public class ClipLabelPosition
{
    private readonly SharpMap.Map _map;
    private GeoAPI.Geometries.IGeometry _clip;

    public ClipLabelPosition(SharpMap.Map map)
    {
        _map = map;
        map.MapViewOnChange += () => _clip = _map.Factory.ToGeometry(_map.Envelope);
    }

    public GeoAPI.Geometries.Coordinate GetClippedPosition(SharpMap.Data.FeatureDataRow row)
    {
        var g = _clip.Intersection(row.Geometry);
        return g.PointOnSurface.Coordinate;
    }
}