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Add F# snippets for System.AppContext (dotnet#7425)
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‎samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/Example4.fs

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module Example4
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// <Snippet4>
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open System
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open System.Reflection
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[<assembly: AssemblyVersion("1.0.0.0")>]
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do ()
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module StringLibrary =
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let substringStartsAt (fullString: string) (substr: string) =
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fullString.IndexOf(substr, StringComparison.Ordinal)
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// </Snippet4>
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// <Snippet5>
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let value = "The archaeologist"
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let substring = "archæ"
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let position =
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StringLibrary.substringStartsAt value substring
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if position >= 0 then
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printfn $"'{substring}' found in '{value}' starting at position {position}"
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else
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printfn $"'{substring}' not found in '{value}'"
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// The example displays the following output:
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// 'archæ' not found in 'The archaeologist'
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// </Snippet5>

‎samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/Example6.fs

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module Example6
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// <Snippet6>
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open System
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open System.Reflection
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[<assembly: AssemblyVersion("2.0.0.0")>]
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do ()
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module StringLibrary =
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let substringStartsAt (fullString: string) (substr: string) =
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fullString.IndexOf(substr, StringComparison.CurrentCulture)
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// </Snippet6>
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// <Snippet7>
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let value = "The archaeologist"
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let substring = "archæ"
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let position =
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StringLibrary.substringStartsAt value substring
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if position >= 0 then
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printfn $"'{substring}' found in '{value}' starting at position {position}"
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else
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printfn $"'{substring}' not found in '{value}'"
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// The example displays the following output:
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// 'archæ' found in 'The archaeologist' starting at position 4
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// </Snippet7>

‎samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/Example8.fs

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module Example8
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// <Snippet8>
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open System
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open System.Reflection
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[<assembly: AssemblyVersion("2.0.0.0")>]
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do ()
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AppContext.SetSwitch("StringLibrary.DoNotUseCultureSensitiveComparison",true)
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module StringLibrary =
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let substringStartsAt (fullString: string) (substr: string) =
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match AppContext.TryGetSwitch "StringLibrary.DoNotUseCultureSensitiveComparison" with
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| true, true -> fullString.IndexOf(substr, StringComparison.Ordinal)
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| _ -> fullString.IndexOf(substr, StringComparison.CurrentCulture)
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// </Snippet8>

‎samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/ForConsumers1.fs

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// <Snippet10>
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module Example
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open System.IO
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open System.Runtime.Versioning
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[<assembly: TargetFramework(".NETFramework,Version=v4.6.2")>]
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do ()
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Path.GetDirectoryName "file://c/temp/dirlist.txt"
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|> printfn "%s"
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// The example displays the following output:
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// Unhandled Exception: System.ArgumentException: The path is not of a legal form.
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// at System.IO.Path.NewNormalizePathLimitedChecks(String path, Int32 maxPathLength, Boolean expandShortPaths)
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// at System.IO.Path.NormalizePath(String path, Boolean fullCheck, Int32 maxPathLength, Boolean expandShortPaths)
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// at System.IO.Path.InternalGetDirectoryName(String path)
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// at <StartupCode$ForConsumers1>.$Example.main@()
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// </Snippet10>

‎samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/ForConsumers1/ForConsumers1.fsproj

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<Project Sdk="Microsoft.NET.Sdk">
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<PropertyGroup>
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<OutputType>Exe</OutputType>
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<TargetFramework>net6.0</TargetFramework>
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</PropertyGroup>
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<ItemGroup>
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<Compile Include="..\ForConsumers1.fs" />
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</ItemGroup>
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</Project>

‎samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/TestValue1.fs

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module TestValue1
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open System
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// <Snippet1>
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AppContext.SetSwitch("Switch.AmazingLib.ThrowOnException", true)
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// </Snippet1>
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// <Snippet2>
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module AmazingLib =
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let performAnOperation () =
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match AppContext.TryGetSwitch "Switch.AmazingLib.ThrowOnException" with
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| false, _ ->
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// This is the case where the switch value was not set by the application.
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// The library can choose to get the value of shouldThrow by other means.
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// If no overrides or default values are specified, the value should be 'false'.
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// A false value implies the latest behavior.
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()
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| true, shouldThrow ->
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// The library can use the value of shouldThrow to throw exceptions or not.
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if shouldThrow then
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// old code
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()
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else
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// new code
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()
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// </Snippet2>
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AmazingLib.performAnOperation ()

