Obsolete Encoding.UTF7 property and UTF7Encoding ctors (#37535)

- Disallow Encoding.GetEncoding("utf-7", ...) from returning an Encoding instance
- Minor code cleanup by removing unused ifdefs
- A compat switch is available to re-enable Encoding.GetEncoding("utf-7", ...)

Author: GrabYourPitchforks

docs/project/list-of-obsoletions.md

@@ -13,5 +13,5 @@ Currently the identifiers `MSLIB0001` through `MSLIB0999` are carved out for obs
 
 | Diagnostic ID    | Description |
 | :--------------- | :---------- |
-|  __`MSLIB0001`__ | (Reserved for `Encoding.UTF7`.) |
+|  __`MSLIB0001`__ | The UTF-7 encoding is insecure and should not be used. Consider using UTF-8 instead. |
 |  __`MSLIB0002`__ | `PrincipalPermissionAttribute` is not honored by the runtime and must not be used. |

src/libraries/System.IO.Compression/src/System/IO/Compression/ZipArchive.cs

@@ -369,14 +369,7 @@ private set
 
                 if (value != null &&
                         (value.Equals(Encoding.BigEndianUnicode)
-                        || value.Equals(Encoding.Unicode)
-#if FEATURE_UTF32
-                        || value.Equals(Encoding.UTF32)
-#endif // FEATURE_UTF32
-#if FEATURE_UTF7
-                        || value.Equals(Encoding.UTF7)
-#endif // FEATURE_UTF7
-                        ))
+                        || value.Equals(Encoding.Unicode)))
                 {
                     throw new ArgumentException(SR.EntryNameEncodingNotSupported, nameof(EntryNameEncoding));
                 }

src/libraries/System.IO.Ports/tests/SerialPort/Encoding.cs

@@ -111,7 +111,7 @@ public void Encoding_ISCIIAssemese()
         public void Encoding_UTF7()
         {
             Debug.WriteLine("Verifying UTF7Encoding Encoding");
-            VerifyException(Encoding.UTF7, ThrowAt.Set, typeof(ArgumentException));
+            VerifyException(LegacyUTF7Encoding, ThrowAt.Set, typeof(ArgumentException));
         }
 
         [ConditionalFact(nameof(HasOneSerialPort))]

src/libraries/System.IO.Ports/tests/SerialPort/ReadTo.cs

@@ -691,7 +691,7 @@ private void PerformReadOnCom1FromCom2(SerialPort com1, SerialPort com2, string
             int totalCharsRead;
             int lastIndexOfNewLine = -newLineStringLength;
             string expectedString;
-            bool isUTF7Encoding = com1.Encoding.EncodingName == Encoding.UTF7.EncodingName;
+            bool isUTF7Encoding = IsUTF7Encoding(com1.Encoding);
 
             char[] charsToWrite = strToWrite.ToCharArray();
             byte[] bytesToWrite = com1.Encoding.GetBytes(charsToWrite);
@@ -805,7 +805,7 @@ private void VerifyReadToWithWriteLine(Encoding encoding, string newLine)
                 Random rndGen = new Random(-55);
                 StringBuilder strBldrToWrite;
                 string strExpected;
-                bool isUTF7Encoding = encoding.EncodingName == Encoding.UTF7.EncodingName;
+                bool isUTF7Encoding = IsUTF7Encoding(encoding);
 
                 Debug.WriteLine("Verifying ReadTo with WriteLine encoding={0}, newLine={1}", encoding, newLine);
 
@@ -861,10 +861,10 @@ private string GenRandomNewLine(bool validAscii)
 
         private int GetUTF7EncodingBytes(char[] chars, int index, int count)
         {
-            byte[] bytes = Encoding.UTF7.GetBytes(chars, index, count);
+            byte[] bytes = LegacyUTF7Encoding.GetBytes(chars, index, count);
             int byteCount = bytes.Length;
 
-            while (Encoding.UTF7.GetCharCount(bytes, 0, byteCount) == count)
+            while (LegacyUTF7Encoding.GetCharCount(bytes, 0, byteCount) == count)
             {
                 --byteCount;
             }

src/libraries/System.IO.Ports/tests/SerialPort/Read_char_int_int.cs

@@ -950,18 +950,9 @@ private void VerifyBytesFollowedByChars(Encoding encoding)
                     Fail("ERROR!!!: Expected to read {0} chars actually read {1}", xmitCharBuffer.Length, numRead);
                 }
 
