Stream that starts off backed by MemoryStream but after threshold will switch to file-backed temp-file stream

SmartStream.cs

using System;
using System.IO;

class SmartStream : Stream
{
    int _maxMemorySize;
    Stream _stream = new MemoryStream();

    public SmartStream(int maxMemorySize)
    {
        _maxMemorySize = maxMemorySize;
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        _stream.Write(buffer, offset, count);
        if (_stream is MemoryStream && _stream.Length > _maxMemorySize)
            switchToTempFile();
    }

    void switchToTempFile()
    {
        var tempFile = new FileStream(Path.Combine(Path.GetTempPath(), Path.GetRandomFileName()), FileMode.Create, FileAccess.ReadWrite, FileShare.Read | FileShare.Delete, 4096, FileOptions.DeleteOnClose);
        var streamPos = _stream.Position;

        _stream.Position = 0;
        _stream.CopyTo(tempFile);
        _stream.Dispose();

        tempFile.Position = streamPos;
        _stream = tempFile;
    }

    #region Wrappers
    public override bool CanRead => _stream.CanRead;

    public override bool CanSeek => _stream.CanSeek;

    public override bool CanWrite => _stream.CanWrite;

    public override long Length => _stream.Length;

    public override long Position { get => _stream.Position; set => _stream.Position = value; }

    public override void Flush() => _stream.Flush();

    public override int Read(byte[] buffer, int offset, int count) => _stream.Read(buffer, offset, count);

    public override long Seek(long offset, SeekOrigin origin) => _stream.Seek(offset, origin);

    public override void SetLength(long value) => _stream.SetLength(value);
    #endregion
}