Default Marshalling Behavior

Interop marshalling operates on rules that dictate how data associated with method parameters behaves as it passes between managed and unmanaged memory. These built-in rules control such marshalling activities as data type transformations, whether a callee can change data passed to it and return those changes to the caller, and under which circumstances the marshaller provides performance optimizations.

This section identifies the default behavioral characteristics of the interop marshalling service. It presents detailed information on marshalling arrays, Boolean types, char types, delegates, classes, objects, strings, and structures.

Memory management with the interop marshaller

The interop marshaller always attempts to free memory allocated by unmanaged code. This behavior complies with COM memory management rules, but differs from the rules that govern native C++.

Confusion can arise if you anticipate native C++ behavior (no memory freeing) when using platform invoke, which automatically frees memory for pointers. For example, calling the following unmanaged method from a C++ DLL does not automatically free any memory.

Unmanaged signature

BSTR MethodOne (BSTR b) {  
     return b;  
}  

However, if you define the method as a platform invoke prototype, replace each BSTR type with a String type, and call MethodOne, the common language runtime attempts to free b twice. You can change the marshalling behavior by using IntPtr types rather than String types.

The runtime always uses the CoTaskMemFree method on Windows and free method on other platforms to free memory. If the memory you are working with was not allocated with the CoTaskMemAlloc method on Windows or malloc method on other platforms, you must use an IntPtr and free the memory manually using the appropriate method. Similarly, you can avoid automatic memory freeing in situations where memory should never be freed, such as when using the GetCommandLine function from Kernel32.dll, which returns a pointer to kernel memory. For details on manually freeing memory, see the Buffers Sample.

Default marshalling for classes

Classes can be marshalled only by COM interop and are always marshalled as interfaces. In some cases the interface used to marshal the class is known as the class interface. For information about overriding the class interface with an interface of your choice, see Introducing the class interface.

Passing Classes to COM

When a managed class is passed to COM, the interop marshaller automatically wraps the class with a COM proxy and passes the class interface produced by the proxy to the COM method call. The proxy then delegates all calls on the class interface back to the managed object. The proxy also exposes other interfaces that are not explicitly implemented by the class. The proxy automatically implements interfaces such as IUnknown and IDispatch on behalf of the class.

Passing Classes to .NET Code

Coclasses are not typically used as method arguments in COM. Instead, a default interface is usually passed in place of the coclass.

When an interface is passed into managed code, the interop marshaller is responsible for wrapping the interface with the proper wrapper and passing the wrapper to the managed method. Determining which wrapper to use can be difficult. Every instance of a COM object has a single, unique wrapper, no matter how many interfaces the object implements. For example, a single COM object that implements five distinct interfaces has only one wrapper. The same wrapper exposes all five interfaces. If two instances of the COM object are created, then two instances of the wrapper are created.

For the wrapper to maintain the same type throughout its lifetime, the interop marshaller must identify the correct wrapper the first time an interface exposed by the object is passed through the marshaller. The marshaller identifies the object by looking at one of the interfaces the object implements.

For example, the marshaller determines that the class wrapper should be used to wrap the interface that was passed into managed code. When the interface is first passed through the marshaller, the marshaller checks whether the interface is coming from a known object. This check occurs in two situations:

• An interface is being implemented by another managed object that was passed to COM elsewhere. The marshaller can readily identify interfaces exposed by managed objects and is able to match the interface with the managed object that provides the implementation. The managed object is then passed to the method and no wrapper is needed.

• An object that has already been wrapped is implementing the interface. To determine whether this is the case, the marshaller queries the object for its IUnknown interface and compares the returned interface to the interfaces of other objects that are already wrapped. If the interface is the same as that of another wrapper, the objects have the same identity and the existing wrapper is passed to the method.

If an interface is not from a known object, the marshaller does the following:

1. The marshaller queries the object for the IProvideClassInfo2 interface. If provided, the marshaller uses the CLSID returned from IProvideClassInfo2.GetGUID to identify the coclass providing the interface. With the CLSID, the marshaller can locate the wrapper from the registry if the assembly has previously been registered.

2. The marshaller queries the interface for the IProvideClassInfo interface. If provided, the marshaller uses the ITypeInfo returned from IProvideClassInfo.GetClassinfo to determine the CLSID of the class exposing the interface. The marshaller can use the CLSID to locate the metadata for the wrapper.

