Stupid Question - Where can I find a searchable API for C#?

This seems ridiculous but I can not seem to find a good searchable API for C# (I think API is the correct term, I'm new).. but the point is when I was learning LUA there were websites that listed every method and class and had examples of use and all that.

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Microsoft has "C## Reference" - https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/

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But when I go there I can not find anything. There is a search button.. but say i search for "random" to see the "random" method/class thing... it gives me no results.. but I know that there is this thing called random?

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