Your work permit is valid both when you apply and when the visa is issued (or you are authorized to work in Canada without a work permit when your visa is issued).

and

CIC issued your work permit based on a positive Labour Market Impact Assessment (LMIA) from Employment and Social Development Canada (ESDC). Your employer would have applied for the LMIA, which you then had to attach to your application to CIC.

and

You are working for an employer named on your work permit who has made a permanent job offer based on you being accepted as a skilled worker.

You currently work in Canada in a job that is exempt from the LMIA requirement under:

• an international agreement (such as, the North America Free Trade Agreement) or
• a federal-provincial agreement.

You currently do not:

• have a work permit, or
• plan to work in Canada before you get a permanent resident visa.

OR

You are currently working in Canada and a different employer has offered to give you a permanent full-time job.

OR

You are currently working in Canada in a job that is exempt from a Labour Market Impact Assessment, but not under an international or federal-provincial agreement.

An employer has made you a permanent job offer based on you being accepted as a skilled worker.

and

The employer has a positive Labour Market Impact Assessment from ESDC.