I’ve been using the Kestrel server from Microsoft.AspNetCore.Server.Kestrel in a Visual Studio 2015 *.csproj project with project.json file (i.e. a portable class library targeting .NET Standard).

If I then try to start Kestrel from my main project (a .NET Framework 4.6.x executable), I receive this error message at run time:

System.InvalidOperationException: Unable to load libuv.

While searching for a solution to this issue, I have come across Kestrel issue 216 which reports the same kind of behaviour. To summarize, this is a known problem with Visual Studio 2015 and the old projects which should be solved in Visual Studio 2017.

Temporary solution while waiting for Visual Studio 2017

For now, the easiest solution is to dig into the NuGet packages folder and copy libuv.dll from runtimes → win7-x86 → native to the main project, mark the DLL as Content and configure the properties so that it gets copied to the output path. Also make sure that the program gets compiled as an x86 assembly.