From 4b3fdcf6c998fc677cd202457211d306da2be5fc Mon Sep 17 00:00:00 2001
From: albert-du <52804499+albert-du@users.noreply.github.com>
Date: Tue, 1 Feb 2022 20:09:31 -0800
Subject: [PATCH 1/2] IObservable<T>, IObserver<T> F# snippets

---
 .../System/IObservableT/Overview/fs.fsproj    | 11 +++++
 .../System/IObservableT/Overview/observer.fs  | 35 ++++++++++++++
 .../System/IObservableT/Overview/program.fs   | 24 ++++++++++
 .../System/IObservableT/Overview/provider.fs  | 47 +++++++++++++++++++
 xml/System/IObservable`1.xml                  |  6 +++
 xml/System/IObserver`1.xml                    |  8 ++++
 6 files changed, 131 insertions(+)
 create mode 100644 snippets/fsharp/System/IObservableT/Overview/fs.fsproj
 create mode 100644 snippets/fsharp/System/IObservableT/Overview/observer.fs
 create mode 100644 snippets/fsharp/System/IObservableT/Overview/program.fs
 create mode 100644 snippets/fsharp/System/IObservableT/Overview/provider.fs

diff --git a/snippets/fsharp/System/IObservableT/Overview/fs.fsproj b/snippets/fsharp/System/IObservableT/Overview/fs.fsproj
new file mode 100644
index 00000000000..131b26313d5
--- /dev/null
+++ b/snippets/fsharp/System/IObservableT/Overview/fs.fsproj
@@ -0,0 +1,11 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="provider.fs" />
+    <Compile Include="observer.fs" />
+    <Compile Include="program.fs" />
+  </ItemGroup>
+</Project>
\ No newline at end of file
diff --git a/snippets/fsharp/System/IObservableT/Overview/observer.fs b/snippets/fsharp/System/IObservableT/Overview/observer.fs
new file mode 100644
index 00000000000..4a03f57310a
--- /dev/null
+++ b/snippets/fsharp/System/IObservableT/Overview/observer.fs
@@ -0,0 +1,35 @@
+namespace global 
+
+// <Snippet8>
+open System
+
+type LocationReporter(name) =
+    let mutable unsubscriber = Unchecked.defaultof<IDisposable>
+
+    member _.Name = name
+
+    member this.Subscribe(provider: IObservable<Location>) =
+        if provider <> null then
+            unsubscriber <- provider.Subscribe this
+
+    member _.Unsubscribe() =
+        unsubscriber.Dispose()
+
+    interface IObserver<Location> with
+        // <Snippet11>
+        member this.OnCompleted() =
+            printfn $"The Location Tracker has completed transmitting data to {name}."
+            this.Unsubscribe()
+        // </Snippet11>
+
+        // <Snippet10>
+        member _.OnError(_) =
+            printfn $"{name}: The location cannot be determined."
+        // </Snippet10>
+
+        // <Snippet12>
+        member _.OnNext(value) =
+            printfn $"{name}: The current location is {value.Latitude}, {value.Longitude}"
+        // </Snippet12>
+
+// </Snippet8>
\ No newline at end of file
diff --git a/snippets/fsharp/System/IObservableT/Overview/program.fs b/snippets/fsharp/System/IObservableT/Overview/program.fs
new file mode 100644
index 00000000000..1ce49115c3e
--- /dev/null
+++ b/snippets/fsharp/System/IObservableT/Overview/program.fs
@@ -0,0 +1,24 @@
+module program
+
+// <Snippet9>
+open System
+
+// Define a provider and two observers.
