Nullable F# snippets (#7795)

Author: albert-du

snippets/fsharp/System/Nullable/GetUnderlyingType/fs.fsproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="gut.fs" />
+  </ItemGroup>
+</Project>

snippets/fsharp/System/Nullable/GetUnderlyingType/gut.fs

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+//<snippet1>
+// This code example demonstrates the
+// Nullable.GetUnderlyingType() method.
+open System
+
+// Declare a type named Example.
+// The MyMethod member of Example returns a Nullable of Int32.
+
+type Example() =
+    member _.MyMethod() =
+        Nullable 0
+
+(*
+   Use reflection to obtain a Type object for the Example type.
+   Use the Type object to obtain a MethodInfo object for the MyMethod method.
+   Use the MethodInfo object to obtain the type of the return value of
+     MyMethod, which is Nullable of Int32.
+   Use the GetUnderlyingType method to obtain the type argument of the
+     return value type, which is Int32.
+*)
+let t = typeof<Example>
+let mi = t.GetMethod "MyMethod"
+let retval = mi.ReturnType
+printfn $"Return value type ... {retval}"
+let answer = Nullable.GetUnderlyingType retval
+printfn $"Underlying type ..... {answer}"
+
+// This code example produces the following results:
+//     Return value type ... System.Nullable`1[System.Int32]
+//     Underlying type ..... System.Int32
+//</snippet1>

snippets/fsharp/System/NullableT/Equals/eq.fs

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+//<snippet1>
+// This code example demonstrates the Nullable<T>.Equals
+// methods.
+open System
+
+let nullInt1 = Nullable 100
+let nullInt2 = Nullable 200
+
+// Determine if two nullable of System.Int32 values are equal.
+// The nullable objects have different values.
+printf "1) nullInt1 and nullInt2 "
+if nullInt1.Equals nullInt1 then
+    printf "are"
+else
+    printf "are not"
+printfn " equal."
+
+// Determine if a nullable of System.Int32 and an object
+// are equal. The object contains the boxed value of the
+// nullable object.
+
+let myObj = box nullInt1
+printf "2) nullInt1 and myObj "
+if nullInt1.Equals myObj then
+    printf "are"
+else
+    printf "are not"
+printfn " equal."
+
+// This code example produces the following results:
+//     1) nullInt1 and nullInt2 are not equal.
+//     2) nullInt1 and myObj are equal.
+//</snippet1>

snippets/fsharp/System/NullableT/Equals/fs.fsproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="eq.fs" />
+  </ItemGroup>
+</Project>

snippets/fsharp/System/NullableT/GetValueOrDefault/fs.fsproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="gvod.fs" />
+  </ItemGroup>
+</Project>

snippets/fsharp/System/NullableT/GetValueOrDefault/gvod.fs

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+//<snippet1>
+// This code example demonstrates the
+// Nullable<T>.GetValueOrDefault methods.
+open System
+
+// Display the values of two nullable of System.Single structures.
+// The printfn string interpolation automatically calls the ToString methods of
+// each input argument to display its values. If no value is defined for a
+// nullable type, the ToString method for that argument returns the empty
+// string ("").
+let display title dspMySingle dspYourSingle =
+    printfn $"{title}) mySingle = [{dspMySingle}], yourSingle = [{dspYourSingle}]"
+
+let mySingle = Nullable 12.34f
+let yourSingle = Nullable -1f
+
+[<EntryPoint>]
+let main _ =
+    printfn "*** Display a value or the default value ***\n"
+    // Display the values of mySingle and yourSingle.
+
+    display "A1" mySingle yourSingle
+
+    // Shadow the value of mySingle to yourSingle, then display the values
+    // of mySingle and yourSingle. The yourSingle variable is assigned the
+    // value 12.34 because mySingle has a value.
+
+    let yourSingle = mySingle.GetValueOrDefault()
+    display "A2" mySingle yourSingle
+
+    // Shadow null (Nothing in Visual Basic) to mySingle, which means no value is
+    // defined for mySingle. Then assign the value of mySingle to yourSingle and
+    // display the values of both variables. The default value of all binary zeroes
+    // is assigned to yourSingle because mySingle has no value.
+
+    let mySingle = Nullable()
+    let yourSingle = mySingle.GetValueOrDefault()
+    display "A3" mySingle yourSingle
+
+    // Shadow the original values of mySingle and yourSingle.
+    let mySingle = Nullable 12.34f
+    let yourSingle = Nullable -1.0f
+
+    printf "\n*** Display a value or the "
+    printfn "specified default value ***\n"
+
+    // Display the values of mySingle and yourSingle.
+    display "B1" mySingle yourSingle
+
+    // Shadow the value of mySingle to yourSingle, then display the values
+    // of mySingle and yourSingle. The yourSingle variable is assigned the
+    // value 12.34 because mySingle has a value.
+
+    let yourSingle = mySingle.GetValueOrDefault -222.22f
+    display "B2" mySingle yourSingle
+
+    // Shadow null (Nothing in Visual Basic) to mySingle, which means no value is
+    // defined for mySingle. Then shadow the value of mySingle to yourSingle and
+    // display the values of both variables. The specified default value of -333.33
+    // is assigned to yourSingle because mySingle has no value.
+
+    let mySingle = Nullable()
+    let yourSingle = mySingle.GetValueOrDefault -333.33f
+    display "B3" mySingle yourSingle
+    0
+
+// This code example produces the following results:
+//     A1) mySingle = [12.34], yourSingle = [-1]
+//     A2) mySingle = [12.34], yourSingle = [12.34]
+//     A3) mySingle = [], yourSingle = [0]
+//     
+//     *** Display a value or the specified default value ***
+//     
+//     B1) mySingle = [12.34], yourSingle = [-1]
+//     B2) mySingle = [12.34], yourSingle = [12.34]
+//     B3) mySingle = [], yourSingle = [-333.33]
+//</snippet1>

