P/Invoke Tutorial: Passing parameters (Part 3)

P/Invoke tries to make your life easier by automatically converting (“marshalling”) data types from managed code to native code and the other way around.

Marshalling Primitive Data Types

Primitive data types (bool, int, double, …) are the easiest to use. They map directly to their native counterparts.

Marshalling Strings

For passing strings, it’s recommended that you pass them as Unicode strings (if possible). For this, you need to specify Char.Unicode like this:

[DllImport("NativeLib.dll", CharSet = CharSet.Unicode)]
private static extern void do_something(string str);

This requires the C/C++ parameter type be a wchar_t*:

void do_something(const wchar_t* str);

For more details, see P/Invoke Tutorial: Passing strings (Part 2).

Marshalling Arrays

Arrays of primitive types can be passed directly.

[DllImport("NativeLib.dll")]
private static extern void do_something(byte[] data);

Marshalling Objects

To be able to pass objects you need to make their memory layout sequential:

[StructLayout(LayoutKind.Sequential)]
class MyClass {
  ...
}

This ensures that the fields are stored in the same order they’re written in code. (Without this attribute the C# compiler reorder fields around to optimize the data structure.)

Then simply use the object’s class directly:

[DllImport("NativeLib.dll")]
private static extern void do_something(MyClass data);

The object will be passed by reference (either as struct* or stuct&) to the C function:

typedef struct {
  ...
} MyClass;

void do_something(MyClass* data);

Note: Obviously the order of the fields in the native struct and the managed class must be exactly the same.

Marshalling Structs

Marshalling managed structs is almost identical to marshalling objects with only one difference: structs are passed by copy by default.

So for structs the C/C++ function signature reads:

void do_something(MyClass data);

Of course, you can pass the struct also by reference. In this case, use (MyClass* data) or (MyClass& data) in C/C++ and (ref MyClass data) in C#.

Marshalling Arbitrary Pointers

Arbitrary pointers (like void*) are marshaled as IntPtr objects.

So this C function:

void do_something(void* ptr);

becomes:

[DllImport("NativeLib.dll")]
private static extern void do_something(IntPtr ptr);