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NotSupportedException F# snippets (#7801)
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‎snippets/fsharp/System/NotSupportedException/Overview/BadState1.fs‎

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module BadState
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// <Snippet1>
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open System.IO
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open System.Text
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let main = task {
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let enc = Encoding.Unicode
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let value = "This is a string to persist."
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let bytes = enc.GetBytes value
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let fs = new FileStream(@".\TestFile.dat", FileMode.Open, FileAccess.Read)
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let t = fs.WriteAsync(enc.GetPreamble(), 0, enc.GetPreamble().Length)
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let t2 = t.ContinueWith(fun a -> fs.WriteAsync(bytes, 0, bytes.Length))
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let! _ = t2
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fs.Close()
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}
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main.Wait()
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// The example displays the following output:
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// Unhandled Exception: System.NotSupportedException: Stream does not support writing.
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// at System.IO.Stream.BeginWriteInternal(Byte[] buffer, Int32 offset, Int32 count, AsyncCallback callback, Object state
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// , Boolean serializeAsynchronously)
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// at System.IO.FileStream.BeginWrite(Byte[] array, Int32 offset, Int32 numBytes, AsyncCallback userCallback, Object sta
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// teObject)
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// at System.IO.Stream.<>c.<BeginEndWriteAsync>b__53_0(Stream stream, ReadWriteParameters args, AsyncCallback callback,
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// Object state)
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// at System.Threading.Tasks.TaskFactory`1.FromAsyncTrim[TInstance,TArgs](TInstance thisRef, TArgs args, Func`5 beginMet
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// hod, Func`3 endMethod)
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// at System.IO.Stream.BeginEndWriteAsync(Byte[] buffer, Int32 offset, Int32 count)
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// at System.IO.FileStream.WriteAsync(Byte[] buffer, Int32 offset, Int32 count, CancellationToken cancellationToken)
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// at System.IO.Stream.WriteAsync(Byte[] buffer, Int32 offset, Int32 count)
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// at <StartupCode:fs>.main()
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// </Snippet1>

‎snippets/fsharp/System/NotSupportedException/Overview/TestProp1.fs‎

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module TestProp1
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// <Snippet3>
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open System
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open System.IO
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module FileUtilities =
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type EncodingType =
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| None = 0
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| Unknown = -1
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| Utf8 = 1
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| Utf16 = 2
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| Utf32 = 3
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let getEncodingType (fs: FileStream) =
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task {
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let bytes = Array.zeroCreate<byte> 4
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let! bytesRead = fs.ReadAsync(bytes, 0, 4)
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if bytesRead < 2 then
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return EncodingType.None
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elif bytesRead >= 3 && bytes[0] = 0xEFuy && bytes[1] = 0xBBuy && bytes[2] = 0xBFuy then
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return EncodingType.Utf8
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else
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let value = BitConverter.ToUInt32(bytes, 0)
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if bytesRead = 4 && (value = 0x0000FEFFu || value = 0xFEFF0000u) then
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return EncodingType.Utf32
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else
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let value16 = BitConverter.ToUInt16(bytes, 0)
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if value16 = 0xFEFFus || value16 = 0xFFFEus then
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return EncodingType.Utf16
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else
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return EncodingType.Unknown
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}
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let main _ =
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task {
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let name = @".\TestFile.dat"
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let fs = new FileStream(name, FileMode.Create, FileAccess.Write)
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let! et = FileUtilities.getEncodingType fs
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printfn $"Filename: {name}, Encoding: {et}"
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}
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// The example displays the following output:
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// Unhandled Exception: System.NotSupportedException: Stream does not support reading.
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// at System.IO.FileStream.BeginRead(Byte[] array, Int32 offset, Int32 numBytes, AsyncCallback callback, Object state)
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// at System.IO.Stream.<>c.<BeginEndReadAsync>b__46_0(Stream stream, ReadWriteParameters args, AsyncCallback callback, Object state)
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// at System.Threading.Tasks.TaskFactory`1.FromAsyncTrim[TInstance, TArgs](TInstance thisRef, TArgs args, Func`5 beginMethod, Func`3 endMethod)
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// at System.IO.Stream.BeginEndReadAsync(Byte[] buffer, Int32 offset, Int32 count)
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// at System.IO.FileStream.ReadAsync(Byte[] buffer, Int32 offset, Int32 count, CancellationToken cancellationToken)
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// at System.IO.Stream.ReadAsync(Byte[] buffer, Int32 offset, Int32 count)
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// at FileUtilities.GetEncodingType(FileStream fs)
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// at Example.Main()
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// at Example.<Main>()
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// </Snippet3>

