Give a Polyline, you should be able to calculate it’s length using Polyline.Length. A polyline is just an array of points, to the calculation is pretty simiple.

Sorry I wasn’t clear. I am trying to find a point at a certain distance along the polyline (i.e. 35m from the start). I thought Evaluate Length would be the easiest way but I cannot find that method in rhino3dm.

You should be able to do that with some simple vector math.

// Some arbitrary line
var point0 = new Point3d(15.660128434922481, 65.84636006795553, 0.0);
var point1 = new Point3d(66.036967855848303, 5.3510956351343566, 0.0);
var line0 = new Line(point0, point1);
var length0 = line0.Length;
// Direction vector
var direction = point1 - point0;
direction.Unitize();
direction *= 35.0; // Your distance
var point2 = point0 + direction; // Create by offsetting point0
var line1 = new Line(point0, point2);
var length1 = line1.Length;