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C# in a Nutshell by Peter Drayton, Ted Neward, Ben Albahari

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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 [a]
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

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

Table C-2 shows the mapping of the C# modifiers to their equivalent COM interface attributes.

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

C# Modifier

COM Attribute

<no modifier>

[in]
out [out]
ref [in, out]

<return value>

[out, retval]

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