byte[] bytes = File.ReadAllBytes(filepath);

byte[] bytes = File.ReadAllBytes(filepath);

C#
FileStream fs = new FileStream(filepath, FileMode.Open, FileAccess.Read);
// not sure why you are creating a binaryReader, as you aren't using it...
using (BinaryReader br = new BinaryReader(fs))
{

    byte[] bytes = new byte[0];
    using (MemoryStream test = new MemoryStream())
    {
        // This will copy the entire file into the memory stream.
        // MemoryStream will dynamically grow, by increasing the internal buffer by 2x
        // every time space is exhausted. This means that you'll ultimately use about 2x the memory
        // when reading the entire file.
        fs.CopyTo(test);
        // This allocates a brand new array of exactly the right size and copies
        // the bytes from memory stream's intenal buffer to the new array.
        bytes = test.ToArray();
    }

    // The rest of this code is opaque to me, but the "positions" array could grow quite large
    // if there are a lot of matches

    byte[] searchBytes = Encoding.UTF8.GetBytes("test");
    List<long> positions = new List<long>();

    foreach (long pos in Extensions.SearchStringInBytes(bytes, searchBytes))
    {
        positions.Add(pos - 4);
    }
}

C#
FileStream fs = new FileStream(filepath, FileMode.Open, FileAccess.Read);
// not sure why you are creating a binaryReader, as you aren't using it...
using (BinaryReader br = new BinaryReader(fs))
{

    byte[] bytes = new byte[0];
    using (MemoryStream test = new MemoryStream())
    {
        // This will copy the entire file into the memory stream.
        // MemoryStream will dynamically grow, by increasing the internal buffer by 2x
        // every time space is exhausted. This means that you'll ultimately use about 2x the memory
        // when reading the entire file.
        fs.CopyTo(test);
        // This allocates a brand new array of exactly the right size and copies
        // the bytes from memory stream's intenal buffer to the new array.
        bytes = test.ToArray();
    }

    // The rest of this code is opaque to me, but the "positions" array could grow quite large
    // if there are a lot of matches

    byte[] searchBytes = Encoding.UTF8.GetBytes("test");
    List<long> positions = new List<long>();

    foreach (long pos in Extensions.SearchStringInBytes(bytes, searchBytes))
    {
        positions.Add(pos - 4);
    }
}

C#
private Stream GetFileStream()
{
    var memoryStream = new MemoryStream();
    using (var streamWriter = new StreamWriter(memoryStream, leaveOpen: true))
    using (var csvWriter = new CsvWriter(streamWriter, culture: CultureInfo.InvariantCulture))
    {
        csvWriter.WriteRecords(objects);
    }

    // Reset memory stream
    memoryStream.Position = 0;

    return memoryStream;
}

C#
private Stream GetFileStream()
{
    var memoryStream = new MemoryStream();
    using (var streamWriter = new StreamWriter(memoryStream, leaveOpen: true))
    using (var csvWriter = new CsvWriter(streamWriter, culture: CultureInfo.InvariantCulture))
    {
        csvWriter.WriteRecords(objects);
    }

    // Reset memory stream
    memoryStream.Position = 0;

    return memoryStream;
}