The default Z3 bindings for .NET are built for the .NET framework version 4.
Should the need arise, it is also possible to build them for .NET 3.5; the
instructions are as follows:

In the project properties of Microsoft.Z3.csproj:
- Under 'Application': Change Target framework to .NET Framework 3.5
- Under 'Build': Add FRAMEWORK_LT_4 to the condidional compilation symbols
- Remove the reference to System.Numerics
- Install the NuGet Package "Microsoft Code Contracts for Net3.5"