And have no fellowship with the unfruitful works of darkness, but rather reprove them.
Already, Paul has established that we are to be followers of God (vs. 1), walking in love (vs. 2), abstaining from fornication, uncleanness, covetousness, filthiness, foolish talking, and jesting (vs. 3-4) and instead give thanks (vs. 4). He has reminded the believer that there should be a clear difference between the darkness and light (vs. 5-10). With all of this background, is added "And have no fellowship with the unfruitful works of darkness, but rather reprove them." (vs. 11) In context, it seems that Paul is exhorting the believer to completely shun the works of darkness - to "have no fellowship (share in company with, co-participate in)" with the works.
It very clearly defines the works of darkness, and not the children of darkness. For example, 1 Corinthians 5:9-11, "I wrote unto you in an epistle not to company with fornicators: Yet not altogether with the fornicators of this world, or with the covetous, or extortioners, or with idolaters; for then must ye needs go out of the world. But now I have written unto you not to keep company, if any man that is called a brother be a fornicator, or covetous, or an idolater, or a railer, or a drunkard, or an extortioner; with such an one no not to eat." In our daily lives, we will come across people who are fornicators of the world, covetous, extortioners, idolaters, etc. If we were to avoid them completely, then we should just all die and go to heaven. No; God has a special plan for mixing His children with the children of darkness. We are to be lights in this dark world ("Do all things without murmurings and disputings: That ye may be blameless and harmless, the sons of God, without rebuke, in the midst of a crooked and perverse nation, among whom ye shine as lights in the world; Holding forth the word of life; that I may rejoice in the day of Christ, that I have not run in vain, neither laboured in vain." Philippians 2:14-16 - "Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven." Matthew 5:16) and by being this light, we are to point others to Christ so that more would come into His kingdom.
But - if we participate in the works of darkness, our light will become dim and lose the powerful glow that could draw men unto Christ. Is our light shining clearly? or do we need our hearts cleansed from some hints of darkness?