Refactor UTF-7 encoding detection

src/Microsoft.PowerShell.Commands.Utility/commands/utility/CsvCommands.cs

@@ -224,7 +224,7 @@ public Encoding Encoding
 
             set
             {
-                EncodingConversion.WarnIfObsolete(this, value);
+                EncodingConversion.WarnIfUtf7Encoding(this, value);
                 _encoding = value;
             }
         }
@@ -610,7 +610,7 @@ public Encoding Encoding
 
             set
             {
-                EncodingConversion.WarnIfObsolete(this, value);
+                EncodingConversion.WarnIfUtf7Encoding(this, value);
                 _encoding = value;
             }
         }

src/Microsoft.PowerShell.Commands.Utility/commands/utility/FormatAndOutput/format-hex/Format-Hex.cs

@@ -80,7 +80,7 @@ public Encoding Encoding
 
             set
             {
-                EncodingConversion.WarnIfObsolete(this, value);
+                EncodingConversion.WarnIfUtf7Encoding(this, value);
                 _encoding = value;
             }
         }

src/Microsoft.PowerShell.Commands.Utility/commands/utility/FormatAndOutput/out-file/Out-File.cs

@@ -86,7 +86,7 @@ public Encoding Encoding
 
             set
             {
-                EncodingConversion.WarnIfObsolete(this, value);
+                EncodingConversion.WarnIfUtf7Encoding(this, value);
                 _encoding = value;
             }
         }

src/Microsoft.PowerShell.Commands.Utility/commands/utility/ImplicitRemotingCommands.cs

@@ -85,7 +85,7 @@ public Encoding Encoding
 
             set
             {
-                EncodingConversion.WarnIfObsolete(this, value);
+                EncodingConversion.WarnIfUtf7Encoding(this, value);
                 _encoding = value;
             }
         }

src/Microsoft.PowerShell.Commands.Utility/commands/utility/MatchString.cs

@@ -1358,7 +1358,7 @@ public Encoding Encoding
 
             set
             {
-                EncodingConversion.WarnIfObsolete(this, value);
+                EncodingConversion.WarnIfUtf7Encoding(this, value);
                 _encoding = value;
             }
         }

src/Microsoft.PowerShell.Commands.Utility/commands/utility/Send-MailMessage.cs

@@ -72,7 +72,7 @@ public Encoding Encoding
 
             set
             {
-                EncodingConversion.WarnIfObsolete(this, value);
+                EncodingConversion.WarnIfUtf7Encoding(this, value);
                 _encoding = value;
             }
         }

src/Microsoft.PowerShell.Commands.Utility/commands/utility/XmlCommands.cs

@@ -123,7 +123,7 @@ public Encoding Encoding
 
             set
             {
-                EncodingConversion.WarnIfObsolete(this, value);
+                EncodingConversion.WarnIfUtf7Encoding(this, value);
                 _encoding = value;
             }
         }

src/System.Management.Automation/namespaces/FileSystemProvider.cs

@@ -7656,15 +7656,22 @@ public Encoding Encoding
 
             set
             {
-                // Check for UTF-7 by checking for code page 65000
-                // See: https://docs.microsoft.com/en-us/dotnet/core/compatibility/corefx#utf-7-code-paths-are-obsolete
-                if (value != null && value.CodePage == 65000)
-                {
-                    _provider.WriteWarning(PathUtilsStrings.Utf7EncodingObsolete);
-                }
+                WarnIfUtf7Encoding(value);
                 _encoding = value;
                 // If an encoding was explicitly set, be sure to capture that.
                 WasStreamTypeSpecified = true;
+
+                void WarnIfUtf7Encoding(Encoding value)
+                {
+                    const int CodePageUtf7 = 65000;
+
+                    // Use the recommended pattern to check for UTF-7.
+                    // https://docs.microsoft.com/dotnet/core/compatibility/core-libraries/5.0/utf-7-code-paths-obsolete
+                    if (value is { CodePage: CodePageUtf7 })
+                    {
+                        _provider.WriteWarning(PathUtilsStrings.Utf7EncodingObsolete);
+                    }
+                }
             }
         }
 

src/System.Management.Automation/utils/EncodingUtils.cs

@@ -63,11 +63,7 @@ internal static Encoding Convert(Cmdlet cmdlet, string encoding)
             Encoding foundEncoding;
             if (encodingMap.TryGetValue(encoding, out foundEncoding))
             {
-                // Write a warning if using utf7 as it is obsolete in .NET5
-                if (string.Equals(encoding, Utf7, StringComparison.OrdinalIgnoreCase))
-                {
-                    cmdlet.WriteWarning(PathUtilsStrings.Utf7EncodingObsolete);
-                }
+                WarnIfUtf7Encoding(cmdlet, foundEncoding);
 
                 return foundEncoding;
             }
@@ -89,15 +85,17 @@ internal static Encoding Convert(Cmdlet cmdlet, string encoding)
         }
 
         /// <summary>
-        /// Warn if the encoding has been designated as obsolete.
+        /// Warn if the encoding is obsolete UTF-7 encoding.
         /// </summary>
         /// <param name="cmdlet">A cmdlet instance which is used to emit the warning.</param>
         /// <param name="encoding">The encoding to check for obsolescence.</param>
-        internal static void WarnIfObsolete(Cmdlet cmdlet, Encoding encoding)
+        internal static void WarnIfUtf7Encoding(Cmdlet cmdlet, Encoding encoding)
         {
-            // Check for UTF-7 by checking for code page 65000
-            // See: https://docs.microsoft.com/en-us/dotnet/core/compatibility/corefx#utf-7-code-paths-are-obsolete
-            if (encoding != null && encoding.CodePage == 65000)
+            const int CodePageUtf7 = 65000;
+
+            // Use the recommended pattern to check for UTF-7.
+            // https://docs.microsoft.com/dotnet/core/compatibility/core-libraries/5.0/utf-7-code-paths-obsolete
+            if (encoding is { CodePage: CodePageUtf7 })
             {
                 cmdlet.WriteWarning(PathUtilsStrings.Utf7EncodingObsolete);
             }