System", gStringFuncs)

Second, we must then ensure that gStringFuncs contains a proper entry for Replace. Note that if a method name has multiple overloads (such as String.Replace(String, String)), then we can specify a signature:

    FCFuncElement("IndexOf", COMString::IndexOfChar)
    FCFuncElementSig("Replace", &gsig_IM_Char_Char_RetStr, COMString::Replace)
    FCFuncElementSig("Replace", &gsig_IM_Str_Str_RetStr, COMString::ReplaceString)

There is a parallel QCFuncElement macro.

Naming convention

Try to use normal name (e.g. no "_", "n" or "native" prefix) for all FCalls and QCalls. It is not good idea to embed that the function is implemented in VM in the name of the function for the following reasons:

  • There are directly exposed public FCalls. These FCalls have to follow the naming convention for public APIs.
  • The implementation of functions do move between CLR and mscorlib.dll. It is painful to change the name of the function in all call sites when this happens.

When necessary you can use "Internal" prefix to disambiguate the name of the FCall or QCall from public entry point (e.g. the public entry point does error checking and then calls shared worker function with exactly same signature). This is no different from how you would deal with this situation in pure managed code in BCL.

Types with a Managed/Unmanaged Duality

Certain managed types must have a representation available in both managed & native code. You could ask whether the canonical definition of a type is in managed code or native code within the CLR, but the answer doesn't matter – the key thing is they must both be identical. This will allow the CLR's native code to access fields within a managed object in a very fast, easy to use manner. There is a more complex way of using essentially the CLR's equivalent of Reflection over MethodTables & FieldDescs to retrieve field values, but this probably doesn't perform as well as you'd like, and it isn't very usable. For commonly used types, it makes sense to declare a data structure in native code & attempt to keep the two in sync.

The CLR provides a binder for this purpose. After you define your managed & native classes, you should provide some clues to the binder to help ensure that the field offsets remain the same, to quickly spot when someone accidentally adds a field to only one definition of a type.

In mscorlib.h, you can use macros ending in "_U" to describe a type, the name of fields in managed code, and the name of fields in a corresponding native data structure. Additionally, you can specify a list of methods, and reference them by name when you attempt to call them later.

DEFINE_CLASS_U(SAFE_HANDLE,         Interop,                SafeHandle,         SafeHandle)
DEFINE_FIELD(SAFE_HANDLE,           HANDLE,                 handle)
DEFINE_FIELD_U(SAFE_HANDLE,         STATE,                  _state,                     SafeHandle,            m_state)
DEFINE_FIELD_U(SAFE_HANDLE,         OWNS_HANDLE,            _ownsHandle,                SafeHandle,            m_ownsHandle)
DEFINE_FIELD_U(SAFE_HANDLE,         INITIALIZED,            _fullyInitialized,          SafeHandle,            m_fullyInitialized)
DEFINE_METHOD(SAFE_HANDLE,          GET_IS_INVALID,         get_IsInvalid,              IM_RetBool)
DEFINE_METHOD(SAFE_HANDLE,          RELEASE_HANDLE,         ReleaseHandle,              IM_RetBool)
DEFINE_METHOD(SAFE_HANDLE,          DISPOSE,                Dispose,                    IM_RetVoid)
DEFINE_METHOD(SAFE_HANDLE,          DISPOSE_BOOL,           Dispose,                    IM_Bool_RetVoid)

Then, you can use the REF template to create a type name like SAFEHANDLEREF. All the error checking from OBJECTREF is built into the REF macro, and you can freely dereference this SAFEHANDLEREF & use fields off of it in native code. You still must GC protect these references.

Calling Into Managed Code From Native

Clearly there are places where the CLR must call into managed code from native. For this purpose, we have added a MethodDescCallSite class to handle a lot of plumbing for you. Conceptually, all you need to do is find the MethodDesc* for the method you want to call, find a managed object for the "this" pointer (if you're calling an instance method), pass in an array of arguments, and deal with the return value. Internally, you'll need to potentially toggle your thread's state to allow the GC to run in preemptive mode, etc.

Here's a simplified example. Note how this instance uses the binder described in the previous section to call SafeHandle's virtual ReleaseHandle method.

void SafeHandle::RunReleaseMethod(SafeHandle* psh)

    SAFEHANDLEREF sh(psh);


    MethodDescCallSite releaseHandle(s_pReleaseHandleMethod, METHOD__SAFE_HANDLE__RELEASE_HANDLE, (OBJECTREF*)&sh, TypeHandle(), TRUE);

    ARG_SLOT releaseArgs[] = { ObjToArgSlot(sh) };
    if (!(BOOL)releaseHandle.Call_RetBool(releaseArgs)) {
        MDA_TRIGGER_ASSISTANT(ReleaseHandleFailed, ReportViolation)(sh->GetTypeHandle(), sh->m_handle);


Interactions with Other Subsystems


One limitation of FCalls today is that you cannot easily debug both managed code and FCalls easily in Visual Studio's Interop (or mixed mode) debugging. Setting a breakpoint today in an FCall and debugging with Interop debugging just doesn't work. This most likely won't be fixed.

Physical Architecture

When the CLR starts up, mscorlib is loaded by a method called LoadBaseSystemClasses. Here, the base data types & other similar classes (like Exception) are loaded, and appropriate global pointers are set up to refer to mscorlib's types.

For FCalls, look in fcall.h for infrastructure, and ecalllist.h to properly inform the runtime about your FCall method.

For QCalls, look in qcall.h for associated infrastructure, and ecalllist.h to properly inform the runtime about your QCall method.

More general infrastructure and some native type definitions can be found in object.h. The binder uses mscorlib.h to associate managed & native classes.