Register encoding provider when GetEncoding throws NotSupportedException

src/Compilers/CSharp/Test/CommandLine/CommandLineTests.cs

@@ -5791,6 +5791,10 @@ public void Codepage()
             parsedArgs.Errors.Verify();
             Assert.Equal("Unicode (UTF-8)", parsedArgs.Encoding.EncodingName);
 
+            parsedArgs = DefaultParse(new[] { "/CodePage:1252", "a.cs" }, WorkingDirectory);
+            parsedArgs.Errors.Verify();
+            Assert.Equal("Western European (Windows)", parsedArgs.Encoding.EncodingName);
+
             //  error
             parsedArgs = DefaultParse(new[] { "/codepage:0", "a.cs" }, WorkingDirectory);
             parsedArgs.Errors.Verify(Diagnostic(ErrorCode.FTL_BadCodepage).WithArguments("0"));

src/Compilers/Core/Portable/CommandLine/CommandLineParser.cs

@@ -24,6 +24,7 @@ public abstract class CommandLineParser
         internal readonly bool IsScriptCommandLineParser;
         private static readonly char[] s_searchPatternTrimChars = new char[] { '\t', '\n', '\v', '\f', '\r', ' ', '\x0085', '\x00a0' };
         internal const string ErrorLogOptionFormat = "<file>[,version={1|1.0|2|2.1}]";
+        private static bool s_registeredEncodingProvider = CodePagesEncodingProvider.Instance == null;
 
         internal CommandLineParser(CommonMessageProvider messageProvider, bool isScriptCommandLineParser)
         {
@@ -1220,10 +1221,49 @@ internal IEnumerable<CommandLineSourceFile> ParseRecurseArgument(string arg, str
                 && long.TryParse(arg, NumberStyles.None, CultureInfo.InvariantCulture, out long codepage)
                 && (codepage > 0))
             {
+try_again:
                 try
                 {
                     return Encoding.GetEncoding((int)codepage);
                 }
+                catch (NotSupportedException) when (!s_registeredEncodingProvider)
+                {
+                    // From documentation:
+                    //   - 'GetEncoding' throws NotSupportedException when codepage is not supported by the underlying platform.
+                    //   - 'EncodingProvider.Instance' gets an encoding provider for code pages supported
+                    //     in the desktop .NET Framework but not by the current underlying platform.
+                    //   - 'Encoding.RegisterProvider' makes character encodings available on a platform that does not otherwise support them.
+                    //      * Once the encoding provider is registered, the encodings that it supports can be retrieved by calling any
+                    //        Encoding.GetEncoding overload.
+                    //      * Registering an encoding provider by using the 'RegisterProvider' method also affects the behavior of
+                    //        GetEncoding(Int32) when passed an argument of 0.
+                    //      * If multiple providers are registered, GetEncoding(Int32) attempts to retrieve the encoding from the most recently
+                    //        registered provider first.
+                    //      * If the 'RegisterProvider' method is called to register multiple providers that handle the same encoding,
+                    //        the last registered provider is the used for all encoding and decoding operations. Any previously registered providers are ignored.
+                    //      * If the same encoding provider is used in multiple calls to the 'RegisterProvider' method,
+                    //        only the first method call registers the provider. Subsequent calls are ignored.
+                    //      
+                    //  Given all that:
+                    //   - We don't call 'Encoding.RegisterProvider' unconditionally to avoid changing environment
+                    //     that is already configured to support the requested codepage. We call it only when we encounter
+                    //     a 'NotSupportedException'.
+                    //   - We also do not attempt to call 'Encoding.RegisterProvider' more than once.
+                    try
+                    {
+                        // Ignore any exceptions from an attempt to register the provider.
+                        Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
+                    }
+                    catch
+                    {
+                    }
+
+                    s_registeredEncodingProvider = true;
+
+                    // Try to get the encoding again after attempting to register the provider.
+                    // Since we set `s_registeredEncodingProvider` to true, we won't get here again.
+                    goto try_again;
+                }
                 catch (Exception)
                 {
                     return null;

src/Compilers/VisualBasic/Test/CommandLine/CommandLineTests.vb

@@ -1863,6 +1863,10 @@ End Module").Path
             parsedArgs.Errors.Verify()
             Assert.Equal("Unicode (UTF-8)", parsedArgs.Encoding.EncodingName)
 
+            parsedArgs = DefaultParse({"/CodePage:1252", "a.vb"}, _baseDirectory)
+            parsedArgs.Errors.Verify()
+            Assert.Equal("Western European (Windows)", parsedArgs.Encoding.EncodingName)
+
             ' errors 
             parsedArgs = DefaultParse({"/codepage:0", "a.vb"}, _baseDirectory)
             parsedArgs.Errors.Verify(Diagnostic(ERRID.ERR_BadCodepage).WithArguments("0"))