Add tests for C# behavior around invalid UTF-8

These are primarily in order to document the current behavior,
rather than asserting it's the "right" behavior.

csharp/src/Google.Protobuf.Test/ParsingPrimitivesTest.cs

@@ -0,0 +1,63 @@
+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2022 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+
+using NUnit.Framework;
+using System;
+using System.Linq;
+
+namespace Google.Protobuf.Test;
+
+internal class ParsingPrimitivesTest
+{
+    // Note: test cases use integers rather than bytes as they're easier
+    // to specify in attributes.
+
+    [Test]
+    [TestCase("\ufffd", 255)]
+    [TestCase("A\ufffd", 65, 255)]
+    [TestCase("A\ufffd\ufffdB", 65, 255, 255, 66)]
+    // Overlong form of "space"
+    [TestCase("\ufffd\ufffd", 0xc0, 0xa0)]
+    public void ReadRawString_NonUtf8(string expectedText, params int[] bytes)
+    {
+        var context = CreateContext(bytes);
+        string text = ParsingPrimitives.ReadRawString(ref context.buffer, ref context.state, bytes.Length);
+        Assert.AreEqual(expectedText, text);
+    }
+
+    private static ParseContext CreateContext(int[] bytes)
+    {
+        byte[] actualBytes = bytes.Select(b => (byte) b).ToArray();
+        ParseContext.Initialize(actualBytes.AsSpan(), out var context);
+        return context;
+    }
+}

csharp/src/Google.Protobuf.Test/WritingPrimitivesTest.cs

@@ -0,0 +1,61 @@
+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2022 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+
+using NUnit.Framework;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Google.Protobuf.Test;
+
+internal class WritingPrimitivesTest
+{
+    [Test]
+    public void WriteRawString_IllFormedUnicodeString()
+    {
+        // See https://codeblog.jonskeet.uk/2014/11/07/when-is-a-string-not-a-string/
+        char c1 = '\u0058';
+        char c2 = '\ud800';
+        char c3 = '\u0059';
+        string text = new string(new[] { c1, c2, c3 });
+        Span<byte> buffer = new byte[10];
+        WriteContext.Initialize(ref buffer, out var context);
+        WritingPrimitives.WriteString(ref context.buffer, ref context.state, text);
+
+        // The high surrogate is written out in a "raw" form, surrounded by the ASCII
+        // characters.
+        byte[] expectedBytes = { 0x5, 0x58, 0xef, 0xbf, 0xbd, 0x59 };
+        Assert.AreEqual(expectedBytes, buffer.Slice(0, context.state.position).ToArray());
+    }
+}