dotnet · BillWagner · Mar 7, 2022 · Mar 5, 2022
@@ -0,0 +1,34 @@
+module BadState
+
+// <Snippet1>
+open System.IO
+open System.Text
+
+let main = task {
+    let enc = Encoding.Unicode
+    let value = "This is a string to persist."
+    let bytes  = enc.GetBytes value
+
+    let fs = new FileStream(@".\TestFile.dat", FileMode.Open, FileAccess.Read)
+    let t = fs.WriteAsync(enc.GetPreamble(), 0, enc.GetPreamble().Length)
+    let t2 = t.ContinueWith(fun a -> fs.WriteAsync(bytes, 0, bytes.Length))
+    let! _ = t2
+    fs.Close()
+}
+main.Wait()
+
+// The example displays the following output:
+//    Unhandled Exception: System.NotSupportedException: Stream does not support writing.
+//       at System.IO.Stream.BeginWriteInternal(Byte[] buffer, Int32 offset, Int32 count, AsyncCallback callback, Object state
+//    , Boolean serializeAsynchronously)
+//       at System.IO.FileStream.BeginWrite(Byte[] array, Int32 offset, Int32 numBytes, AsyncCallback userCallback, Object sta
+//    teObject)
+//       at System.IO.Stream.<>c.<BeginEndWriteAsync>b__53_0(Stream stream, ReadWriteParameters args, AsyncCallback callback,
+//    Object state)
+//       at System.Threading.Tasks.TaskFactory`1.FromAsyncTrim[TInstance,TArgs](TInstance thisRef, TArgs args, Func`5 beginMet
+//    hod, Func`3 endMethod)
+//       at System.IO.Stream.BeginEndWriteAsync(Byte[] buffer, Int32 offset, Int32 count)
+//       at System.IO.FileStream.WriteAsync(Byte[] buffer, Int32 offset, Int32 count, CancellationToken cancellationToken)
+//       at System.IO.Stream.WriteAsync(Byte[] buffer, Int32 offset, Int32 count)
+//       at <StartupCode:fs>.main()
+// </Snippet1>
@@ -0,0 +1,55 @@
+module TestProp1
+
+// <Snippet3>
+open System
+open System.IO
+
+module FileUtilities =
+    type EncodingType =
+        | None = 0
+        | Unknown = -1
+        | Utf8 = 1
+        | Utf16 = 2
+        | Utf32 = 3
+
+    let getEncodingType (fs: FileStream) = 
+        task {
+            let bytes = Array.zeroCreate<byte> 4
+            let! bytesRead = fs.ReadAsync(bytes, 0, 4)
+            if bytesRead < 2 then
+                return EncodingType.None
+
+            elif bytesRead >= 3 && bytes[0] = 0xEFuy && bytes[1] = 0xBBuy && bytes[2] = 0xBFuy then
+                return EncodingType.Utf8
+            else
+                let value = BitConverter.ToUInt32(bytes, 0)
+                if bytesRead = 4 && (value = 0x0000FEFFu || value = 0xFEFF0000u) then
+                    return EncodingType.Utf32
+                else
+                    let value16 = BitConverter.ToUInt16(bytes, 0)
+                    if value16 = 0xFEFFus || value16 = 0xFFFEus then
+                        return EncodingType.Utf16
+                    else
+                        return EncodingType.Unknown
+        }
+
+let main _ = 
+    task {
+        let name = @".\TestFile.dat"
+        let fs = new FileStream(name, FileMode.Create, FileAccess.Write)
+        let! et = FileUtilities.getEncodingType fs
+        printfn $"Filename: {name}, Encoding: {et}"
+    }
+
+// The example displays the following output:
+//    Unhandled Exception: System.NotSupportedException: Stream does not support reading.
+//       at System.IO.FileStream.BeginRead(Byte[] array, Int32 offset, Int32 numBytes, AsyncCallback callback, Object state)
+//       at System.IO.Stream.<>c.<BeginEndReadAsync>b__46_0(Stream stream, ReadWriteParameters args, AsyncCallback callback, Object state)
+//       at System.Threading.Tasks.TaskFactory`1.FromAsyncTrim[TInstance, TArgs](TInstance thisRef, TArgs args, Func`5 beginMethod, Func`3 endMethod)
+//       at System.IO.Stream.BeginEndReadAsync(Byte[] buffer, Int32 offset, Int32 count)