snippets/fsharp/System/NullableT/HasValue/fs.fsproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="hasvalue2.fs" />
+  </ItemGroup>
+</Project>

snippets/fsharp/System/NullableT/HasValue/hasvalue2.fs

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+// <Snippet1>
+open System
+
+let n1 = Nullable 10
+let n2 = Nullable()
+let mutable n3 = Nullable 20
+n3 <- Nullable()
+let items = [| n1; n2; n3 |]
+
+for item in items do
+    printfn $"Has a value: {item.HasValue}"
+    if item.HasValue then
+        printfn $"Type: {item.GetType().Name}"
+        printfn $"Value: {item.Value}"
+    else
+        printfn $"Null: {item = Nullable()}"
+        printfn $"Default Value: {item.GetValueOrDefault()}"
+    printfn ""
+// The example displays the following output:
+//       Has a value: True
+//       Type: Int32
+//       Value: 10
+//
+//       Has a value: False
+//       Null: True
+//       Default Value: 0
+//
+//       Has a value: False
+//       Null: True
+//       Default Value: 0
+// </Snippet1>

snippets/fsharp/System/NullableT/Overview/fs.fsproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="tarow.fs" />
+  </ItemGroup>
+</Project>

snippets/fsharp/System/NullableT/Overview/tarow.fs

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+//<snippet1>
+open System
+
+// Define the "titleAuthor" table of the Microsoft "pubs" database.
+
+type titleAuthor =
+  struct
+    // Author ID format ###-##-####
+    val mutable au_id: string
+    // Title ID format AA####
+    val mutable title_id: string
+    // Author ORD is nullable.
+    val mutable au_ord: Nullable<int16>
+    // Royalty Percent is nullable.
+    val mutable royaltyper: Nullable<int>
+  end
+
+// Display the values of the titleAuthor array elements.
+let display dspTitle (dspAllTitleAuthors: #seq<titleAuthor>) =
+    printfn $"*** {dspTitle} ***"
+    for dspTA in dspAllTitleAuthors do
+        printfn $"Author ID ... {dspTA.au_id}"
+        printfn $"Title ID .... {dspTA.title_id}"
+        printfn $"Author ORD .. {dspTA.au_ord.GetValueOrDefault -1s}"
+        printfn $"Royalty %% ... {dspTA.royaltyper.GetValueOrDefault -1}\n"
+
+// Declare and initialize the titleAuthor array.
+let ta = Array.zeroCreate<titleAuthor> 3
+ta[0].au_id <- "712-32-1176"
+ta[0].title_id <- "PS3333"
+ta[0].au_ord <- Nullable 1s
+ta[0].royaltyper <- Nullable 100
+
+ta[1].au_id <- "213-46-8915"
+ta[1].title_id <- "BU1032"
+ta[1].au_ord <- Nullable()
+ta[1].royaltyper <- Nullable()
+
+ta[2].au_id <- "672-71-3249"
+ta[2].title_id <- "TC7777"
+ta[2].au_ord <- Nullable()
+ta[2].royaltyper <- Nullable 40
+
+// Display the values of the titleAuthor array elements, and
+// display a legend.
+display "Title Authors Table" ta
+printfn "Legend:"
+printfn "An Author ORD of -1 means no value is defined."
+printfn "A Royalty %% of 0 means no value is defined."
+
+// The example displays the following output:
+//     *** Title Authors Table ***
+//     Author ID ... 712-32-1176
+//     Title ID .... PS3333
+//     Author ORD .. 1
+//     Royalty % ... 100
+//
+//     Author ID ... 213-46-8915
+//     Title ID .... BU1032
+//     Author ORD .. -1
+//     Royalty % ... 0
+//
+//     Author ID ... 672-71-3249
+//     Title ID .... TC7777
+//     Author ORD .. -1
+//     Royalty % ... 40
+//
+//     Legend:
+//     An Author ORD of -1 means no value is defined.
+//     A Royalty % of 0 means no value is defined.
+//</snippet1>