issue-50839 handle large int write input.

src/libraries/System.IO/tests/BufferedStream/BufferedStreamTests.cs

@@ -53,6 +53,54 @@ public void BufferSize()
             Assert.Equal(1234, bufferedStream.BufferSize);
         }
 
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.Is64BitProcess))]
+        [OuterLoop]
+        [InlineData(int.MaxValue / 2 + 1)]
+        public void WriteFromByte_InputSizeLargerThanHalfOfMaxInt_ShouldSuccess(int inputSize)
+        {
+            byte[] bytes;
+
+            try
+            {
+                bytes = new byte[inputSize];
+            }
+            catch (OutOfMemoryException)
+            {
+                return;
+            }
+
+            var writableStream = new WriteOnlyStream();
+            using (var bs = new BufferedStream(writableStream))
+            {
+                bs.Write(bytes, 0, inputSize);
+                Assert.Equal(inputSize, writableStream.Position);
+            }
+        }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.Is64BitProcess))]
+        [OuterLoop]
+        [InlineData(int.MaxValue / 2 + 1)]
+        public void WriteFromSpan_InputSizeLargerThanHalfOfMaxInt_ShouldSuccess(int inputSize)
+        {
+            byte[] bytes;
+
+            try
+            {
+                bytes = new byte[inputSize];
+            }
+            catch (OutOfMemoryException)
+            {
+                return;
+            }
+
+            var writableStream = new WriteOnlyStream();
+            using (var bs = new BufferedStream(writableStream))
+            {
+                bs.Write(new ReadOnlySpan<byte>(bytes));
+                Assert.Equal(inputSize, writableStream.Position);
+            }
+        }
+
         [Theory]
         [InlineData(true)]
         [InlineData(false)]
@@ -369,4 +417,49 @@ public override Task WriteAsync(byte[] buffer, int offset, int count, Cancellati
         public override Task FlushAsync(CancellationToken cancellationToken) =>
             throw new InvalidOperationException("Exception from FlushAsync");
     }
+
+    internal sealed class WriteOnlyStream : Stream
+    {
+        private long _pos;
+
+        public override void Flush()
+        {
+        }
+
+        public override int Read(byte[] buffer, int offset, int count)
+        {
+            throw new NotSupportedException();
+        }
+
+        public override long Seek(long offset, SeekOrigin origin)
+        {
+            throw new NotSupportedException();
+        }
+
+        public override void SetLength(long value)
+        {
+            throw new NotSupportedException();
+        }
+
+        public override void Write(byte[] buffer, int offset, int count)
+        {
+            _pos += count;
+        }
+
+        public override void Write(ReadOnlySpan<byte> buffer)
+        {
+            _pos += buffer.Length;
+        }
+
+        public override bool CanRead => false;
+        public override bool CanSeek => false;
+        public override bool CanWrite => true;
+        public override long Length => _pos;
+
+        public override long Position
+        {
+            get => _pos;
+            set => throw new NotSupportedException();
+        }
+    }
 }

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

@@ -889,7 +889,8 @@ public override void Write(byte[] buffer, int offset, int count)
             checked
             {  // We do not expect buffer sizes big enough for an overflow, but if it happens, lets fail early:
                 totalUserbytes = _writePos + count;
-                useBuffer = (totalUserbytes + count < (_bufferSize + _bufferSize));
+                // Allow current totalUserbytes up to int.MaxValue by using uint arithmetic operation for totalUserbytes + count
+                useBuffer = ((uint)totalUserbytes + count < (_bufferSize + _bufferSize));
             }
 
             if (useBuffer)
@@ -959,7 +960,8 @@ public override void Write(ReadOnlySpan<byte> buffer)
             {
                 // We do not expect buffer sizes big enough for an overflow, but if it happens, lets fail early:
                 totalUserbytes = _writePos + buffer.Length;
-                useBuffer = (totalUserbytes + buffer.Length < (_bufferSize + _bufferSize));
+                // Allow current totalUserbytes up to int.MaxValue by using uint arithmetic operation for totalUserbytes + buffer.Length
+                useBuffer = ((uint)totalUserbytes + buffer.Length < (_bufferSize + _bufferSize));
             }
 
             if (useBuffer)