P/Invoke cheat sheet

I’m working a lot with p/invoke, and know how it’s hard to produce correct signature for unmanaged method. So, today I decided to publish basic cheat sheet for methods, parameters and attributes you should use in order to invoke unmanaged methods from managed code without a lot of problems. We start with data type translations. Here the table to understand it.

Data type from unmanaged signature	Data type in managed signature
int	int the same with all other simple types such as double, uint, etc or private objects
void*	IntPtr
int*	ref int the same with all other simple types such as double, uint, etc or private objects
char**	ref IntPtr later, you should get ascii string by using System.Runtime.InteropServices.Marshal.PtrToStringAnsi() method
wcar_t**	ref IntPtr later, you should get ascii string by using System.Runtime.InteropServices.Marshal.PtrToStringUni() method
const int*	ref int
const char*	[System.Runtime.InteropServices.In()] [System.Runtime.InteropServices.MarshalAs(System.Runtime.InteropServices.UnmanagedType.LPWStr)] string
… (variable argument)	[System.Runtime.InteropServices.In()] [System.Runtime.InteropServices.MarshalAs(System.Runtime.InteropServices.UnmanagedType.AsAny)] object

You can use either System.Runtime.InteropServices.In or System.Runtime.InteropServices.Out attribute to specify how arguments should be used.

Now we done with simple arguments, let’s see what can be done when argument is actually callback or delegate?

Unmanaged definition

Managed definition

typedef void (*MyCallback)(int Arg)

[System.Runtime.InteropServices.UnmanagedFunctionPointer(System.Runtime.InteropServices.CallingConvention.Cdecl)]delegate void MyCallback(int Arg) Caller cleans stack argument is used to assure, that we can call varargs type function, usually used by API provider. It is very similar to C# overrides for methods. Also you can use StdCall (this is default), ThisCall – stores this first and pushes other parameters on the stack, FastCall – not very supported :(

 

To call all those methods, we should know managed equivalents of unmanaged types. Here the table. The rule is simple – know how many bytes unmanaged type has and find managed type with the same number of bytes. Other words, you can marshal int into IntPtr too…

Unmanaged type	Managed equivalent
bool	bool
char	sbyte (signed), byte (unsigned)
wchar_t	char
double	double
float	single
int, long (signed)	Int32
int, long (unsigned)	UInt32
__int64 (signed)	Int64
__int64	UInt64
short (signed)	Int16
short (unsigned)	UInt16
void	void

But not only types are problem in managed/unmanaged transitions. Also structures are aligned differently. For this purpose we can use StructLayout attribute. Even if unmanaged classes are sequential and you used correct managed data types, you can find you with problems in Pack. What “pack” is? Pack is actually slot size in bytes for members of your structure. It can be 0, 1, 2, 4, 8, 16, 32, 64, or 128 and depends on the platform and application setting.

Now you can see, that it is not very complicated to create managed signatures when you have header of unmanaged assemblies. So go ahead and ask, if I missed something.

That’s all by now. Have a nice day and be good people.