3. If the marshaller still cannot identify the class, it wraps the interface with a generic wrapper class called System.__ComObject.

Default marshalling for delegates

A managed delegate is marshalled as a COM interface or as a function pointer, based on the calling mechanism:

• For platform invoke, a delegate is marshalled as an unmanaged function pointer by default.

• For COM interop, a delegate is marshalled as a COM interface of type _Delegate by default. The _Delegate interface is defined in the Mscorlib.tlb type library and contains the Delegate.DynamicInvoke method, which enables you to call the method that the delegate references.

The following table shows the marshalling options for the managed delegate data type. The MarshalAsAttribute attribute provides several UnmanagedType enumeration values to marshal delegates.

Consider the following example code in which the methods of DelegateTestInterface are exported to a COM type library. Notice that only delegates marked with the ref (or ByRef) keyword are passed as In/Out parameters.

using System;  
using System.Runtime.InteropServices;  
  
public interface DelegateTest {  
void m1(Delegate d);  
void m2([MarshalAs(UnmanagedType.Interface)] Delegate d);
void m3([MarshalAs(UnmanagedType.Interface)] ref Delegate d);
void m4([MarshalAs(UnmanagedType.FunctionPtr)] Delegate d);
void m5([MarshalAs(UnmanagedType.FunctionPtr)] ref Delegate d);
}  

Type library representation

importlib("mscorlib.tlb");  
interface DelegateTest : IDispatch {  
[id(…)] HRESULT m1([in] _Delegate* d);  
[id(…)] HRESULT m2([in] _Delegate* d);  
[id(…)] HRESULT m3([in, out] _Delegate** d);  
[id()] HRESULT m4([in] int d);  
[id()] HRESULT m5([in, out] int *d);  
   };  

A function pointer can be dereferenced, just as any other unmanaged function pointer can be dereferenced.

In this example, when the two delegates are marshalled as UnmanagedType.FunctionPtr, the result is an int and a pointer to an int. Because delegate types are being marshalled, int here represents a pointer to a void (void*), which is the address of the delegate in memory. In other words, this result is specific to 32-bit Windows systems, since int here represents the size of the function pointer.

For example, the following code is incorrect because the reference to the cb object, passed to the SetChangeHandler method, does not keep cb alive beyond the life of the Test method. Once the cb object is garbage collected, the function pointer passed to SetChangeHandler is no longer valid.

public class ExternalAPI {  
   [DllImport("External.dll")]  
   public static extern void SetChangeHandler(  
      [MarshalAs(UnmanagedType.FunctionPtr)]ChangeDelegate d);  
}  
public delegate bool ChangeDelegate([MarshalAs(UnmanagedType.LPWStr) string S);  
public class CallBackClass {  
   public bool OnChange(string S){ return true;}  
}  
internal class DelegateTest {  
   public static void Test() {  
      CallBackClass cb = new CallBackClass();  
      // Caution: The following reference on the cb object does not keep the
      // object from being garbage collected after the Main method
      // executes.  
      ExternalAPI.SetChangeHandler(new ChangeDelegate(cb.OnChange));
   }  
}  

To compensate for unexpected garbage collection, the caller must ensure that the cb object is kept alive as long as the unmanaged function pointer is in use. Optionally, you can have the unmanaged code notify the managed code when the function pointer is no longer needed, as the following example shows.

internal class DelegateTest {  
   CallBackClass cb;  
   // Called before ever using the callback function.  
   public static void SetChangeHandler() {  
      cb = new CallBackClass();  
      ExternalAPI.SetChangeHandler(new ChangeDelegate(cb.OnChange));  
   }  
   // Called after using the callback function for the last time.  
   public static void RemoveChangeHandler() {  
      // The cb object can be collected now. The unmanaged code is
      // finished with the callback function.  
      cb = null;  
   }  
}  

Default marshalling for value types

Most value types, such as integers and floating-point numbers, are blittable and do not require marshalling. Other non-blittable types have dissimilar representations in managed and unmanaged memory and do require marshalling. Still other types require explicit formatting across the interoperation boundary.

This section provides information on the following formatted value types:

• Value Types Used in Platform Invoke

• Value Types Used in COM Interop

In addition to describing formatted types, this topic identifies System Value Types that have unusual marshalling behavior.

