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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
"engine" is a term that's mostly used for games. If you're interested in writing games in C# then you should look at Unity.
An alternative (that, unlike Unity, I have no experience of) is Godot, which I think can use C# as its scripting language.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.