+let provider = LocationTracker()
+let reporter1 = LocationReporter "FixedGPS"
+reporter1.Subscribe provider
+let reporter2 = LocationReporter "MobileGPS"
+reporter2.Subscribe provider
+
+provider.TrackLocation { Latitude = 47.6456; Longitude = -122.1312 }
+reporter1.Unsubscribe()
+provider.TrackLocation { Latitude = 47.6677; Longitude = -122.1199 }
+provider.TrackLocation(Nullable())
+provider.EndTransmission()
+// The example displays output similar to the following:
+//      FixedGPS: The current location is 47.6456, -122.1312
+//      MobileGPS: The current location is 47.6456, -122.1312
+//      MobileGPS: The current location is 47.6677, -122.1199
+//      MobileGPS: The location cannot be determined.
+//      The Location Tracker has completed transmitting data to MobileGPS.
+// </Snippet9>
\ No newline at end of file
diff --git a/snippets/fsharp/System/IObservableT/Overview/provider.fs b/snippets/fsharp/System/IObservableT/Overview/provider.fs
new file mode 100644
index 00000000000..933e2cfb7e5
--- /dev/null
+++ b/snippets/fsharp/System/IObservableT/Overview/provider.fs
@@ -0,0 +1,47 @@
+namespace global
+
+open System
+open System.Collections.Generic
+
+// <Snippet5>
+[<Struct>]
+type Location =
+    { Latitude: double
+      Longitude: double }
+// </Snippet5>
+
+// <Snippet7>
+exception LocationUnknownException
+// </Snippet7>
+
+// <Snippet6>
+type Unsubscriber(observers: List<IObserver<Location>>, observer: IObserver<Location>) =
+    interface IDisposable with
+        member _.Dispose() =
+            if observer <> null && observers.Contains observer then
+                observers.Remove observer |> ignore
+
+type LocationTracker() =
+    // <Snippet13>
+    let observers = ResizeArray<IObserver<Location>>()
+
+    interface IObservable<Location> with
+        member _.Subscribe(observer) =
+            if observers.Contains observer |> not then
+                observers.Add observer
+            new Unsubscriber(observers, observer)
+
+    // </Snippet13>
+    member _.TrackLocation(loc: Nullable<Location>) =
+        for observer in observers do
+            if not loc.HasValue then
+                observer.OnError LocationUnknownException
+            else
+                observer.OnNext loc.Value
+
+    member _.EndTransmission() =
+        for observer in observers.ToArray() do
+            if observers.Contains observer then
+                observer.OnCompleted()
+        observers.Clear()
+// </Snippet6>
\ No newline at end of file
diff --git a/xml/System/IObservable`1.xml b/xml/System/IObservable`1.xml
index 615b33151a9..d5bfeb44db2 100644
--- a/xml/System/IObservable`1.xml
+++ b/xml/System/IObservable`1.xml
@@ -67,16 +67,19 @@
  The following example illustrates the observer design pattern. It defines a `Location` class that contains latitude and longitude information.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/provider.cs" id="Snippet5":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/provider.fs" id="Snippet5":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/provider.vb" id="Snippet5":::  
   
