The following sample code demonstrates how determine the maximum Z coordinate value of a surface or polysurface given some X,Y coordinate.

C++

  CRhinoCommand::result CCommandTest::RunCommand( const CRhinoCommandContext& context )
  {
    double x = 0.0, y = 0.0;
    ON_wString sx, sy, sz;


    // Pick a brep to evaluate
    CRhinoGetObject go;
    go.SetCommandPrompt( L"Select surface or polysurface to evaluate" );
    go.SetGeometryFilter( CRhinoGetObject::surface_object|CRhinoGetObject::polysrf_object );
    go.GetObjects( 1, 1 );
    if( go.CommandResult() != success )
      return go.CommandResult();


    // Get the brep geometry
    const CRhinoObjRef& ref = go.Object(0);
    const ON_Brep* brep = ref.Brep();
    if( !brep )
      return failure;


    // Prompt for an X coordinate value
    CRhinoGetNumber gn;
    gn.SetCommandPrompt( L"Value of X coordinate" );
    gn.SetDefaultNumber( x );
    gn.GetNumber();
    if( gn.CommandResult() != success )
      return gn.CommandResult();


    x = gn.Number();
    RhinoFormatNumber( x, sx );


    // Prompt for a Y coordinate value
    gn.SetCommandPrompt( L"Value of Y coordinate" );
    gn.SetDefaultNumber( y );
    gn.GetNumber();
    if( gn.CommandResult() != success )
      return gn.CommandResult();


    y = gn.Number();
    RhinoFormatNumber( y, sy );


    // Now that we have all of the input, we want to intersect
    // a line curve with the brep. To determine the magnitude of
    // the line, we will first get the brep's bounding box.
    ON_BoundingBox bbox;
    if( !brep->GetTightBoundingBox(bbox) )
      return failure;


    // Calculate the height of the bounding box
    ON_3dVector v = bbox.Corner(0,0,1) - bbox.Corner(0,0,0);


    // Starting point of line
    ON_3dPoint p0( x, y, 0.0 );


    // Ending point of line. To make sure the line
    // completely intersects the brep, we will double
    // the height.
    ON_3dPoint p1( x, y, v.Length() * 2 );


    // Create the line curve
    ON_LineCurve line( p0, p1 );


    // Intersect the line with the brep
    ON_SimpleArray<ON_Curve*> curves;
    ON_3dPointArray points;
    bool rc = RhinoCurveBrepIntersect( line, *brep, context.m_doc.AbsoluteTolerance(), curves, points );
    if( false == rc || 0 == points.Count() ) 
    {
      RhinoApp().Print( L"No maximum surface Z coordinate at %s,%s found.\n", sx, sy );
      return nothing;
    }


    // Because it is possible that our line might intersect
    // the brep in more than one place, find the intersection
    // point that is farthest from our input coordinate.
    int i;
    ON_3dPoint pt = p0;
    for( i = 0; i < points.Count(); i++ )
    {
      if( p0.DistanceTo(points[i]) > p0.DistanceTo(pt) )
        pt = points[i];
    }


    // Print results
    RhinoFormatNumber( pt.z, sz );
    RhinoApp().Print( L"Maximum surface Z coordinate at %s,%s is %s.\n", sx, sy, sz );


    // Optional, add a point object
    context.m_doc.AddPointObject( pt );


    // Delete any overlap intersection curves that might have been calculated
    for( i = 0; i < curves.Count(); i++ )
    {
      delete curves[i];
      curves[i] = 0;
    }
    context.m_doc.Redraw();
    return success;
  }

C# (Rhino 4)

  public override IRhinoCommand.result RunCommand(IRhinoCommandContext context)
  {
    // Pick a brep to evaluate
    MRhinoGetObject go = new MRhinoGetObject();
    go.SetCommandPrompt( "Select surface or polysurface to evaluate" );
    go.SetGeometryFilter( IRhinoGetObject.GEOMETRY_TYPE_FILTER.surface_object |
                          IRhinoGetObject.GEOMETRY_TYPE_FILTER.polysrf_object );
    go.GetObjects( 1, 1 );
    if( go.CommandResult() != IRhinoCommand.result.success )
      return go.CommandResult();


