I’m writing a small runtime-assertions library for C#. I wouldn’t call it production-ready just yet, but it’s probably complete enough to be useful.

PM> Install-Package Demands
Demand.That(snowflake.Specialness < 42)
      .OrThrow<SpecialSnowflakeException>("Snowflake too special");

Demand.ThatFor(someEnumerable)
      .AtLeast(1).Match(x => x % 2 == 0)
      .Because("someEnumerable must be able to even");

More information is available on the GitHub project page.