Evaluate $\oint_{C} \mathbf{F} \cdot d \mathbf{r}$ for the following vector fields and closed oriented curves $C$ by parameterizing $C .$ If the integral is not zero, give an explanation.
$$\mathbf{F}=\langle x, y, z\rangle ; \quad C: \mathbf{r}(t)=\langle\cos t, \sin t, 2\rangle, \text { for } 0 \leq t \leq 2 \pi$$