Esme Povirk (@madewokherd) commented about dlls/gdiplus/tests/image.c:
{ case 0: paramsize = sizeof(struct BlurParams);
expect(paramsize, size);
blur.radius = 2.0;
blur.expandEdge = TRUE;
stat = pGdipSetEffectParameters(effect, (void *)&blur, 5);
expect(InvalidParameter, stat);
stat = pGdipSetEffectParameters(effect, (void *)&blur, paramsize);
A declarative approach would probably make more sense here: declare a struct with the value you want and put that in the test_data structure. Then you also have an easy way to test that reading the data gives you the same result.