dotnet · BillWagner · Mar 7, 2022 · Mar 6, 2022
@@ -0,0 +1,14 @@
+module Array1
+
+// <Snippet8>
+open System
+
+let values = [| "one"; null; "two" |]
+for i = 0 to values.GetUpperBound 0 do
+    printfn $"""{values[i].Trim()}{if i = values.GetUpperBound 0 then "" else ", "}"""
+printfn ""
+// The example displays the following output:
+//    Unhandled Exception:
+//       System.NullReferenceException: Object reference not set to an instance of an object.
+//       at <StartupCode$fs>.main()
+// </Snippet8>
@@ -0,0 +1,12 @@
+module Array2
+
+// <Snippet9>
+open System
+
+let values = [| "one"; null; "two" |]
+for i = 0 to values.GetUpperBound 0 do
+    printf $"""{if values[i] <> null then values[i].Trim() else ""}{if i = values.GetUpperBound 0 then "" else ", "}"""
+Console.WriteLine()
+// The example displays the following output:
+//       one, , two
+// </Snippet9>
@@ -0,0 +1,14 @@
+module Array3
+
+// <Snippet10>
+let values: int[] = null
+for i = 0 to 9 do
+    values[i] <- i * 2
+
+for value in values do
+    printfn $"{value}"
+// The example displays the following output:
+//    Unhandled Exception:
+//       System.NullReferenceException: Object reference not set to an instance of an object.
+//       at <StartupCode$fs>.main()
+// </Snippet10>
@@ -0,0 +1,21 @@
+module Array4
+
+// <Snippet11>
+let values = Array.zeroCreate<int> 10
+for i = 0 to 9 do
+    values[i] <- i * 2
+
+for value in values do
+    printfn $"{value}"
+// The example displays the following output:
+//    0
+//    2
+//    4
+//    6
+//    8
+//    10
+//    12
+//    14
+//    16
+//    18
+// </Snippet11>
@@ -0,0 +1,48 @@
+module Chain1
+
+// <Snippet6>
+open System
+
+type Page() =
+    [<DefaultValue>]
+    val mutable public URL: Uri
+    [<DefaultValue>]
+    val mutable public Title: string
+
+type Pages() =
+    let pages = Array.zeroCreate<Page> 10
+    let mutable i = 0
+
+    member _.CurrentPage
+        with get () = pages[i]
+        and set (value) =
+            // Move all the page objects down to accommodate the new one.
+            if i > pages.GetUpperBound 0 then
+                for ndx = 1 to pages.GetUpperBound 0 do
+                    pages[ndx - 1] <- pages[ndx]
+
+            pages[i] <- value
+            if i < pages.GetUpperBound 0 then
+                i <- i + 1
+
+    member _.PreviousPage =
+        if i = 0 then
+            if box pages[0] = null then
+                Unchecked.defaultof<Page>
+            else
+                pages[0]
+        else
+            i <- i - 1
+            pages[i + 1]
+
+let pages = Pages()
+if String.IsNullOrEmpty pages.CurrentPage.Title |> not then
+    let title = pages.CurrentPage.Title
+    printfn $"Current title: '{title}'"
+
+
+// The example displays the following output:
+//    Unhandled Exception:
+//       System.NullReferenceException: Object reference not set to an instance of an object.
+//       at <StartupCode$fs>.main()
+// </Snippet6>
@@ -0,0 +1,46 @@
+module Chain2
+
+type Page() =
+    [<DefaultValue>]
+    val mutable public URL: System.Uri
+    [<DefaultValue>]
+    val mutable public Title: string
+
+type Pages() =
+    let pages = Array.zeroCreate<Page> 10
+    let mutable i = 0
+
+    member _.CurrentPage
+        with get () = pages[i]
+        and set (value) =
+            // Move all the page objects down to accommodate the new one.
+            if i > pages.GetUpperBound 0 then
+                for ndx = 1 to pages.GetUpperBound 0 do
+                    pages[ndx - 1] <- pages[ndx]
+
+            pages[i] <- value
+            if i < pages.GetUpperBound 0 then
+                i <- i + 1
+
+    member _.PreviousPage =
+        if i = 0 then
+            if box pages[0] = null then
+                Unchecked.defaultof<Page>
+            else
+                pages[0]
+        else
+            i <- i - 1
+            pages[i + 1]
+
+// <Snippet7>
+let pages = Pages()
+let current = pages.CurrentPage
+if box current <> null then
+    let title = current.Title
+    printfn $"Current title: '{title}'"
+else
+    printfn "There is no page information in the cache."
