there is a project in github
Wrapper for libwebp in C#. The most complete wrapper in pure managed C#.
Exposes Simple Decoding API and Encoding API, Advanced Decoding and
Encoding API (with stadistis of compresion), Get version library and
WebPGetFeatures (info of any WebP file). Exposed get PSNR, SSIM or
LSIM distortion metrics.
The wrapper is in safe managed code in one class. No need external dll
except libwebp_x86.dll and libwebp_x64.dll (included v6.1). The
wrapper work in 32, 64 bit or ANY (auto swith to the apropiate
library).
The code is full comented and include simple example for using the
wrapper.
see project page: https://github.com/JosePineiro/WebP-wrapper