R v Darwish, 2010 ONCA 124 (Doherty):

[29] An accused does not have a freestanding constitutional right to an adequate investigation of
the charges against him or her: R. v. Barnes, 2009 ONCA 432, at para. 1. Inadequacies in an investigation may lead to the ultimate failure of the prosecution, to a specific breach of a Charter
right or to a civil remedy. Those inadequacies do not, however, in-and-of-themselves constitute a
denial of the right to make full answer and defence.

[30] An accused also does not have a constitutional right to direct the conduct of the criminal
investigation of which he or she is the target. As Hill J. put it in R. v. West, [2001] O.J. No. 3406
(S.C.), at para. 75, the defence cannot, through a disguised-disclosure demand, “conscript the police to undertake investigatory work for the accused”. See also: R. v. Schmidt (2001), 151 C.C.C. (3d) 74 (B.C.C.A.), at para. 19. That is not to say that the police and the Crown should not give serious consideration to investigative requests made on behalf of an accused. Clearly, they must. However, it is the prosecutorial authorities that carry the ultimate responsibility for determining the course of the investigation. Criminal investigations involve the use of public resources and the exercise of intrusive powers in the public interest. Responsibility for the proper use of those resources and powers rests with those in the service of the prosecution, and not with the defence.

[31] Nor does the disclosure right, as broad as that right is, extend so far as to require the police to investigate potential defences. The Crown’s disclosure obligation was recently described in R. v. McNeil, [2009] 1 S.C.R. 66. The court, at para. 22, reiterated the Crown’s obligation, subject to very limited exceptions, to make timely disclosure to an accused of all relevant material “in the possession or control of the Crown”. The Crown’s disclosure obligation will also require the Crown, in response to defence requests, to take reasonable steps to inquire about and obtain relevant information in the possession of some third parties. Charron J. described this aspect of the disclosure obligation at para. 49:

The Crown is not an ordinary litigant. As a minister of justice, the Crown’s
undivided loyalty is to the proper administration of justice. As such, Crown
counsel who is put on notice of the existence of relevant information cannot
simply disregard the matter. Unless the notice appears unfounded, Crown
counsel will not be able to fully assess the merits of the case and fulfil its duty as
an officer of the court without inquiring further and obtaining the information if
it is reasonably feasible to do so.