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Using strings in COM

Most COM interfaces use regular C strings, which are allocated and used using the techniques described in Allocating strings.

However, some COM APIs use the BSTR type, in particular those that support late binding (often known as IDispatch) interfaces.

BSTR types differ from Pointer<Utf16> in that they include a four byte prefix stored immediately prior to the string itself that represents its length in bytes. The pointer points to the first character of the data string, not to the length prefix.


BSTRs should never be created using Dart's memory allocation functions. For instance, the following code is incorrect, since it does not allocate and store the length prefix.

final bstr = 'I am a happy BSTR'.toNativeUtf16();

The win32 package instead offers a wrapper for the COM memory allocation functions so that BSTR types can be created without concern. Instead of the above code, you can write:

final bstr = BSTR.fromString('I am a happy BSTR');

A debugger that examines the four bytes prior to this location will see a 32-bit int containing the value 34, representing the length of the string encoded in UTF-16 (two bytes per char).

For example:

  final bstr = BSTR.fromString('I am a happy BSTR');
final len = bstr.ptr.cast<Uint32>()[-1];
print('Length of bstr: $len');;

As shown above, you can access the pointer for a BSTR using its .ptr property.

As with other manually allocated memory, Dart does not garbage collect BSTR objects; instead, you are responsible for freeing the memory allocated for a BSTR when it is no longer used.


To release its memory, don't use the global free() function, since that will not free the length prefix. Instead, either use the .free() method on the BSTR object itself, or use the Windows API SysFreeString:

free(bstr);              // BAD!!! This will leak memory; // Good
SysFreeString(bstr.ptr); // Also good