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Cooperative diversity achieves enhanced data rates and throughput using distributed spatial diversity techniques. We consider a cooperative diversity system comprising of a source, a relay, and a destination, with decode and forward (DF) protocol at the relay. The drawback of conventional instantaneous signal-to-noise ratio (SNR) based selection combining is that it fails to use the instantaneous SNR of the source-relay channel at the destination while selecting the best channel from source-destination and relay-destination channels. Full channel state information (CSI) selection combining, which effectively utilizes the instantaneous SNR of the source-relay channel at the destination, is used to overcome this drawback. The exact end-to-end symbol error probability (SEP) of full CSI selection combining for this DF system, with M-ary phase-shift keying in a flat Rayleigh fading environment, is derived. The analysis is validated through simulations. The diversity gain analysis is also studied for this scheme. Further, we compare the full CSI selection combining scheme with some other existing schemes. Numerical results show that full CSI selection combining has performance similar to that of instantaneous SEP based selection combining, but offers SNR gain over instantaneous SNR based selection combining, max-min selection, and selection cooperation.