Type marshalling

Marshalling is the process of transforming types when they need to cross between managed and native code.

Marshalling is needed because the types in the managed and unmanaged code are different. In managed code, for instance, you have a String, while in the unmanaged world strings can be Unicode ("wide"), non-Unicode, null-terminated, ASCII, etc. By default, the P/Invoke subsystem tries to do the right thing based on the default behavior, described on this article. However, for those situations where you need extra control, you can employ the MarshalAs attribute to specify what is the expected type on the unmanaged side. For instance, if you want the string to be sent as a null-terminated ANSI string, you could do it like this:

[DllImport("somenativelibrary.dll")]
static extern int MethodA([MarshalAs(UnmanagedType.LPStr)] string parameter);

If you apply the System.Runtime.CompilerServices.DisableRuntimeMarshallingAttribute attribute to the assembly, the rules in the following section don't apply. For information on how .NET values are exposed to native code when this attribute is applied, see disabled runtime marshalling.

Default rules for marshalling common types

Generally, the runtime tries to do the "right thing" when marshalling to require the least amount of work from you. The following tables describe how each type is marshalled by default when used in a parameter or field. The C99/C++11 fixed-width integer and character types are used to ensure that the following table is correct for all platforms. You can use any native type that has the same alignment and size requirements as these types.

This first table describes the mappings for various types for whom the marshalling is the same for both P/Invoke and field marshalling.

C# keyword	.NET Type	Native Type
byte	System.Byte	uint8_t
sbyte	System.SByte	int8_t
short	System.Int16	int16_t
ushort	System.UInt16	uint16_t
int	System.Int32	int32_t
uint	System.UInt32	uint32_t
long	System.Int64	int64_t
ulong	System.UInt64	uint64_t
char	System.Char	Either char or char16_t depending on the CharSet of the P/Invoke or structure. See the charset documentation.
	System.Char	Either char* or char16_t* depending on the CharSet of the P/Invoke or structure. See the charset documentation.
nint	System.IntPtr	intptr_t
nuint	System.UIntPtr	uintptr_t
	.NET Pointer types (ex. void*)	void*
	Type derived from System.Runtime.InteropServices.SafeHandle	void*
	Type derived from System.Runtime.InteropServices.CriticalHandle	void*
bool	System.Boolean	Win32 BOOL type
decimal	System.Decimal	COM DECIMAL struct
	.NET Delegate	Native function pointer
	System.DateTime	Win32 DATE type
	System.Guid	Win32 GUID type

A few categories of marshalling have different defaults if you're marshalling as a parameter or structure.

.NET Type	Native Type (Parameter)	Native Type (Field)
.NET array	A pointer to the start of an array of native representations of the array elements.	Not allowed without a [MarshalAs] attribute
A class with a LayoutKind of Sequential or Explicit	A pointer to the native representation of the class	The native representation of the class

The following table includes the default marshalling rules that are Windows-only. On non-Windows platforms, you cannot marshal these types.

.NET Type	Native Type (Parameter)	Native Type (Field)
System.Object	VARIANT	IUnknown*
System.Array	COM interface	Not allowed without a [MarshalAs] attribute
System.ArgIterator	va_list	Not allowed
System.Collections.IEnumerator	IEnumVARIANT*	Not allowed
System.Collections.IEnumerable	IDispatch*	Not allowed
System.DateTimeOffset	int64_t representing the number of ticks since midnight on January 1, 1601	int64_t representing the number of ticks since midnight on January 1, 1601

Some types can only be marshalled as parameters and not as fields. These types are listed in the following table:

.NET Type	Native Type (Parameter Only)
System.Text.StringBuilder	Either char* or char16_t* depending on the CharSet of the P/Invoke. See the charset documentation.
System.ArgIterator	va_list (on Windows x86/x64/arm64 only)
System.Runtime.InteropServices.ArrayWithOffset	void*
System.Runtime.InteropServices.HandleRef	void*

If these defaults don't do exactly what you want, you can customize how parameters are marshalled. The parameter marshalling article walks you through how to customize how different parameter types are marshalled.

Default marshalling in COM scenarios

When you are calling methods on COM objects in .NET, the .NET runtime changes the default marshalling rules to match common COM semantics. The following table lists the rules that .NET runtimes uses in COM scenarios:

.NET Type	Native Type (COM method calls)
System.Boolean	VARIANT_BOOL
StringBuilder	LPWSTR
System.String	BSTR
Delegate types	_Delegate* in .NET Framework. Disallowed in .NET Core and .NET 5+.
System.Drawing.Color	OLECOLOR
.NET array	SAFEARRAY
System.String[]	SAFEARRAY of BSTRs

Marshalling classes and structs

Another aspect of type marshalling is how to pass in a struct to an unmanaged method. For instance, some of the unmanaged methods require a struct as a parameter. In these cases, you need to create a corresponding struct or a class in managed part of the world to use it as a parameter. However, just defining the class isn't enough, you also need to instruct the marshaller how to map fields in the class to the unmanaged struct. Here the StructLayout attribute becomes useful.

[DllImport("kernel32.dll")]
static extern void GetSystemTime(SystemTime systemTime);

[StructLayout(LayoutKind.Sequential)]
class SystemTime {
    public ushort Year;
    public ushort Month;
    public ushort DayOfWeek;
    public ushort Day;
    public ushort Hour;
    public ushort Minute;
    public ushort Second;
    public ushort Millisecond;
}

public static void Main(string[] args) {
    SystemTime st = new SystemTime();
    GetSystemTime(st);
    Console.WriteLine(st.Year);
}

The previous code shows a simple example of calling into GetSystemTime() function. The interesting bit is on line 4. The attribute specifies that the fields of the class should be mapped sequentially to the struct on the other (unmanaged) side. This means that the naming of the fields isn't important, only their order is important, as it needs to correspond to the unmanaged struct, shown in the following example:

typedef struct _SYSTEMTIME {
  WORD wYear;
  WORD wMonth;
  WORD wDayOfWeek;
  WORD wDay;
  WORD wHour;
  WORD wMinute;
  WORD wSecond;
  WORD wMilliseconds;
} SYSTEMTIME, *PSYSTEMTIME;

Sometimes the default marshalling for your structure doesn't do what you need. The Customizing structure marshalling article teaches you how to customize how your structure is marshalled.