+// The example displays the following output:
+//       There is no page information in the cache.
+// </Snippet7>
+
@@ -0,0 +1,23 @@
+module Example
+
+// <Snippet1>
+open System
+
+[<EntryPoint>]
+let main args =
+    let value = Int32.Parse args[0]
+    // Set names to null, don't initialize it. 
+    let mutable names = Unchecked.defaultof<ResizeArray<string>>
+    if value > 0 then
+        names <- ResizeArray()
+    names.Add "Major Major Major"
+    0
+// Compilation does not display a warning as this is an extremely rare occurance in F#.
+// Creating a value without initalizing either requires using 'null' (not possible
+// on types defined in F# without [<AllowNullLiteral>]) or Unchecked.defaultof.
+//
+// The example displays output like the following output:
+//    Unhandled Exception: System.NullReferenceException: Object reference
+//    not set to an instance of an object.
+//       at Example.main()
+// </Snippet1>
@@ -0,0 +1,6 @@
+module example1a
+
+// <Snippet2>
+let names = ResizeArray()
+names.Add "Major Major Major"
+// </Snippet2>
@@ -0,0 +1,22 @@
+module example2
+
+// <Snippet3>
+let populateNames (names: ResizeArray<string>) =
+    let arrNames =
+        [ "Dakota"; "Samuel"; "Nikita"
+          "Koani"; "Saya"; "Yiska"; "Yumaevsky" ]
+    for arrName in arrNames do
+        names.Add arrName
+
+let getData () : ResizeArray<string> =
+    null
+
+let names = getData ()
+populateNames names
+
+// The example displays output like the following:
+//    Unhandled Exception: System.NullReferenceException: Object reference
+//    not set to an instance of an object.
+//       at Example.PopulateNames(List`1 names)
+//       at <StartupCode$fs>.main()
+// </Snippet3>
@@ -0,0 +1,20 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="example1a.fs" />
+    <Compile Include="Array3.fs" />
+    <Compile Include="Array4.fs" />
+    <Compile Include="nullreturn2.fs" />
+    <Compile Include="Chain1.fs" />
+    <Compile Include="Chain2.fs" />
+    <Compile Include="Array1.fs" />
+    <Compile Include="Array2.fs" />
+    <Compile Include="example2.fs" />
+    <Compile Include="nullreturn2a.fs" />
+    <Compile Include="example1.fs" />
+  </ItemGroup>
+</Project>
@@ -0,0 +1,26 @@
+module nullreturn2
+
+// <Snippet4>
+open System
+
+type Person(firstName) =
+    member _.FirstName = firstName
+
+    static member AddRange(firstNames) =
+        Array.map Person firstNames
+
+let persons = 
+    [| "Abigail"; "Abra"; "Abraham"; "Adrian"
+       "Ariella"; "Arnold"; "Aston"; "Astor" |]
+    |> Person.AddRange
+
+let nameToFind = "Robert"
+let found = Array.Find(persons, fun p -> p.FirstName = nameToFind)
+
+printfn $"{found.FirstName}"
+
+// The example displays the following output:
+//       Unhandled Exception: System.NullReferenceException:
+//       Object reference not set to an instance of an object.
+//          at <StartupCode$fs>.main()
+// </Snippet4>
@@ -0,0 +1,40 @@
+module nullreturn2a
+
+// <Snippet5>
+open System
+
+[<AllowNullLiteral>]
+type Person(firstName) =
+    member _.FirstName = firstName
+
+    static member AddRange(firstNames) =
+        Array.map Person firstNames
+
+let persons = 
+    [| "Abigail"; "Abra"; "Abraham"; "Adrian"
+       "Ariella"; "Arnold"; "Aston"; "Astor" |]
+    |> Person.AddRange
+
+let nameToFind = "Robert"
+let found = Array.Find(persons, fun p -> p.FirstName = nameToFind)
+
+if found <> null then
+    printfn $"{found.FirstName}"
+else 
+    printfn $"{nameToFind} not found."
+
+// Using F#'s Array.tryFind function
+// This does not require a null check or [<AllowNullLiteral>]
+let found2 = 
+    persons |> Array.tryFind (fun p -> p.FirstName = nameToFind)
+
+match found2 with
+| Some firstName ->
+    printfn $"{firstName}"
+| None ->
+    printfn $"{nameToFind} not found."
+
+// The example displays the following output:
+//        Robert not found.
+//        Robert not found.
+// </Snippet5>
@@ -66,21 +66,25 @@
 -   You've forgotten to instantiate a reference type. In the following example, `names` is declared but never instantiated:  
 
