Reads X lines from a file, stopping each line after Y characters.

TrimFileLines.cs

void Main()
{
	var path = @"R:\test.txt";

	const int MAX_LINE_LENGTH = 15;
	const int MAX_LINE_COUNT = 15;

	using (var stream = GetTruncatedFile(path, MAX_LINE_LENGTH, MAX_LINE_COUNT))
	using (var reader = new StreamReader(stream))
	{
		reader.ReadToEnd().Dump();
	}
}

public MemoryStream GetTruncatedFile(string path, int maxLineLength, int maxLineCount)
{
	if (maxLineLength == 0) throw new ArgumentException("Line length must be greater than 0.", nameof(maxLineLength));

	var outputStream = new MemoryStream((maxLineCount * maxLineLength) + maxLineCount - 1);

	using (var mmf = MemoryMappedFile.CreateFromFile(path))
	using (var mmfStream = mmf.CreateViewStream())
	{
		var lineBuffer = new byte[maxLineLength];

		while (maxLineCount-- > 0)
		{
			var position = 0;

			while (position < maxLineLength)
			{
				var c = mmfStream.ReadByte();

				if (c == '\n' || c <= 0)
				{
					break;
				}

				lineBuffer[position] = (byte)c;
				++position;
			}

			outputStream.Write(lineBuffer, 0, position);
			outputStream.WriteByte((byte)'\n');

			--mmfStream.Position;

			int last;
			while ((last = mmfStream.ReadByte()) != '\n' && last > 0) ;

			if (last <= 0) break;
		}
	}

	// Trim off final \n
	outputStream.SetLength(outputStream.Length - 1);
	outputStream.Flush();
	outputStream.Position = 0;
	return outputStream;
}