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Mar 28, 2022
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System.ThreadStaticAttribute F# snippet #7877

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10 changes: 10 additions & 0 deletions snippets/fsharp/System/ThreadStaticAttribute/Overview/fs.fsproj
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<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>net6.0</TargetFramework>
</PropertyGroup>

<ItemGroup>
<Compile Include="threadsafe2a.fs" />
</ItemGroup>
</Project>
96 changes: 96 additions & 0 deletions snippets/fsharp/System/ThreadStaticAttribute/Overview/threadsafe2a.fs
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// <Snippet1>
open System
open System.Threading

type Example() =
[<ThreadStatic; DefaultValue>]
static val mutable private previous : double

[<ThreadStatic; DefaultValue>]
static val mutable private sum : double

[<ThreadStatic; DefaultValue>]
static val mutable private calls : int

[<ThreadStatic; DefaultValue>]
static val mutable private abnormal : bool

static let mutable totalNumbers = 0
static let countdown = new CountdownEvent(1)
static let lockObj = obj ()
let rand = Random()


member this.Execute() =
for threads = 1 to 10 do
let newThread = new Thread(ThreadStart this.GetRandomNumbers)
countdown.AddCount()
newThread.Name <- threads.ToString()
newThread.Start()
this.GetRandomNumbers()
countdown.Wait()
printfn $"{totalNumbers:N0} random numbers were generated."

member _.GetRandomNumbers() =
let mutable i = 0
while i < 2000000 do
lock lockObj (fun () ->
let result = rand.NextDouble()
Example.calls <- Example.calls + 1
Interlocked.Increment &totalNumbers |> ignore
// We should never get the same random number twice.
if result = Example.previous then
Example.abnormal <- true
i <- 2000001 // break
else
Example.previous <- result
Example.sum <- Example.sum + result )
i <- i + 1
// get last result
if Example.abnormal then
printfn $"Result is {Example.previous} in {Thread.CurrentThread.Name}"

printfn $"Thread {Thread.CurrentThread.Name} finished random number generation."
printfn $"Sum = {Example.sum:N4}, Mean = {Example.sum / float Example.calls:N4}, n = {Example.calls:N0}\n"
countdown.Signal() |> ignore

let ex = Example()
Thread.CurrentThread.Name <- "Main"
ex.Execute()

// The example displays output similar to the following:
// Thread 1 finished random number generation.
// Sum = 1,000,556.7483, Mean = 0.5003, n = 2,000,000
//
// Thread 6 finished random number generation.
// Sum = 999,704.3865, Mean = 0.4999, n = 2,000,000
//
// Thread 2 finished random number generation.
// Sum = 999,680.8904, Mean = 0.4998, n = 2,000,000
//
// Thread 10 finished random number generation.
// Sum = 999,437.5132, Mean = 0.4997, n = 2,000,000
//
// Thread 8 finished random number generation.
// Sum = 1,000,663.7789, Mean = 0.5003, n = 2,000,000
//
// Thread 4 finished random number generation.
// Sum = 999,379.5978, Mean = 0.4997, n = 2,000,000
//
// Thread 5 finished random number generation.
// Sum = 1,000,011.0605, Mean = 0.5000, n = 2,000,000
//
// Thread 9 finished random number generation.
// Sum = 1,000,637.4556, Mean = 0.5003, n = 2,000,000
//
// Thread Main finished random number generation.
// Sum = 1,000,676.2381, Mean = 0.5003, n = 2,000,000
//
// Thread 3 finished random number generation.
// Sum = 999,951.1025, Mean = 0.5000, n = 2,000,000
//
// Thread 7 finished random number generation.
// Sum = 1,000,844.5217, Mean = 0.5004, n = 2,000,000
//
// 22,000,000 random numbers were generated.
// </Snippet1>
5 changes: 3 additions & 2 deletions xml/System/ThreadStaticAttribute.xml
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## Remarks
A `static` field marked with <xref:System.ThreadStaticAttribute> is not shared between threads. Each executing thread has a separate instance of the field, and independently sets and gets values for that field. If the field is accessed on a different thread, it will contain a different value.

Note that in addition to applying the <xref:System.ThreadStaticAttribute> attribute to a field, you must also define it as a `static` field (in C#) or a `Shared` field (in Visual Basic).
Note that in addition to applying the <xref:System.ThreadStaticAttribute> attribute to a field, you must also define it as a `static` field (in C# or F#) or a `Shared` field (in Visual Basic).

> [!NOTE]
> Do not specify initial values for fields marked with `ThreadStaticAttribute`, because such initialization occurs only once, when the class constructor executes, and therefore affects only one thread. If you do not specify an initial value, you can rely on the field being initialized to its default value if it is a value type, or to `null` if it is a reference type.
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The following example instantiates a random number generator, creates ten threads in addition to the main thread, and then generates two million random numbers in each thread. It uses the <xref:System.ThreadStaticAttribute> attribute to calculate the sum and the count of random numbers per thread. It also defines two additional per-thread fields, `previous` and `abnormal`, that allows it to detect corruption of the random number generator.

:::code language="csharp" source="~/snippets/csharp/System/ThreadStaticAttribute/Overview/threadsafe2a.cs" id="Snippet1":::
:::code language="fsharp" source="~/snippets/fsharp/System/ThreadStaticAttribute/Overview/threadsafe2a.fs" id="Snippet1":::
:::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.threadstaticattribute/vb/threadsafe2a.vb" id="Snippet1":::

The example uses the `lock` statement in C# and the `SyncLock` construct in Visual Basic to synchronize access to the random number generator. This prevents corruption of the random number generator, which typically results in its returning a value of zero for all subsequent calls.
The example uses the `lock` statement in C#, the `lock` function in F#, and the `SyncLock` construct in Visual Basic to synchronize access to the random number generator. This prevents corruption of the random number generator, which typically results in its returning a value of zero for all subsequent calls.

The example also uses the <xref:System.Threading.CountdownEvent> class to ensure that each thread has finished generating random numbers before it displays the total number of calls. Otherwise, if the main thread completes execution before the additional threads that it spawns, it displays an inaccurate value for the total number of method calls.

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