Top 3 C# Key Value Store Projects

FASTER

8 6,199 7.2 C#

Fast persistent recoverable log and key-value store + cache, in C# and C++.

Project mention: A MySQL compatible database engine written in pure Go | news.ycombinator.com | 2024-04-09

You would be surprised by performance of modern .NET :)
Writing no-alloc is oftentimes done by reducing complexity and not doing "stupid" tricks that actually work against JIT and CoreLib features.
For databases specifically, .NET is actually positioned very well with its low-level features (intrisics incl. SIMD, FFI, struct generics though not entirely low-level) and high-throughput GC.
Interesting example of this applied in practice is Garnet[0]/FASTER[1]. Keep in mind that its codebase still consist of un-idiomatic C# and you can do way better by further simplification, but it already does the job well enough.
[0] https://github.com/microsoft/garnet
[1] https://github.com/microsoft/FASTER

ZoneTree

8 269 6.7 C#

ZoneTree is a persistent, high-performance, transactional, ACID-compliant ordered key-value database for NET. It can operate in memory or on local/cloud storage.
WorkOS

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The modern identity platform for B2B SaaS. The APIs are flexible and easy-to-use, supporting authentication, user identity, and complex enterprise features like SSO and SCIM provisioning.
key-value-store

3 5 7.6 C#

Basic Key-Value Store built on top of YugabyteDB, .Net 7.0, and Entity Framework Core 7.0.

NOTE: The open source projects on this list are ordered by number of github stars. The number of mentions indicates repo mentiontions in the last 12 Months or since we started tracking (Dec 2020).