if you have a refugee status in Canada, which is then a safe country for you, your asylum application will probably be rejected as "inadmissible" because it is presumed, you don't need any new protection, Canada doesn't danger your life and can't return you to your dangerous country of origin (a so called prohibition of refoulement, Art. 33 Geneva refugee convention). You can only avoid it, if you have a very good proof that your rights as a human and as a refugee are strongly violated in Canada (which, I presume, would be very difficult). Naturally this only happens, if Germany finds out about your refugee status in Canada, but I don't think you can hide this fact for a long time. And if you give a false evidence about your personality, then you will be really in a bad situation referring to your chance of legal staying in Germany.
If you leave Germany immediately after this "inadmissible" rejection, you won't normally have an official re-entry ban. The last part of your negative decision will stipulate that you will be banned from re-entering Germany for a period of time in case of deportation - so that if you go voluntarily, you can, from the legal point of view, apply for a visa directly after that. Naturally, the embassy must check your file and they are also obliged to ask the Ausländerbehörde in the place, in which you want to stay in Germany, for their agreement. They will know that you had a rejected asylum procedure, it can influence the decision about a student visa. So sometimes people have practical difficulty to get a visa after leaving Germany even if it is not legally prohibited.
With any other kind of a negative decision it is also very important to see what kind of a re-entry ban is there. As Meike said, some kinds of “negatives” include a re-entry ban (for example for 10 months but it can be different) even for the case of voluntarily return.
A re-entry ban is valid in the whole Schengen area.
Having a refugee status in Canada I wouldn’t risk making an asylum application in Germany because the chance looks really very bad. But we don’t know what problems lead you to this decision. Please look for a personal advice in a councelling center for migrants and refugees in the place where you stay in Germany or go to a lawyer who hat an expertise in asylum questions (councelling centers must have the names of such lawyers).