... and to save re-inventing the wheel ..
Autodesk.AutoCAD.Runtime.Converter
{
// Methods
public static string AngleToString(double value);
public static string AngleToString(double value, AngularUnitFormat units, int precision);
public static string DistanceToString(double value);
public static string DistanceToString(double value, DistanceUnitFormat units, int precision);
public static double StringToAngle(string value);
public static double StringToAngle(string value, AngularUnitFormat units);
public static double StringToDistance(string value);
public static double StringToDistance(string value, DistanceUnitFormat units);
}