Unify BufferedFileStreamStrategy seeking

src/libraries/System.IO.FileSystem/tests/FileStream/Seek.cs

@@ -7,16 +7,18 @@ namespace System.IO.Tests
 {
     public class FileStream_Seek : FileSystemTest
     {
-        [Fact]
-        public void SeekAppendModifyThrows()
+        [Theory]
+        [InlineData(0)]
+        [InlineData(10)]
+        public void SeekAppendModifyThrows(int bufferSize)
         {
             string fileName = GetTestFilePath();
             using (FileStream fs = new FileStream(fileName, FileMode.Create))
             {
                 fs.Write(TestBuffer, 0, TestBuffer.Length);
             }
 
-            using (FileStream fs = new FileStream(fileName, FileMode.Append))
+            using (FileStream fs = new FileStream(fileName, FileMode.Append, FileAccess.Write, FileShare.Read, bufferSize))
             {
                 long length = fs.Length;
                 Assert.Throws<IOException>(() => fs.Seek(length - 1, SeekOrigin.Begin));
@@ -33,6 +35,9 @@ public void SeekAppendModifyThrows()
                 Assert.Throws<IOException>(() => fs.Seek(-length, SeekOrigin.End));
                 Assert.Equal(length, fs.Position);
 
+                Assert.Throws<IOException>(() => fs.Position = length - 1);
+                Assert.Equal(length, fs.Position);
+
                 fs.Write(TestBuffer);
                 Assert.Equal(length, fs.Seek(length, SeekOrigin.Begin));
             }

src/libraries/System.Private.CoreLib/src/System/IO/FileStream.cs

@@ -487,8 +487,17 @@ public override long Position
                 {
                     ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.value, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
                 }
+                else if (!CanSeek)
+                {
+                    if(_strategy.IsClosed)
+                    {
+                        ThrowHelper.ThrowObjectDisposedException_FileClosed();
+                    }
+
+                    ThrowHelper.ThrowNotSupportedException_UnseekableStream();
+                }
 
-                _strategy.Seek(value, SeekOrigin.Begin);
+                _strategy.Position = value;
             }
         }
 
@@ -575,7 +584,24 @@ public override void EndWrite(IAsyncResult asyncResult)
 
         public override bool CanSeek => _strategy.CanSeek;
 
-        public override long Seek(long offset, SeekOrigin origin) => _strategy.Seek(offset, origin);
+        public override long Seek(long offset, SeekOrigin origin)
+        {
+            if (origin < SeekOrigin.Begin || origin > SeekOrigin.End)
+            {
+                throw new ArgumentException(SR.Argument_InvalidSeekOrigin, nameof(origin));
+            }
+            else if (!CanSeek)
+            {
+                if (_strategy.IsClosed)
+                {
+                    ThrowHelper.ThrowObjectDisposedException_FileClosed();
+                }
+
+                ThrowHelper.ThrowNotSupportedException_UnseekableStream();
+            }
+
+            return _strategy.Seek(offset, origin);
+        }
 
         internal Task BaseFlushAsync(CancellationToken cancellationToken)
             => base.FlushAsync(cancellationToken);

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/BufferedFileStreamStrategy.cs

@@ -65,15 +65,7 @@ public override long Position
             }
             set
             {
-                if (_writePos > 0)
-                {
-                    FlushWrite();
-                }
-
-                _readPos = 0;
-                _readLen = 0;
-
-                _strategy.Position = value;
+                Seek(value, SeekOrigin.Begin);
             }
         }
 
@@ -904,9 +896,6 @@ public override void CopyTo(Stream destination, int bufferSize)
 
         public override long Seek(long offset, SeekOrigin origin)
         {
-            EnsureNotClosed();
-            EnsureCanSeek();
-
             // If we have bytes in the write buffer, flush them out, seek and be done.
             if (_writePos > 0)
             {

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/OSFileStreamStrategy.cs

@@ -85,7 +85,7 @@ internal void OnIncompleteOperation(int expectedBytesTransferred, int actualByte
         public sealed override long Position
         {
             get => _filePosition;
-            set => _filePosition = value;
+            set => Seek(value, SeekOrigin.Begin);
         }
 
         internal sealed override string Name => _fileHandle.Path ?? SR.IO_UnknownFileName;
@@ -144,11 +144,6 @@ internal sealed override void Flush(bool flushToDisk)
 
         public sealed override long Seek(long offset, SeekOrigin origin)
         {
-            if (origin < SeekOrigin.Begin || origin > SeekOrigin.End)
-                throw new ArgumentException(SR.Argument_InvalidSeekOrigin, nameof(origin));
-            if (_fileHandle.IsClosed) ThrowHelper.ThrowObjectDisposedException_FileClosed();
-            if (!CanSeek) ThrowHelper.ThrowNotSupportedException_UnseekableStream();
-
             long oldPos = _filePosition;
             long pos = origin switch
             {