SketchPlane

public void ShowSketchPlane(SketchPlane sketchPlane)
{
    // get the information of the created sketch plane
    string information = "Sketch Plane: ";
    //get the corresponding Plane
    Autodesk.Revit.DB.Plane plane = sketchPlane.GetPlane();
    //show the plane's origin point
    information += "\norigin of the plane:  " + XYZToString(plane.Origin);
    //show the plane's normal vector
    information += "\nnormal of the plane:  " + XYZToString(plane.Normal);
    //show the plane's Xvec vector
    information += "\nXvec of the plane: (" + XYZToString(plane.XVec);
    //show the plane's Yvec vector
    information += "\nYvec of the plane: (" + XYZToString(plane.YVec);

    TaskDialog.Show("Revit",information);
}

// output the point's three coordinates
string XYZToString(XYZ point)
{
    return "(" + point.X + ", " + point.Y + ", " + point.Z + ")";
}