F# - converting a C# dictionary to a Map

gistfile1.fs

let toMap dictionary = 
    (dictionary :> seq<_>)
    |> Seq.map (|KeyValue|)
    |> Map.ofSeq

you don't need the () around the top part.

Two suggestions:

1. Mark the function as inline so that callers of toMap will not affect the F# type inference process and fix the generic arguments
2. The upcast is not necessary as IDictionary<K, V> already implements IEnumerable<KeyValuePair<K, V>>

let inline toMap kvps =
    kvps
    |> Seq.map (|KeyValue|)
    |> Map.ofSeq

I'm fairly new to F# and wondering - how to get to know with built-in active patterns like this?

I'm fairly new to F# and wondering - how to get to know with built-in active patterns like this?

Same

I'm fairly new to F# and wondering - how to get to know with built-in active patterns like this?

This site can be helpful.
https://fsharp.github.io/fsharp-core-docs/

For example, here you can check all Map operations.
https://fsharp.github.io/fsharp-core-docs/reference/fsharp-collections-mapmodule.html