How to create a Polygon around a Linestring as a GeoFence in C# .NET

I'm looking for a .net library that will help to create a polygon that I can use as a Geo-fence around a route on a map.

The following image demonstrates the result I'm after. Imagine that this polygon was projected from a line string that defined a route on a map.

I've not done any GIS mapping work before, so I'm really quite lost even as where or what to look for.

UPDATE

Ok, I've had a quick look and seem to have something working. The code to generate a polygon as a buffer around the linestring is pretty simple.

var line = new LineString(points);
var bufferOp = new BufferOp(line);
var buffer = (Polygon)bufferOp.GetResultGeometry(.00001);

But what confuses me now is the distance parameter to GetResultGeomotry. In the above code I'm specifying .00001 as the distance.. this is the only way I could get a reasonable result.

What unit of measure is that?