      :::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/example1.cs" id="Snippet1":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/example1.fs" id="Snippet1":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/example1.vb" id="Snippet1":::  
 
      Some compilers issue a warning when they compile this code. Others issue an error, and the compilation fails. To address this problem, instantiate the object so that its value is no longer `null`. The following example does this by calling a type's class constructor.  
 
      :::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/example1a.cs" id="Snippet2":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/example1a.fs" id="Snippet2":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/example1a.vb" id="Snippet2":::  
 
 -   You've forgotten to dimension an array before initializing it. In the following example, `values` is declared to be an integer array, but the number of elements that it contains is never specified. The attempt to initialize its values therefore thrown a  <xref:System.NullReferenceException> exception.  
 
      :::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/Array3.cs" id="Snippet10":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/Array3.fs" id="Snippet10":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/Array3.vb" id="Snippet10":::  
 
      You can eliminate the exception by declaring the number of elements in the array before initializing it, as the following example does.  
 
      :::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/Array4.cs" id="Snippet11":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/Array4.fs" id="Snippet11":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/Array4.vb" id="Snippet11":::  
 
      For more information on declaring and initializing arrays, see [Arrays](/dotnet/csharp/programming-guide/arrays/) and [Arrays](/dotnet/visual-basic/programming-guide/language-features/arrays/).  
@@ -90,40 +94,47 @@
      The code in the following example assumes that the <xref:System.Array.Find%2A?displayProperty=nameWithType> method always returns `Person` object whose `FirstName` field matches a search string. Because there is no match, the runtime throws a <xref:System.NullReferenceException> exception.  
 
      :::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/nullreturn2.cs" id="Snippet4":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/nullreturn2.fs" id="Snippet4":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/nullreturn2.vb" id="Snippet4":::  
 
      To address this problem, test the method's return value to ensure that it is not `null` before calling any of its members, as the following example does.  
 
      :::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/nullreturn2a.cs" id="Snippet5":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/nullreturn2a.fs" id="Snippet5":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/nullreturn2a.vb" id="Snippet5":::  
 
 -   You're using an expression (for example, you're chaining a list of methods or properties together) to retrieve a value and, although you're checking whether the value is `null`, the runtime still throws a <xref:System.NullReferenceException> exception. This occurs because one of the intermediate values in the expression returns `null`. As a result, your test for `null` is never evaluated.  
 
      The following example defines a `Pages` object that  caches information about web pages, which are presented by `Page` objects. The `Example.Main` method checks whether the current web page has a non-null title and, if it does, displays the title. Despite this check, however, the method throws a <xref:System.NullReferenceException> exception.  
 
      :::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/Chain1.cs" id="Snippet6":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/Chain1.fs" id="Snippet6":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/Chain1.vb" id="Snippet6":::  
 
      The exception is thrown because `pages.CurrentPage` returns `null` if no page information is  stored in the cache. This exception can be corrected by testing the value of the `CurrentPage` property before retrieving the current `Page` object's `Title` property, as the following example does:  
 
      :::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/Chain2.cs" id="Snippet7":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/Chain2.fs" id="Snippet7":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/Chain2.vb" id="Snippet7":::  
 
 -   You're enumerating the elements of an array that contains reference types, and your attempt to process one of the elements throws a <xref:System.NullReferenceException> exception.  
 
      The following example defines a string array. A `for` statement enumerates the elements in the array and calls each string's <xref:System.String.Trim%2A> method before displaying the string.  
 
      :::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/Array1.cs" id="Snippet8":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/Array1.fs" id="Snippet8":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/Array1.vb" id="Snippet8":::  
 
      This exception occurs if you assume that each element of the array must contain a non-null value, and the value of the array element is in fact `null`. The exception can be eliminated by testing whether the element is `null` before performing any operation on that element, as the following example shows.  
 
      :::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/Array2.cs" id="Snippet9":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/Array2.fs" id="Snippet9":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/Array2.vb" id="Snippet9":::  
 
 -   A <xref:System.NullReferenceException> exception is thrown by a method that is passed `null`. Some methods validate the arguments that are passed to them. If they do and one of the arguments is `null`, the method throws an <xref:System.ArgumentNullException?displayProperty=nameWithType> exception. Otherwise, it throws a <xref:System.NullReferenceException> exception. The following example illustrates this scenario.  
 
      :::code language="csharp" source="~/snippets/csharp/System/NullReferenceException/Overview/example2.cs" id="Snippet3":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NullReferenceException/Overview/example2.fs" id="Snippet3":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/system.nullreferenceexception.class/vb/example2.vb" id="Snippet3":::  
 
      To address this issue, make sure that the argument passed to the method is not `null`, or handle the thrown exception in a `try…catch…finally` block. For more information, see [Exceptions](/dotnet/standard/exceptions/).