Contract of service can easily be termed as ' when one person has agreed to employ another person as an employee, and the other person has agreed to the terms of employment and of an employee, then the party have said to be entered in Contract of Service'
Section 27 of Indian contract Act, 1972 states that,
"Agreement in restraint of trade, void – Every agreement by which any one is restrained from exercising a lawful profession, trade or business of any kind, is to that extent void.
The constitution of India in article 19(1) (g) provides an individual to practice any profession, vocation or trade of his own choice. The legal position with regard to the post- contractual covenants is well settled in India. As a general rule, negative covenant restricting the employee beyond the period of employment are considered to be unreasonable but there are some exception to it. The exceptions include confidential information and non-solicitation agreements (to a reasonable period of time).
Section 27 of the Indian Contract act is something different to your issue and not applicable to the subject query.
While in employment of one company you should not get employed with another company. All employers shall draft text of employment contracts and if required bring interpretations to suit them, to deter the employee from any gainful activity and activities that shall consume his/her time attention