A formatted type is a complex type that contains information that explicitly controls the layout of its members in memory. The member layout information is provided using the StructLayoutAttribute attribute. The layout can be one of the following LayoutKind enumeration values:

• LayoutKind.Auto

  Indicates that the common language runtime is free to reorder the members of the type for efficiency. However, when a value type is passed to unmanaged code, the layout of the members is predictable. An attempt to marshal such a structure automatically causes an exception.

• LayoutKind.Sequential

  Indicates that the members of the type are to be laid out in unmanaged memory in the same order in which they appear in the managed type definition.

• LayoutKind.Explicit

  Indicates that the members are laid out according to the FieldOffsetAttribute supplied with each field.

Value Types Used in Platform Invoke

In the following example the Point and Rect types provide member layout information using the StructLayoutAttribute.

Imports System.Runtime.InteropServices  
<StructLayout(LayoutKind.Sequential)> Public Structure Point  
   Public x As Integer  
   Public y As Integer  
End Structure  
<StructLayout(LayoutKind.Explicit)> Public Structure Rect  
   <FieldOffset(0)> Public left As Integer  
   <FieldOffset(4)> Public top As Integer  
   <FieldOffset(8)> Public right As Integer  
   <FieldOffset(12)> Public bottom As Integer  
End Structure  

using System.Runtime.InteropServices;  
[StructLayout(LayoutKind.Sequential)]  
public struct Point {  
   public int x;  
   public int y;  
}
  
[StructLayout(LayoutKind.Explicit)]  
public struct Rect {  
   [FieldOffset(0)] public int left;  
   [FieldOffset(4)] public int top;  
   [FieldOffset(8)] public int right;  
   [FieldOffset(12)] public int bottom;  
}  

When marshalled to unmanaged code, these formatted types are marshalled as C-style structures. This provides an easy way of calling an unmanaged API that has structure arguments. For example, the POINT and RECT structures can be passed to the Microsoft Windows API PtInRect function as follows:

BOOL PtInRect(const RECT *lprc, POINT pt);  

You can pass structures using the following platform invoke definition:

Friend Class NativeMethods
    Friend Declare Auto Function PtInRect Lib "User32.dll" (
        ByRef r As Rect, p As Point) As Boolean
End Class

internal static class NativeMethods
{
   [DllImport("User32.dll")]
   internal static extern bool PtInRect(ref Rect r, Point p);
}

The Rect value type must be passed by reference because the unmanaged API is expecting a pointer to a RECT to be passed to the function. The Point value type is passed by value because the unmanaged API expects the POINT to be passed on the stack. This subtle difference is very important. References are passed to unmanaged code as pointers. Values are passed to unmanaged code on the stack.

Classes can also be marshalled to unmanaged code as C-style structures, provided they have fixed member layout. The member layout information for a class is also provided with the StructLayoutAttribute attribute. The main difference between value types with fixed layout and classes with fixed layout is the way in which they are marshalled to unmanaged code. Value types are passed by value (on the stack) and consequently any changes made to the members of the type by the callee are not seen by the caller. Reference types are passed by reference (a reference to the type is passed on the stack); consequently, all changes made to blittable-type members of a type by the callee are seen by the caller.

In the following example, the SystemTime class has sequential member layout and can be passed to the Windows API GetSystemTime function.

<StructLayout(LayoutKind.Sequential)> Public Class SystemTime  
   Public wYear As System.UInt16  
   Public wMonth As System.UInt16  
   Public wDayOfWeek As System.UInt16  
   Public wDay As System.UInt16  
   Public wHour As System.UInt16  
   Public wMinute As System.UInt16  
   Public wSecond As System.UInt16  
   Public wMilliseconds As System.UInt16  
End Class  

[StructLayout(LayoutKind.Sequential)]  
   public class SystemTime {  
   public ushort wYear;
   public ushort wMonth;  
   public ushort wDayOfWeek;
   public ushort wDay;
   public ushort wHour;
   public ushort wMinute;
   public ushort wSecond;
   public ushort wMilliseconds;
}  

The GetSystemTime function is defined as follows:

void GetSystemTime(SYSTEMTIME* SystemTime);  

The equivalent platform invoke definition for GetSystemTime is as follows:

Friend Class NativeMethods
    Friend Declare Auto Sub GetSystemTime Lib "Kernel32.dll" (
        ByVal sysTime As SystemTime)
End Class

internal static class NativeMethods
{
   [DllImport("Kernel32.dll", CharSet = CharSet.Auto)]
   internal static extern void GetSystemTime(SystemTime st);
}

Notice that the SystemTime argument is not typed as a reference argument because SystemTime is a class, not a value type. Unlike value types, classes are always passed by reference.

