${\mathrm{SnO}}_2$ has attracted considerable attention due to its wide bandgap, large exciton binding energy, and outstanding electrical and optoelectronic features. Owing to the lack of reliable and reproducible p-type ${\mathrm{SnO}}_2$, many challenges on developing ${\mathrm{SnO}}_2$-based optoelectronic devices and their practical applications still remain. Herein, single-crystal ${\mathrm{SnO}}_2$ microwires (MWs) are acquired via the self-catalyzed approach. As a strategic alternative, ${\mathrm{n}\text{-}\mathrm{SnO}}_2$ MW/p-GaN heterojunction was constructed, which exhibited selectable dual-functionalities of light-emitting and photodetection when operated by applying an appropriate voltage. The device illustrated a distinct near-ultraviolet light-emission peaking at $\sim 395.0\mathrm{nm}$ and a linewidth $\sim 50\mathrm{nm}$. Significantly, the device characteristics, in terms of the main peak positions and linewidth, are nearly invariant as functions of various injection current, suggesting that quantum-confined Stark effect is essentially absent. Meanwhile, the identical $\mathrm{n}\text{-}{\mathrm{SnO}}_2$ MW/p-GaN heterojunction can also achieve photovoltaic-type light detection. The device can steadily feature ultraviolet photodetecting ability, including the ultraviolet/visible rejection ratio ($R_{360\mathrm{nm}}/R_{400\mathrm{nm}}$) $\sim 1.5\times {10}^3$, high photodark current ratio of ${10}^5$, fast response speed of 9.2/51 ms, maximum responsivity of 1.5 A/W, and detectivity of $1.3\times {10}^{13}$ Jones under 360 nm light at $-3\mathrm{V}$ bias. Therefore, the bifunctional device not only displays distinct near-ultraviolet light emission, but also has the ability of high-sensitive ultraviolet photodetection. The novel design of ${\mathrm{n}\text{-}\mathrm{SnO}}_2$ MW/p-GaN heterojunction bifunctional systems is expected to open doors to practical application of ${\mathrm{SnO}}_2$ microstructures/nanostructures for large-scale device miniaturization, integration and multifunction in next-generation high-performance photoelectronic devices.