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Default Marshaling for Value Types 

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

This topic provides the follow information on formatted value types:

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

    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.

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 marshaled to unmanaged code, these formatted types are marshaled 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 Win32 API PtInRect function as follows:

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

You can pass structures using the following platform invoke definition:

class Win32API {
   [DllImport("User32.dll")]
   public 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.

NoteNote

When a formatted type is marshaled as a structure, only the fields within the type are accessible. If the type has methods, properties, or events, they are inaccessible from unmanaged code.

Classes can also be marshaled 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 marshaled 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.

NoteNote

If a reference type has members of non-blittable types, conversion is required twice: the first time when an argument is passed to the unmanaged side and the second time on return from the call. Due to this added overhead, In/Out parameters must be explicitly applied to an argument if the caller wants to see changes made by the callee.

In the following example, the SystemTime class has sequential member layout and can be passed to the Win32 API GetSystemTime function.

[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:

class Win32API {
   [DllImport("Kernel32.dll", CharSet=CharSet.Auto)]
   public 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 marshaled 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 {
   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 marshaling 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 marshaler does not support higher levels of indirection (Point **) in either direction.

NoteNote

Structures having the LayoutKind enumeration value set to Explicit cannot be used in COM interop because the exported type library cannot express an explicit layout.

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 marshaling 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 marshaled 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.

System value type Element type

System.Boolean

ELEMENT_TYPE_BOOLEAN

System.SByte

ELEMENT_TYPE_I1

System.Byte

ELEMENT_TYPE_UI1

System.Char

ELEMENT_TYPE_CHAR

System.Int16

ELEMENT_TYPE_I2

System.UInt16

ELEMENT_TYPE_U2

System.Int32

ELEMENT_TYPE_I4

System.UInt32

ELEMENT_TYPE_U4

System.Int64

ELEMENT_TYPE_I8

System.UInt64

ELEMENT_TYPE_U8

System.Single

ELEMENT_TYPE_R4

System.Double

ELEMENT_TYPE_R8

System.String

ELEMENT_TYPE_STRING

System.IntPtr

ELEMENT_TYPE_I

System.UIntPtr

ELEMENT_TYPE_U

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

System value type IDL type

System.DateTime

DATE

System.Decimal

DECIMAL

System.Guid

GUID

System.Drawing.Color

OLE_COLOR

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 {
   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);
};

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