-                if (encoding.EncodingName == Encoding.UTF7.EncodingName)
+                if (IsUTF7Encoding(encoding))
                 {
-                    //If UTF7Encoding is being used we might leave a - in the stream
-                    if (com1.BytesToRead == xmitByteBuffer.Length + 1)
-                    {
-                        int byteRead;
-
-                        if ('-' != (char)(byteRead = com1.ReadByte()))
-                        {
-                            Fail("Err_29282naie Expected '-' to be left in the stream with UTF7Encoding and read {0}", byteRead);
-                        }
-                    }
+                    Fail("UTF-7 encoding not expected to be passed to this test.");
                 }
 
                 if (xmitByteBuffer.Length != (numRead = com1.Read(rcvByteBuffer, 0, rcvByteBuffer.Length)))

src/libraries/System.IO.Ports/tests/Support/PortsTest.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Text;
 using System.IO;
 using Legacy.Support;
 using Xunit;
@@ -36,5 +37,20 @@ public static void Fail(string format, params object[] args)
         {
             Assert.True(false, string.Format(format, args));
         }
+
+#pragma warning disable MSLIB0001 // Encoding.UTF7 property is obsolete
+        protected static Encoding LegacyUTF7Encoding => Encoding.UTF7;
+#pragma warning restore MSLIB0001
+
+        /// <summary>
+        /// Returns a value stating whether <paramref name="encoding"/> is UTF-7.
+        /// </summary>
+        /// <remarks>
+        /// This method checks only for the code page 65000.
+        /// </remarks>
+        internal static bool IsUTF7Encoding(Encoding encoding)
+        {
+            return (encoding.CodePage == LegacyUTF7Encoding.CodePage);
+        }
     }
 }

src/libraries/System.Net.Http.Json/tests/UnitTests/TranscodingReadStreamTests.cs

@@ -232,15 +232,6 @@ public Task ReadAsync_Works_WhenInputIs_Unicode(string message)
             return ReadAsyncTest(sourceEncoding, message);
         }
 
-        [Theory]
-        [MemberData(nameof(ReadAsyncInputLatin), "utf-7")]
-        [MemberData(nameof(ReadAsyncInputUnicode), "utf-7")]
-        public Task ReadAsync_Works_WhenInputIs_UTF7(string message)
-        {
-            Encoding sourceEncoding = Encoding.UTF7;
-            return ReadAsyncTest(sourceEncoding, message);
-        }
-
         [Theory]
         [MemberData(nameof(ReadAsyncInputLatin), "iso-8859-1")]
         public Task ReadAsync_Works_WhenInputIs_WesternEuropeanEncoding(string message)

src/libraries/System.Net.Http.Json/tests/UnitTests/TranscodingWriteStreamTests.cs

@@ -42,14 +42,6 @@ public Task WriteAsync_Works_WhenOutputIs_Unicode(string message)
             return WriteAsyncTest(targetEncoding, message);
         }
 
-        [Theory]
-        [MemberData(nameof(WriteAsyncInputLatin))]
-        public Task WriteAsync_Works_WhenOutputIs_UTF7(string message)
-        {
-            Encoding targetEncoding = Encoding.UTF7;
-            return WriteAsyncTest(targetEncoding, message);
-        }
-
         [Theory]
         [MemberData(nameof(WriteAsyncInputLatin))]
         public Task WriteAsync_Works_WhenOutputIs_WesternEuropeanEncoding(string message)

src/libraries/System.Private.CoreLib/src/ILLink/ILLink.Substitutions.Shared.xml

@@ -9,5 +9,8 @@
     <type fullname="System.Globalization.GlobalizationMode">
       <method signature="System.Boolean get_Invariant()" body="stub" value="true" feature="System.Globalization.Invariant" featurevalue="true" />
     </type>
+    <type fullname="System.LocalAppContextSwitches">
+      <method signature="System.Boolean get_EnableUnsafeUTF7Encoding()" body="stub" value="false" feature="System.Text.Encoding.EnableUnsafeUTF7Encoding" featurevalue="false" />
+    </type>
   </assembly>
 </linker>

src/libraries/System.Private.CoreLib/src/Resources/Strings.resx

@@ -3742,6 +3742,9 @@
   <data name="SwitchExpressionException_UnmatchedValue" xml:space="preserve">
     <value>Unmatched value was {0}.</value>
   </data>
+  <data name="Encoding_UTF7_Disabled" xml:space="preserve">
+    <value>Support for UTF-7 is disabled. See {0} for more information.</value>
+  </data>
   <data name="IDynamicInterfaceCastable_DoesNotImplementRequested" xml:space="preserve">
     <value>Type '{0}' returned by IDynamicInterfaceCastable does not implement the requested interface '{1}'.</value>
   </data>