‎snippets/fsharp/System/NotSupportedException/Overview/TestProp2.fs‎

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module TestProp2
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open System
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open System.IO
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module FileUtilities =
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type EncodingType =
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| None = 0
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| Unknown = -1
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| Utf8 = 1
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| Utf16 = 2
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| Utf32 = 3
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// <Snippet4>
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let getEncodingType (fs: FileStream) =
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task {
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if not fs.CanRead then
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return EncodingType.Unknown
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else
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let bytes = Array.zeroCreate<byte> 4
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let! bytesRead = fs.ReadAsync(bytes, 0, 4)
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if bytesRead < 2 then
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return EncodingType.None
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elif bytesRead >= 3 && bytes[0] = 0xEFuy && bytes[1] = 0xBBuy && bytes[2] = 0xBFuy then
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return EncodingType.Utf8
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else
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let value = BitConverter.ToUInt32(bytes, 0)
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if bytesRead = 4 && (value = 0x0000FEFFu || value = 0xFEFF0000u) then
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return EncodingType.Utf32
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else
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let value16 = BitConverter.ToUInt16(bytes, 0)
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if value16 = 0xFEFFus || value16 = 0xFFFEus then
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return EncodingType.Utf16
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else
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return EncodingType.Unknown
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}
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// The example displays the following output:
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// Filename: .\TestFile.dat, Encoding: Unknown
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// </Snippet4>
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let main _ =
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task {
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let name = @".\TestFile.dat"
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let fs = new FileStream(name, FileMode.Create, FileAccess.Write)
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let! et = FileUtilities.getEncodingType fs
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printfn $"Filename: {name}, Encoding: {et}"
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}

‎snippets/fsharp/System/NotSupportedException/Overview/fs.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="BadState1.fs" />
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<Compile Include="TestProp1.fs" />
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<Compile Include="TestProp2.fs" />
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</ItemGroup>
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</Project>

‎xml/System/NotSupportedException.xml‎

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- You know the state of the object in advance (usually because your code has instantiated it), but the object is mis-configured. The following example illustrates this issue. It creates a read-only <xref:System.IO.FileStream> object and then attempts to write to it.
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:::code language="csharp" source="~/snippets/csharp/System/NotSupportedException/Overview/BadState1.cs" id="Snippet1":::
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:::code language="fsharp" source="~/snippets/fsharp/System/NotSupportedException/Overview/BadState1.fs" id="Snippet1":::
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:::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/System.NotSupportedException/vb/BadState1.vb" id="Snippet1":::
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You can eliminate the exception by ensuring that the instantiated object supports the functionality you intend. The following example addresses the problem of the read-only <xref:System.IO.FileStream> object by providing the correct arguments to the <xref:System.IO.FileStream.%23ctor%28System.String%2CSystem.IO.FileMode%2CSystem.IO.FileAccess%29?displayProperty=nameWithType> constructor.
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The following example defines a `DetectEncoding` method that throws a <xref:System.NotSupportedException> exception when it attempts to read from the beginning of a stream that does not support read access.
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:::code language="csharp" source="~/snippets/csharp/System/NotSupportedException/Overview/TestProp1.cs" id="Snippet3":::
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:::code language="fsharp" source="~/snippets/fsharp/System/NotSupportedException/Overview/TestProp1.fs" id="Snippet3":::
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:::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/System.NotSupportedException/vb/TestProp1.vb" id="Snippet3":::
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You can eliminate the exception by examining the value of the <xref:System.IO.FileStream.CanRead%2A?displayProperty=nameWithType> property and exiting the method if the stream is read-only.
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:::code language="csharp" source="~/snippets/csharp/System/NotSupportedException/Overview/TestProp2.cs" id="Snippet4":::
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:::code language="fsharp" source="~/snippets/fsharp/System/NotSupportedException/Overview/TestProp2.fs" id="Snippet4":::
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:::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/System.NotSupportedException/vb/TestProp2.vb" id="Snippet4":::
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## Related exception types

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