1. A method, comprising: indicating an association of parameter definitions with extensions in a runtime environment in a computer system in which a computer program is executing, wherein each extension comprises executable code and is selectable to process at least one input object as part of an operation in the computer program executing in the runtime environment; receiving a call, in the runtime environment, for a dispatch interface method, wherein the dispatch interface method is at an extension point in the executing computer program and is called with information identifying at least one input object to process and with requirements for processing the at least one input object; and determining, by the called dispatch interface method, a set of candidate extensions capable of processing the at least one input object at the extension point based on the information on the input object in the call and a criteria for determining extensions whose parameter definitions indicate that the extensions are capable of processing the at least one input object while satisfying the requirements.