From 4956021086999670674f6632bb37d8db4022182e Mon Sep 17 00:00:00 2001
From: Dmitry Dembinsky <2384012+xloggr@users.noreply.github.com>
Date: Sun, 11 Apr 2021 23:26:08 -0400
Subject: [PATCH 1/2] add StartImmediate method to the MailboxProcessor
 (#11370)

---
 src/fsharp/FSharp.Core/mailbox.fs | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/src/fsharp/FSharp.Core/mailbox.fs b/src/fsharp/FSharp.Core/mailbox.fs
index 96782fa1e15..899e71255a2 100644
--- a/src/fsharp/FSharp.Core/mailbox.fs
+++ b/src/fsharp/FSharp.Core/mailbox.fs
@@ -325,6 +325,11 @@ namespace Microsoft.FSharp.Control
     type AsyncReplyChannel<'Reply>(replyf : 'Reply -> unit) =
         member x.Reply value = replyf value
 
+    module private AsyncStarter =
+        type Func = Async<unit> -> CancellationToken -> unit
+        let asyncStart c ct = Async.Start(computation=c, cancellationToken=ct)
+        let asyncStartImmediate c ct = Async.StartImmediate(computation=c, cancellationToken=ct)
+
     [<Sealed>]
     [<AutoSerializable(false)>]
     [<CompiledName("FSharpMailboxProcessor`1")>]
@@ -350,7 +355,7 @@ namespace Microsoft.FSharp.Control
         member _.UnsafeMessageQueueContents = mailbox.UnsafeContents
 #endif
 
-        member x.Start() =
+        member private x.StartWith(startAsync: AsyncStarter.Func) =
             if started then
                 raise (new InvalidOperationException(SR.GetString(SR.mailboxProcessorAlreadyStarted)))
             else
@@ -365,7 +370,13 @@ namespace Microsoft.FSharp.Control
                             with exn ->
                                 errorEvent.Trigger exn }
 
-                Async.Start(computation=p, cancellationToken=cancellationToken)
+                startAsync p cancellationToken
+
+        member x.Start() =
+            x.StartWith(AsyncStarter.asyncStart)
+
+        member x.StartImmediate() =
+            x.StartWith(AsyncStarter.asyncStartImmediate)
 
         member _.Post message = mailbox.Post message
 
@@ -438,3 +449,8 @@ namespace Microsoft.FSharp.Control
             let mailboxProcessor = new MailboxProcessor<'Msg>(body, ?cancellationToken=cancellationToken)
             mailboxProcessor.Start()
             mailboxProcessor
+
+        static member StartImmediate(body, ?cancellationToken) =
+            let mailboxProcessor = new MailboxProcessor<'Msg>(body, ?cancellationToken=cancellationToken)
+            mailboxProcessor.StartImmediate()
+            mailboxProcessor

From 5ff26eda88a8e70a1cee3f420912042f511a9904 Mon Sep 17 00:00:00 2001
From: Dmitry Dembinsky <2384012+xloggr@users.noreply.github.com>
Date: Fri, 7 May 2021 07:41:06 -0400
Subject: [PATCH 2/2] address review comments:

- remove AsyncStarter module
- add unit test
---
 src/fsharp/FSharp.Core/mailbox.fs         | 24 +++++++++--------------
 tests/fsharp/core/controlMailbox/test.fsx |  8 ++++++++
 2 files changed, 17 insertions(+), 15 deletions(-)

diff --git a/src/fsharp/FSharp.Core/mailbox.fs b/src/fsharp/FSharp.Core/mailbox.fs
index 899e71255a2..20b426cd0eb 100644
--- a/src/fsharp/FSharp.Core/mailbox.fs
+++ b/src/fsharp/FSharp.Core/mailbox.fs
@@ -325,11 +325,6 @@ namespace Microsoft.FSharp.Control
     type AsyncReplyChannel<'Reply>(replyf : 'Reply -> unit) =
         member x.Reply value = replyf value
 
-    module private AsyncStarter =
-        type Func = Async<unit> -> CancellationToken -> unit
-        let asyncStart c ct = Async.Start(computation=c, cancellationToken=ct)
-        let asyncStartImmediate c ct = Async.StartImmediate(computation=c, cancellationToken=ct)
-
     [<Sealed>]
     [<AutoSerializable(false)>]
     [<CompiledName("FSharpMailboxProcessor`1")>]
@@ -355,7 +350,7 @@ namespace Microsoft.FSharp.Control
         member _.UnsafeMessageQueueContents = mailbox.UnsafeContents
 #endif
 
-        member private x.StartWith(startAsync: AsyncStarter.Func) =
+        member private x.PrepareStart() =
             if started then
                 raise (new InvalidOperationException(SR.GetString(SR.mailboxProcessorAlreadyStarted)))
             else
@@ -364,19 +359,18 @@ namespace Microsoft.FSharp.Control
                 // Protect the execution and send errors to the event.
                 // Note that exception stack traces are lost in this design - in an extended design
                 // the event could propagate an ExceptionDispatchInfo instead of an Exception.
-                let p =
-                    async { try
-                                do! body x
-                            with exn ->
-                                errorEvent.Trigger exn }
-
-                startAsync p cancellationToken
+                async { try
+                            do! body x
+                        with exn ->
+                            errorEvent.Trigger exn }
 
         member x.Start() =
-            x.StartWith(AsyncStarter.asyncStart)
+            let p = x.PrepareStart()
+            Async.Start(computation=p, cancellationToken=cancellationToken)
 
         member x.StartImmediate() =
-            x.StartWith(AsyncStarter.asyncStartImmediate)
+            let p = x.PrepareStart()
+            Async.StartImmediate(computation=p, cancellationToken=cancellationToken)
 
         member _.Post message = mailbox.Post message
 
diff --git a/tests/fsharp/core/controlMailbox/test.fsx b/tests/fsharp/core/controlMailbox/test.fsx
index fc8a1cc7aea..31eed776a9b 100644
--- a/tests/fsharp/core/controlMailbox/test.fsx
+++ b/tests/fsharp/core/controlMailbox/test.fsx
@@ -66,6 +66,14 @@ module MailboxProcessorBasicTests =
             100
 
 
+        check 
+            "c32398u6: MailboxProcessor null"
+            (let mb1 = new MailboxProcessor<int>(fun inbox -> async { return () })
+             mb1.StartImmediate();
+             100)
+            100
+
+
         check 
             "c32398u7: MailboxProcessor Receive/PostAndReply"
             (let mb1 = new MailboxProcessor<AsyncReplyChannel<int>>(fun inbox -> async { let! msg = inbox.Receive()