  The `LocationTracker` class provides the <xref:System.IObservable%601> implementation. Its `TrackLocation` method is passed a nullable `Location` object that contains the latitude and longitude data. If the `Location` value is not `null`, the `TrackLocation` method calls the <xref:System.IObserver%601.OnNext%2A> method of each observer.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/provider.cs" id="Snippet6":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/provider.fs" id="Snippet6":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/provider.vb" id="Snippet6":::  
   
  If the `Location` value is `null`, the `TrackLocation` method instantiates a `LocationUnknownException` object, which is shown in the following example. It then calls each observer's <xref:System.IObserver%601.OnError%2A> method and passes it the `LocationUnknownException` object. Note that `LocationUnknownException` derives from <xref:System.Exception>, but does not add any new members.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/provider.cs" id="Snippet7":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/provider.fs" id="Snippet7":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/provider.vb" id="Snippet7":::  
   
  Observers register to receive notifications from a `TrackLocation` object by calling its <xref:System.IObservable%601.Subscribe%2A?displayProperty=nameWithType> method, which assigns a reference to the observer object to a private generic <xref:System.Collections.Generic.List%601> object. The method returns an `Unsubscriber` object, which is an <xref:System.IDisposable> implementation that enables observers to stop receiving notifications. The `LocationTracker` class also includes an `EndTransmission` method. When no further location data is available, the method calls each observer's <xref:System.IObserver%601.OnCompleted%2A> method and then clears the internal list of observers.  
@@ -84,11 +87,13 @@
  In this example, the `LocationReporter` class provides the <xref:System.IObserver%601> implementation. It displays information about the current location to the console. Its constructor includes a `name` parameter, which enables the `LocationReporter` instance to identify itself in its string output. It also includes a `Subscribe` method, which wraps a call to the provider's <xref:System.IObservable%601.Subscribe%2A> method. This allows the method to assign the returned <xref:System.IDisposable> reference to a private variable. The `LocationReporter` class also includes an `Unsubscribe` method, which calls the <xref:System.IDisposable.Dispose%2A?displayProperty=nameWithType> method of the object that is returned by the <xref:System.IObservable%601.Subscribe%2A?displayProperty=nameWithType> method. The following code defines the `LocationReporter` class.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/observer.cs" id="Snippet8":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/observer.fs" id="Snippet8":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/observer.vb" id="Snippet8":::  
   
  The following code then instantiates the provider and the observer.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/program.cs" id="Snippet9":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/program.fs" id="Snippet9":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/module1.vb" id="Snippet9":::  
   
  ]]></format>
@@ -155,6 +160,7 @@
  The following example illustrates the <xref:System.IObservable%601.Subscribe%2A> method for an application that reports latitude and longitude information. It defines an <xref:System.Collections.Generic.IList%601> collection object that stores references to all observers. It also returns a private class named `Unsubscriber` that implements the <xref:System.IDisposable> interface and enables subscribers to stop receiving event notifications.  See the Example section of the <xref:System.IObservable%601> topic for the complete example.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/provider.cs" id="Snippet13":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/provider.fs" id="Snippet13":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/provider.vb" id="Snippet13":::  
   
  ]]></format>
diff --git a/xml/System/IObserver`1.xml b/xml/System/IObserver`1.xml
index bbdc9c9bff1..38eb29d97d7 100644
--- a/xml/System/IObserver`1.xml
+++ b/xml/System/IObserver`1.xml
@@ -65,21 +65,25 @@
  The following example illustrates the observer design pattern. It defines a `Location` class that contains latitude and longitude information.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/provider.cs" id="Snippet5":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/provider.fs" id="Snippet5":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/provider.vb" id="Snippet5":::  
   
  The `LocationReporter` class provides the <xref:System.IObserver%601> implementation. It displays information about the current location to the console. Its constructor includes a `name` parameter, which allows the `LocationReporter` instance to identify itself in its string output. It also includes a `Subscribe` method, which wraps a call to the provider's <xref:System.IObservable%601.Subscribe%2A> method. This enables the method to assign the returned <xref:System.IDisposable> reference to a private variable. The `LocationReporter` class also includes an `Unsubscribe` method, which calls the <xref:System.IDisposable.Dispose%2A?displayProperty=nameWithType> method of the object returned by the <xref:System.IObservable%601.Subscribe%2A?displayProperty=nameWithType> method. The following code defines the `LocationReporter` class.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/observer.cs" id="Snippet8":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/observer.fs" id="Snippet8":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/observer.vb" id="Snippet8":::  
   
  The `LocationTracker` class provides the <xref:System.IObservable%601> implementation. Its `TrackLocation` method is passed a nullable `Location` object that contains the latitude and longitude data. If the `Location` value is not `null`, the `TrackLocation` method calls the <xref:System.IObserver%601.OnNext%2A> method of each observer.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/provider.cs" id="Snippet6":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/provider.fs" id="Snippet6":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/provider.vb" id="Snippet6":::  
   
  If the `Location` value is `null`, the `TrackLocation` method instantiates a `LocationNotFoundException` object, which is shown in the following example. It then calls each observer's <xref:System.IObserver%601.OnError%2A> method and passes it the `LocationNotFoundException` object. Note that `LocationNotFoundException` derives from <xref:System.Exception> but does not add any new members.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/provider.cs" id="Snippet7":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/provider.fs" id="Snippet7":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/provider.vb" id="Snippet7":::  
   