‎samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/appcontext.fsproj

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<Project Sdk="Microsoft.NET.Sdk">
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<PropertyGroup>
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<OutputType>Exe</OutputType>
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<TargetFramework>net6.0</TargetFramework>
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</PropertyGroup>
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<ItemGroup>
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<Compile Include="TestValue1.fs" />
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<Compile Include="Example4.fs" />
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<Compile Include="Example6.fs" />
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<Compile Include="Example8.fs" />
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<Compile Include="ForConsumers1.fs" />
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</ItemGroup>
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</Project>

‎xml/System/AppContext.xml

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The following example illustrates the use of the <xref:System.AppContext> class to allow the customer to choose the original behavior of a library method. The following is version 1.0 of a library named `StringLibrary`. It defines a `SubstringStartsAt` method that performs an ordinal comparison to determine the starting index of a substring within a larger string.
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:::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/System.AppContext.Class/cs/Example4.cs" id="Snippet4":::
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:::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/Example4.fs" id="Snippet4":::
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:::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR_System/System.AppContext.Class/vb/Example4.vb" id="Snippet4":::
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The following example then uses the library to find the starting index of the substring "archæ" in "The archaeologist". Because the method performs an ordinal comparison, the substring cannot be found.
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:::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/System.AppContext.Class/cs/Example4.cs" id="Snippet5":::
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:::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/Example4.fs" id="Snippet5":::
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:::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR_System/System.AppContext.Class/vb/Example4.vb" id="Snippet5":::
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Version 2 of the library, however, changes the `SubstringStartsAt` method to use culture-sensitive comparison.
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:::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/System.AppContext.Class/cs/Example6.cs" id="Snippet6":::
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:::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/Example6.fs" id="Snippet6":::
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:::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR_System/System.AppContext.Class/vb/Example6.vb" id="Snippet6":::
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When the app is recompiled to run against the new version of the library, it now reports that the substring "archæ" is found at index 4 in "The archaeologist".
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:::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/System.AppContext.Class/cs/Example6.cs" id="Snippet7":::
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:::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/Example6.fs" id="Snippet7":::
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:::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR_System/System.AppContext.Class/vb/Example6.vb" id="Snippet7":::
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This change can be prevented from breaking the applications that depend on the original behavior by defining an [\<AppContextSwitchOverrides>](/dotnet/framework/configure-apps/file-schema/runtime/appcontextswitchoverrides-element) switch. In this case, the switch is named `StringLibrary.DoNotUseCultureSensitiveComparison`. Its default value, `false`, indicates that the library should perform its version 2.0 culture-sensitive comparison. `true` indicates that the library should perform its version 1.0 ordinal comparison. A slight modification of the previous code allows the library consumer to set the switch to determine the kind of comparison the method performs.
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:::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/System.AppContext.Class/cs/Example8.cs" id="Snippet8":::
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:::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/Example8.fs" id="Snippet8":::
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:::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR_System/System.AppContext.Class/vb/Example8.vb" id="Snippet8":::
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If application can then use the following configuration file to restore the version 1.0 behavior.
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The following is a simple application that passes a file URI to the <xref:System.IO.Path.GetDirectoryName%2A?displayProperty=nameWithType> method. When run under the .NET Framework 4.6, it throws an <xref:System.ArgumentException> because `file://` is no longer a valid part of a file path.
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:::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/System.AppContext.Class/cs/ForConsumers1.cs" id="Snippet10":::
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:::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/ForConsumers1.fs" id="Snippet10":::
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:::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR_System/System.AppContext.Class/vb/ForConsumers1.vb" id="Snippet10":::
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To restore the method's previous behavior and prevent the exception, you can add the `Switch.System.IO.UseLegacyPathHandling` switch to the application configuration file for the example:
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The following line of code sets a switch named `Switch.AmazingLib.ThrowOnException` to `true`, which enables a legacy behavior. The library can then check whether a library consumer has set the value of the switch by calling the <xref:System.AppContext.TryGetSwitch%2A> method.
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:::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/System.AppContext.Class/cs/TestValue1.cs" id="Snippet1":::
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:::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/TestValue1.fs" id="Snippet1":::
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:::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR_System/System.AppContext.Class/vb/TestValue1.vb" id="Snippet1":::
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]]></format>
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The following example determines whether a library consumer has set a switch named `Switch.AmazingLib.ThrowOnException`.
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:::code language="csharp" source="~/samples/snippets/csharp/VS_Snippets_CLR_System/System.AppContext.Class/cs/TestValue1.cs" id="Snippet2":::
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:::code language="fsharp" source="~/samples/snippets/fsharp/VS_Snippets_CLR_System/System.AppContext.Class/fs/TestValue1.fs" id="Snippet2":::
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:::code language="vb" source="~/samples/snippets/visualbasic/VS_Snippets_CLR_System/System.AppContext.Class/vb/TestValue1.vb" id="Snippet2":::
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]]></format>

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