Add TextReader/Writer Memory-based virtuals (#24434)

* Add Memory-based Read/WriteAsync virtuals to TextReader/Writer

* Override Span/Memory-based APIs on StringReader/Writer

* Address PR feedback

2 files changed, 123 insertions, 1 deletions

diff --git a/src/System.IO/tests/StringReader/StringReader.CtorTests.cs b/src/System.IO/tests/StringReader/StringReader.CtorTests.cs
index 2144054af3..cec91adbb6 100644
--- a/src/System.IO/tests/StringReader/StringReader.CtorTests.cs
+++ b/src/System.IO/tests/StringReader/StringReader.CtorTests.cs
@@ -8,7 +8,7 @@ using Xunit;
 
 namespace System.IO.Tests
 {
-    public class ReaderTests
+    public partial class StringReaderTests
     {
         [Fact]
         public static void StringReaderWithNullString()
diff --git a/src/System.IO/tests/StringReader/StringReaderTests.netcoreapp.cs b/src/System.IO/tests/StringReader/StringReaderTests.netcoreapp.cs
new file mode 100644
index 0000000000..cf9ea48ba2
--- /dev/null
+++ b/src/System.IO/tests/StringReader/StringReaderTests.netcoreapp.cs
@@ -0,0 +1,122 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// 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.Threading;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace System.IO.Tests
+{
+    public partial class StringReaderTests
+    {
+        [Fact]
+        public void ReadSpan_Success()
+        {
+            string input = "abcdef";
+            var reader = new StringReader(input);
+            Span<char> s = new char[2];
+
+            Assert.Equal(2, reader.Read(s));
+            Assert.Equal("ab", new string(s.ToArray()));
+
+            Assert.Equal(1, reader.Read(s.Slice(0, 1)));
+            Assert.Equal("cb", new string(s.ToArray()));
+
+            Assert.Equal(2, reader.Read(s));
+            Assert.Equal("de", new string(s.ToArray()));
+
+            Assert.Equal(1, reader.Read(s));
+            Assert.Equal("f", new string(s.Slice(0, 1).ToArray()));
+
+            Assert.Equal(0, reader.Read(s));
+        }
+
+        [Fact]
+        public void ReadBlockSpan_Success()
+        {
+            string input = "abcdef";
+            var reader = new StringReader(input);
+            Span<char> s = new char[2];
+
+            Assert.Equal(2, reader.ReadBlock(s));
+            Assert.Equal("ab", new string(s.ToArray()));
+
+            Assert.Equal(1, reader.ReadBlock(s.Slice(0, 1)));
+            Assert.Equal("cb", new string(s.ToArray()));
+
+            Assert.Equal(2, reader.ReadBlock(s));
+            Assert.Equal("de", new string(s.ToArray()));
+
+            Assert.Equal(1, reader.ReadBlock(s));
+            Assert.Equal("f", new string(s.Slice(0, 1).ToArray()));
+
+            Assert.Equal(0, reader.ReadBlock(s));
+        }
+
+        [Fact]
+        public async Task ReadMemoryAsync_Success()
+        {
+            string input = "abcdef";
+            var reader = new StringReader(input);
+            Memory<char> m = new char[2];
+
+            Assert.Equal(2, await reader.ReadAsync(m));
+            Assert.Equal("ab", new string(m.ToArray()));
+
+            Assert.Equal(1, await reader.ReadAsync(m.Slice(0, 1)));
+            Assert.Equal("cb", new string(m.ToArray()));
+
+            Assert.Equal(2, await reader.ReadAsync(m));
+            Assert.Equal("de", new string(m.ToArray()));
+
+            Assert.Equal(1, await reader.ReadAsync(m));
+            Assert.Equal("f", new string(m.Slice(0, 1).ToArray()));
+
+            Assert.Equal(0, await reader.ReadAsync(m));
+        }
+
+        [Fact]
+        public async Task ReadBlockMemoryAsync_Success()
+        {
+            string input = "abcdef";
+            var reader = new StringReader(input);
+            Memory<char> m = new char[2];
+
+            Assert.Equal(2, await reader.ReadBlockAsync(m));
+            Assert.Equal("ab", new string(m.ToArray()));
+
+            Assert.Equal(1, await reader.ReadBlockAsync(m.Slice(0, 1)));
+            Assert.Equal("cb", new string(m.ToArray()));
+
+            Assert.Equal(2, await reader.ReadBlockAsync(m));
+            Assert.Equal("de", new string(m.ToArray()));
+
+            Assert.Equal(1, await reader.ReadBlockAsync(m));
+            Assert.Equal("f", new string(m.Slice(0, 1).ToArray()));
+
+            Assert.Equal(0, await reader.ReadBlockAsync(m));
+        }
+
+        [Fact]
+        public void Disposed_ThrowsException()
+        {
+            var reader = new StringReader("abc");
+            reader.Dispose();
+
+            Assert.Throws<ObjectDisposedException>(() => reader.Read(Span<char>.Empty));
+            Assert.Throws<ObjectDisposedException>(() => reader.ReadBlock(Span<char>.Empty));
+            Assert.Throws<ObjectDisposedException>(() => { reader.ReadAsync(Memory<char>.Empty); });
+            Assert.Throws<ObjectDisposedException>(() => { reader.ReadBlockAsync(Memory<char>.Empty); });
+        }
+
+        [Fact]
+        public async Task Precanceled_ThrowsException()
+        {
+            var reader = new StringReader("abc");
+
+            await Assert.ThrowsAnyAsync<OperationCanceledException>(() => reader.ReadAsync(Memory<char>.Empty, new CancellationToken(true)).AsTask());
+            await Assert.ThrowsAnyAsync<OperationCanceledException>(() => reader.ReadBlockAsync(Memory<char>.Empty, new CancellationToken(true)).AsTask());
+        }
+    }
+}