  Observers register to receive notifications from a `TrackLocation` object by calling its <xref:System.IObservable%601.Subscribe%2A?displayProperty=nameWithType> method, which assigns a reference to the observer object to a private generic <xref:System.Collections.Generic.List%601> object. The method returns an `Unsubscriber` object, which is an <xref:System.IDisposable> implementation that enables observers to stop receiving notifications. The `LocationTracker` class also includes an `EndTransmission` method. When no further location data is available, the method calls each observer's <xref:System.IObserver%601.OnCompleted%2A> method and then clears the internal list of observers.  
@@ -87,6 +91,7 @@
  The following code then instantiates the provider and the observer.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/program.cs" id="Snippet9":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/program.fs" id="Snippet9":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/module1.vb" id="Snippet9":::  
   
  ]]></format>
@@ -142,6 +147,7 @@
  The following example provides an implementation of the <xref:System.IObserver%601.OnCompleted%2A> method in a latitude/longitude tracking application. The method simply reports that no further data is available and calls the provider's <xref:System.IDisposable.Dispose%2A?displayProperty=nameWithType> implementation. See the Example section of the <xref:System.IObserver%601> topic for the complete example.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/observer.cs" id="Snippet11":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/observer.fs" id="Snippet11":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/observer.vb" id="Snippet11":::  
   
  ]]></format>
@@ -199,6 +205,7 @@
  The following example provides an implementation of the <xref:System.IObserver%601.OnError%2A> method in a latitude/longitude tracking application. The method simply reports that data is currently unavailable; it does not make use of the <xref:System.Exception> object passed to it as a parameter. See the Example section of the <xref:System.IObserver%601> topic for the complete example.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/observer.cs" id="Snippet10":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/observer.fs" id="Snippet10":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/observer.vb" id="Snippet10":::  
   
  ]]></format>
@@ -256,6 +263,7 @@
  The following example provides an implementation of the <xref:System.IObserver%601.OnNext%2A> method in a latitude/longitude tracking application. See the Example section of the <xref:System.IObserver%601> topic for the complete example.  
   
  :::code language="csharp" source="~/snippets/csharp/System/IObservableT/Overview/observer.cs" id="Snippet12":::
+ :::code language="fsharp" source="~/snippets/fsharp/System/IObservableT/Overview/observer.fs" id="Snippet12":::
  :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.iobserver.class/vb/observer.vb" id="Snippet12":::  
   
  ]]></format>

From f41b8c1048f56d8f723c666e18c705c0eaa4ba05 Mon Sep 17 00:00:00 2001
From: albert-du <52804499+albert-du@users.noreply.github.com>
Date: Tue, 1 Feb 2022 20:12:17 -0800
Subject: [PATCH 2/2] List -> ResizeArray

---
 snippets/fsharp/System/IObservableT/Overview/provider.fs | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/snippets/fsharp/System/IObservableT/Overview/provider.fs b/snippets/fsharp/System/IObservableT/Overview/provider.fs
index 933e2cfb7e5..63417c3689f 100644
--- a/snippets/fsharp/System/IObservableT/Overview/provider.fs
+++ b/snippets/fsharp/System/IObservableT/Overview/provider.fs
@@ -1,7 +1,6 @@
 namespace global
 
 open System
-open System.Collections.Generic
 
 // <Snippet5>
 [<Struct>]
@@ -15,7 +14,7 @@ exception LocationUnknownException
 // </Snippet7>
 
 // <Snippet6>
-type Unsubscriber(observers: List<IObserver<Location>>, observer: IObserver<Location>) =
+type Unsubscriber(observers: ResizeArray<IObserver<Location>>, observer: IObserver<Location>) =
     interface IDisposable with
         member _.Dispose() =
             if observer <> null && observers.Contains observer then