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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.

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16 Responses to “P/Invoke cheat sheet”

  1. Just code - Tamir Khason Says:

    Recently I wrote a cheat sheet for pinvoking in .NET . Shortly after I got a question in comments about

  2. Ofer Says:

    all,

    i have a C++ function which I need to use from a C# project.

    the prototype of the funtion is:

    void complexFunction(char* arg1, …);

    i don’t have any problem with the first argument but I’m having some issues with the variable arguments.

    i used the "… (variable argument)" format specified here but i don’t know how to define the actual parameter. in the format it says "object" however i’m not able to pass any number of arguments, as the C++ function expects.

    I tried to change from "object" to "params string[]" but the marshaler refused to accept this and i got an exception.

    any idea?

    thanks a lot!

    Ofer

  3. Tamir Khason Says:

    pelister, it also, however dot net marshals variable pointer to var as reference to var. Actually every variable in managed code is passed by reference, rather then by value. So from this point of view there is no difference between int and int&

    Tim, It’s absolutely aversome tool, however, it seemed, that it has it own problems too :)

  4. Tim Van Wassenhove Says:

    A lesser known tool, http://www.codeplex.com/clrinterop was released by the ms clr team somewhere around techEd US promises to take away a lot of the pain…

  5. Dew Drop - October 21, 2008 | Alvin Ashcraft's Morning Dew Says:

    Pingback from  Dew Drop – October 21, 2008 | Alvin Ashcraft’s Morning Dew

  6. Tamir Khason Says:

    Hi, Clint

    Working hard to prep to PDC :) ?

    Yes, I know this site. This is great time saver, however the problem with it is that it contains unverified content and sometimes (most in less known methods) it just wrong :)

  7. pelister Says:

    as far as i know, the native equivalent of ref int should be int& and not int* am i wrong?

  8. Clint Rutkas Says:

    This is awesome and extremely useful.  Have you checked out http://www.pinvoke.net also?  If it is a documented call, that site will give you the exact breakdown.

  9. DotNetKicks.com Says:

    You’ve been kicked (a good thing) – Trackback from DotNetKicks.com

  10. Audio CD operation including CD-Text reading in pure C# | Tamir Khason - Just code Says:

    [...] P/Invoke it! But how to marshal unknown array? We should create wrapper object. Also there is very fun BitVector, used in this structure! What’s the problem? Pin it with some Math! [...]

  11. Mike Says:

    But what it GC try to move the delegate?

  12. Aaron Says:

    it is bad, very very bad to use IntPtr. Since .net 2.0, it is better to adopt a user-defined subtype of SafeHandle. Search “writing reliable code” on MSDN.

    BTW, i keep your webside in my favorites. So many wonderful posts of WPF. I was infoed that i am going to need WPF in just past friday. :(

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