System.AsyncCallback F# snippet (#7518)

Author: albert-du

samples/snippets/fsharp/VS_Snippets_CLR/AsyncDesignPattern/FS/AsyncDelegateNoStateObject.fs

@@ -0,0 +1,80 @@
+// <Snippet4>
+// The following example demonstrates using asynchronous methods to
+// get Domain Name System information for the specified host computers.
+// This example uses a delegate to obtain the results of each asynchronous
+// operation.
+
+open System
+open System.Net
+open System.Net.Sockets
+open System.Threading
+open System.Collections.Specialized
+
+let mutable requestCounter = 0
+let hostData = ResizeArray<Result<IPHostEntry, string>>()
+let hostNames = StringCollection()
+
+let updateUserInterface () =
+    // Print a message to indicate that the application
+    // is still working on the remaining requests.
+    printfn $"{requestCounter} requests remaining."
+
+// The following function is called when each asynchronous operation completes.
+let processDnsInformation (result: IAsyncResult) =
+    string result.AsyncState
+    |> hostNames.Add 
+    |> ignore
+    try
+        try
+            // Get the results.
+            Dns.EndGetHostEntry result
+            |> Ok
+            |> hostData.Add
+        // Store the exception message.
+        with :? SocketException as e ->
+            Error e.Message
+            |> hostData.Add 
+    finally
+        // Decrement the request counter in a thread-safe manner.
+        Interlocked.Decrement &requestCounter 
+        |> ignore
+
+// Create the delegate that will process the results of the asynchronous request.
+let callBack = AsyncCallback processDnsInformation
+
+let mutable host = " "
+while host.Length > 0 do
+    printf " Enter the name of a host computer or <enter> to finish: "
+    host <- stdin.ReadLine()
+    if host.Length > 0 then
+        // Increment the request counter in a thread safe manner.
+        Interlocked.Increment &requestCounter |> ignore
+        // Start the asynchronous request for DNS information.
+        Dns.BeginGetHostEntry(host, callBack, host) |> ignore
+
+// The user has entered all of the host names for lookup.
+// Now wait until the threads complete.
+while requestCounter > 0 do
+    updateUserInterface ()
+
+// Display the results.
+for i = 0 to hostNames.Count - 1 do
+    match hostData[i] with 
+    | Error message ->
+        // A SocketException was thrown.
+        printfn $"Request for {hostNames[i]} returned message: {message}"
+    | Ok entry ->    
+        // Get the results.
+        let aliases = entry.Aliases
+        let addresses = entry.AddressList
+        if aliases.Length > 0 then
+            printfn $"Aliases for {hostNames[i]}"
+            for alias in aliases do
+                printfn $"{alias}"
+
+        if addresses.Length > 0 then
+            printfn $"Addresses for {hostNames[i]}"
+            for address in addresses do
+                printfn $"{address}"
+
+//</Snippet4>

samples/snippets/fsharp/VS_Snippets_CLR/AsyncDesignPattern/FS/fs.fsproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="AsyncDelegateNoStateObject.fs" />
+  </ItemGroup>
+</Project>

xml/System/AsyncCallback.xml

@@ -71,6 +71,7 @@
  The following code example demonstrates using asynchronous methods in the <xref:System.Net.Dns> class to retrieve Domain Name System (DNS) information for user-specified computers. This example creates an <xref:System.AsyncCallback> delegate that references the `ProcessDnsInformation` method. This method is called once for each asynchronous request for DNS information.  
   
  :::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR/AsyncDesignPattern/CS/AsyncDelegateNoStateObject.cs" id="Snippet4":::
+ :::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR/AsyncDesignPattern/FS/AsyncDelegateNoStateObject.fs" id="Snippet4":::
  :::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR/AsyncDesignPattern/VB/AsyncDelegateNoState.vb" id="Snippet4":::  
   
  ]]></format>