+//       at System.IO.FileStream.ReadAsync(Byte[] buffer, Int32 offset, Int32 count, CancellationToken cancellationToken)
+//       at System.IO.Stream.ReadAsync(Byte[] buffer, Int32 offset, Int32 count)
+//       at FileUtilities.GetEncodingType(FileStream fs)
+//       at Example.Main()
+//       at Example.<Main>()
+// </Snippet3>
@@ -0,0 +1,48 @@
+module TestProp2
+
+open System
+open System.IO
+
+module FileUtilities =
+    type EncodingType =
+        | None = 0
+        | Unknown = -1
+        | Utf8 = 1
+        | Utf16 = 2
+        | Utf32 = 3
+
+    // <Snippet4>
+    let getEncodingType (fs: FileStream) = 
+        task {
+            if not fs.CanRead then
+                return EncodingType.Unknown
+            else
+                let bytes = Array.zeroCreate<byte> 4
+                let! bytesRead = fs.ReadAsync(bytes, 0, 4)
+                if bytesRead < 2 then
+                    return EncodingType.None
+
+                elif bytesRead >= 3 && bytes[0] = 0xEFuy && bytes[1] = 0xBBuy && bytes[2] = 0xBFuy then
+                    return EncodingType.Utf8
+                else
+                    let value = BitConverter.ToUInt32(bytes, 0)
+                    if bytesRead = 4 && (value = 0x0000FEFFu || value = 0xFEFF0000u) then
+                        return EncodingType.Utf32
+                    else
+                        let value16 = BitConverter.ToUInt16(bytes, 0)
+                        if value16 = 0xFEFFus || value16 = 0xFFFEus then
+                            return EncodingType.Utf16
+                        else
+                            return EncodingType.Unknown
+        }
+    // The example displays the following output:
+    //       Filename: .\TestFile.dat, Encoding: Unknown
+    // </Snippet4>
+
+let main _ = 
+    task {
+        let name = @".\TestFile.dat"
+        let fs = new FileStream(name, FileMode.Create, FileAccess.Write)
+        let! et = FileUtilities.getEncodingType fs
+        printfn $"Filename: {name}, Encoding: {et}"
+    }
@@ -0,0 +1,12 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="BadState1.fs" />
+    <Compile Include="TestProp1.fs" />
+    <Compile Include="TestProp2.fs" />
+  </ItemGroup>
+</Project>
@@ -98,6 +98,7 @@
 -   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.  
 
      :::code language="csharp" source="~/snippets/csharp/System/NotSupportedException/Overview/BadState1.cs" id="Snippet1":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NotSupportedException/Overview/BadState1.fs" id="Snippet1":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/System.NotSupportedException/vb/BadState1.vb" id="Snippet1":::  
 
      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.  
@@ -107,11 +108,13 @@
      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.  
 
      :::code language="csharp" source="~/snippets/csharp/System/NotSupportedException/Overview/TestProp1.cs" id="Snippet3":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NotSupportedException/Overview/TestProp1.fs" id="Snippet3":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/System.NotSupportedException/vb/TestProp1.vb" id="Snippet3":::  
 
      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.  
 
      :::code language="csharp" source="~/snippets/csharp/System/NotSupportedException/Overview/TestProp2.cs" id="Snippet4":::
+     :::code language="fsharp" source="~/snippets/fsharp/System/NotSupportedException/Overview/TestProp2.fs" id="Snippet4":::
      :::code language="vb" source="~/snippets/visualbasic/VS_Snippets_CLR_System/System.NotSupportedException/vb/TestProp2.vb" id="Snippet4":::  
 
 ## Related exception types