Reading a large file and dealing with Unity Mono versus VS .Net

I'm having a lot of trouble with very basic tasks when trying to get the Windows RT/UWP API to work in Unity. I have about zero experience with Windows dev and I'm unclear on the difference between .NET, the Windows RT, UWP, and the hololens API. So please bear with my muddled nomenclature.

I wanted to read a large file (i.e., with buffering) to do some data visualization. So I read the basic article on reading/writing files quick start. Apparently the chosen model for UWP is to do all IO asynchronously. So I write my code in VS and am able to generate an executable. However, Unity won't let me edit the scene further because my scripts don't compile in Unity because Unity uses C# 3.0 (?) and an older version of .NET.

There appears to be no solution to this problem, so I'll just have to learn to write in this older version of C# if I want to use Unity. I can't find UWP articles though that describe something as basic as reading a file WITHOUT async. Where can I learn how to write C# code that is valid for both Unity and Windows RT/UWP? Like is there an archive somewhere of the Windows RT/UWP API for this 2009-ish version of C# used in Unity?

As a side note, it seems strangely difficult to read a file without reading it all at once in Windows RT/UWP. I had to create a StorageFolder, then get a IOFile, then get a stream, then an input stream, then a DataReader, then read from that asynchronously for a fixed size, then read the string from the DataReader, then split the lines along a newline and then manage the fact that my buffer will end in the middle of a string. The ReadLines IEnumerable from the usual System.IO would have been splendid for this task but that doesn't seem to exist in the Windows RT/UWP API.

Here's what I came up with, but of course it won't work with unity:

    //create a stream and get a data reader for it. 
    var stream = await file.OpenAsync(Windows.Storage.FileAccessMode.ReadWrite);
    var inputStream = stream.GetInputStreamAt(0);
    var dataReader = new Windows.Storage.Streams.DataReader(inputStream);

    uint bufferSize = 32; //can tune this to balance better
    while (...)
    {
        uint numBytesLoaded = await dataReader.LoadAsync(bufferSize);
        //completed read?
        if (numBytesLoaded == 0) {
            finishedFile = true;
            break;
         }
        string text =dataReader.ReadString(numBytesLoaded);
        var splitLines = text.Split(new string[] { Environment.NewLine }, StringSplitOptions.None);

        /*
         *  We're reading a fixed buffer and breaking it into lines. We need to keep empty strings where breaks were so we
         *  know when we reach the end of stream vs end of line (which may be identical). 
         */
        string partialLine = "";
        string line;
        for (int i = 0; i < splitLines.Length; i++)
        {
            //did not finish exactly on line. Keep the partial line.
            if (i == splitLines.Length && splitLines[i].Length != 0)
            {
                partialLine = splitLines[i];
                Debug.Log("Finished on incomplete line:" + splitLines[i]);
                break;
            }

            //resume from what was left over, or use current split line.
            if (i == 0) {
                line = partialLine + splitLines[i];
                Debug.Log("Re-using left over from previously incomplete read: " + partialLine + splitLines[i]);
            } else {
                line = splitLines[i];
            }
            if (line.Length == 0)
                continue;

            //FINALLY! I have a line from the file to parse
           ....