[ CCode ( cname = "GCallback" , instance_pos = 2.9 ) ]
public delegate void ClassSetPropertyCb<T> (Class instance, T value)
The type used for callback functions in structure definitions and function signatures.
This doesn't mean that all callback functions must take no parameters and return void. The required signature of a callback function is determined by the context in which is used (e.g. the signal to which it is connected).
Use G_CALLBACK to cast the callback function to a ClassSetPropertyCb.