The following code example shows a different Point class that has a method called SetXY. Because the type has sequential layout, it can be passed to unmanaged code and marshalled as a structure. However, the SetXY member is not callable from unmanaged code, even though the object is passed by reference.

<StructLayout(LayoutKind.Sequential)> Public Class Point  
   Private x, y As Integer  
   Public Sub SetXY(x As Integer, y As Integer)  
      Me.x = x  
      Me.y = y  
   End Sub  
End Class  

[StructLayout(LayoutKind.Sequential)]  
public class Point {  
   int x, y;  
   public void SetXY(int x, int y){
      this.x = x;  
      this.y = y;  
   }  
}  

Value Types Used in COM Interop

Formatted types can also be passed to COM interop method calls. In fact, when exported to a type library, value types are automatically converted to structures. As the following example shows, the Point value type becomes a type definition (typedef) with the name Point. All references to the Point value type elsewhere in the type library are replaced with the Point typedef.

Type library representation

typedef struct tagPoint {  
   int x;  
   int y;  
} Point;  
interface _Graphics {  
   …  
   HRESULT SetPoint ([in] Point p)  
   HRESULT SetPointRef ([in,out] Point *p)  
   HRESULT GetPoint ([out,retval] Point *p)  
}  

The same rules used to marshal values and references to platform invoke calls are used when marshalling through COM interfaces. For example, when an instance of the Point value type is passed from the .NET Framework to COM, the Point is passed by value. If the Point value type is passed by reference, a pointer to a Point is passed on the stack. The interop marshaller does not support higher levels of indirection (Point **) in either direction.

System Value Types

The System namespace has several value types that represent the boxed form of the runtime primitive types. For example, the value type System.Int32 structure represents the boxed form of ELEMENT_TYPE_I4. Instead of marshalling these types as structures, as other formatted types are, you marshal them in the same way as the primitive types they box. System.Int32 is therefore marshalled as ELEMENT_TYPE_I4 instead of as a structure containing a single member of type long. The following table contains a list of the value types in the System namespace that are boxed representations of primitive types.

Some other value types in the System namespace are handled differently. Because the unmanaged code already has well-established formats for these types, the marshaller has special rules for marshalling them. The following table lists the special value types in the System namespace, as well as the unmanaged type they are marshalled to.

The following code shows the definition of the unmanaged types DATE, GUID, DECIMAL, and OLE_COLOR in the Stdole2 type library.

Type library representation

typedef double DATE;  
typedef DWORD OLE_COLOR;  
  
typedef struct tagDEC {  
    USHORT    wReserved;  
    BYTE      scale;  
    BYTE      sign;  
    ULONG     Hi32;  
    ULONGLONG Lo64;  
} DECIMAL;  
  
typedef struct tagGUID {  
    DWORD Data1;  
    WORD  Data2;  
    WORD  Data3;  
    BYTE  Data4[ 8 ];  
} GUID;  

The following code shows the corresponding definitions in the managed IValueTypes interface.

Public Interface IValueTypes  
   Sub M1(d As System.DateTime)  
   Sub M2(d As System.Guid)  
   Sub M3(d As System.Decimal)  
   Sub M4(d As System.Drawing.Color)  
End Interface  

public interface IValueTypes {  
   void M1(System.DateTime d);  
   void M2(System.Guid d);  
   void M3(System.Decimal d);  
   void M4(System.Drawing.Color d);  
}  

Type library representation

[…]  
interface IValueTypes : IDispatch {  
   HRESULT M1([in] DATE d);  
   HRESULT M2([in] GUID d);  
   HRESULT M3([in] DECIMAL d);  
   HRESULT M4([in] OLE_COLOR d);  
};  

See also

• Blittable and Non-Blittable Types
• Copying and Pinning
• Default Marshalling for Arrays
• Default Marshalling for Objects
• Default Marshalling for Strings