Appendix C. Data Marshaling

When calling between the runtime environment and existing COM interfaces, the CLR performs automatic data marshaling for CLR types into compatible COM types. Table C-1 describes the C# to COM default data type mapping.

Table C-1. C# type to COM type mapping

C# type	COM type
bool	VARIANT_BOOL
char	unsigned short
sbyte	Char
byte	Unsigned char
short	Short
ushort	Unsigned short
int	Int
uint	Unsigned int
long	Hyper
ulong	Unsigned hyper
float	Single
double	Double
decimal	DECIMAL
object	VARIANT
string	BSTR
System.DateTime	DATE[1]
System.Guid	GUID
System.Currency	CURRENCY
1-dimensional arrays	SAFEARRAY
Value types	Equivalently named struct
enum	Equivalently named enum
interface	Equivalently named interface
class	Equivalently named CoClass

^[1] COM dates are less precise, causing comparison problems.

Table C-2. C# modifier/COM attribute mapping

C# modifier	COM attribute
<no modifier>	[in]
out	[out]
ref	[in, out]
<return value>	[out, retval]