Correct and Incorrect Way to Use ConcurrentDictionary

Incorrect way

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/// INCORRECT WAY:
let cachedCreateBad key =
    match myCache.TryGetValue key with
    | true, item -> item
    | _ -> 
        let newItem = createType()
        myCache.[key] <- newItem
        newItem

// Using like normal Dictionary is not correct.
// Because while calling createType() the 
// myCache is not locked so the addition can be out of sync.
// Also because of this, createType() call may execute in parallel 
// from multiple threads simultaneously for no reason.

Correct way

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/// CORRECT WAY:
let cachedCreateGood key =
    myCache.GetOrAdd(key, fun key ->
        let newItem = createType()
        newItem)

// While the lambda is called there
// is a lightweight lock inside ConcurrentDictionary

// Notice that the code change between these is quite easy!

val cachedCreateBad : key:string -> string

Full name: Script.cachedCreateBad

INCORRECT WAY:

val key : string

val myCache : System.Collections.Concurrent.ConcurrentDictionary<string,string>

Full name: Script.myCache

System.Collections.Concurrent.ConcurrentDictionary.TryGetValue(key: string, value: byref<string>) : bool

val item : string

val newItem : string

val createType : unit -> string

Full name: Script.createType

val cachedCreateGood : key:string -> string

Full name: Script.cachedCreateGood

CORRECT WAY:

System.Collections.Concurrent.ConcurrentDictionary.GetOrAdd(key: string, value: string) : string
System.Collections.Concurrent.ConcurrentDictionary.GetOrAdd(key: string, valueFactory: System.Func<string,string>) : string

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More information

Link:	http://fssnip.net/7Qr
Posted:	8 years ago
Author:	Tuomas Hietanen
Tags:	concurrency , concurrent , directory