    // Get the brep geometry
    IRhinoObjRef objref = go.Object(0);
    IOnBrep brep = objref.Brep();
    if( brep == null )
      return IRhinoCommand.result.failure;


    double x = 0.0, y = 0.0;
    // Prompt for an X coordinate value
    MRhinoGetNumber gn = new MRhinoGetNumber();
    gn.SetCommandPrompt( "Value of X coordinate" );
    gn.SetDefaultNumber( x );
    gn.GetNumber();
    if( gn.CommandResult() != IRhinoCommand.result.success )
      return gn.CommandResult();


    x = gn.Number();
    string sx = "";
    RhUtil.RhinoFormatNumber( x, ref sx);


    // Prompt for a Y coordinate value
    gn.SetCommandPrompt( "Value of Y coordinate" );
    gn.SetDefaultNumber( y );
    gn.GetNumber();
    if( gn.CommandResult() != IRhinoCommand.result.success )
      return gn.CommandResult();


    y = gn.Number();
    string sy = "";
    RhUtil.RhinoFormatNumber( y, ref sy );


    // Now that we have all of the input, we want to intersect
    // a line curve with the brep. To determine the magnitude of
    // the line, we will first get the brep's bounding box.
    OnBoundingBox bbox = new OnBoundingBox();
    if( !brep.GetTightBoundingBox( ref bbox) )
      return IRhinoCommand.result.failure;


    // Calculate the height of the bounding box
    On3dVector v = bbox.Corner(0,0,1) - bbox.Corner(0,0,0);


    // Starting point of line
    On3dPoint p0 = new On3dPoint( x, y, 0.0 );


    // Ending point of line. To make sure the line completely
    // intersects the brep, we will double the height.
    On3dPoint p1 = new On3dPoint( x, y, v.Length() * 2 );


    // Create the line curve
    OnLineCurve line = new OnLineCurve( p0, p1 );


    // Intersect the line with the brep
    OnCurve[] curves = null;
    On3dPointArray points = null;
    bool rc = RhUtil.RhinoCurveBrepIntersect( line, brep,
                context.m_doc.AbsoluteTolerance(), out curves, out points);
    if( false == rc || points == null || points.Count() == 0 ) 
    {
      string msg = string.Format("No maximum surface Z coordinate at {0},{1} found.\n", sx, sy );
      RhUtil.RhinoApp().Print( msg );
      return IRhinoCommand.result.nothing;
    }


    // Because it is possible that our line might intersect
    // the brep in more than one place, find the intersection
    // point that is farthest from our input coordinate.
    On3dPoint pt = new On3dPoint(p0);
    for( int i = 0; i < points.Count(); i++ )
    {
      On3dPoint testPt = points[i];
      if( p0.DistanceTo(testPt) > p0.DistanceTo(pt) )
        pt.Set(testPt.x, testPt.y, testPt.z );
    }


    // Print results
    string sz = "";
    RhUtil.RhinoFormatNumber( pt.z, ref sz );
    string msg2 = string.Format("Maximum surface Z coordinate at {0},{1} is {2}.\n", sx, sy, sz);
    RhUtil.RhinoApp().Print( msg2 );


    // Optional, add a point object
    context.m_doc.AddPointObject( pt );
    context.m_doc.Redraw();


    return IRhinoCommand.result.success;
  }

VB.Net (Rhino 4)

  Public Overrides Function RunCommand(ByVal context As RMA.Rhino.IRhinoCommandContext) _
    As RMA.Rhino.IRhinoCommand.result
    ' Pick a brep to evaluate
    Dim go As New MRhinoGetObject()
    go.SetCommandPrompt("Select surface or polysurface to evaluate")
    go.SetGeometryFilter(IRhinoGetObject.GEOMETRY_TYPE_FILTER.surface_object Or _
                          IRhinoGetObject.GEOMETRY_TYPE_FILTER.polysrf_object)
    go.GetObjects(1, 1)
    If (go.CommandResult() <> IRhinoCommand.result.success) Then
      Return go.CommandResult()
    End If


    ' Get the brep geometry
    Dim objref As IRhinoObjRef = go.Object(0)
    Dim brep As IOnBrep = objref.Brep()
    If (brep Is Nothing) Then
      Return IRhinoCommand.result.failure
    End If


    Dim x As Double = 0.0, y As Double = 0.0


    ' Prompt for an X coordinate value
    Dim gn As New MRhinoGetNumber()
    gn.SetCommandPrompt("Value of X coordinate")
    gn.SetDefaultNumber(x)
    gn.GetNumber()
    If (gn.CommandResult() <> IRhinoCommand.result.success) Then
      Return gn.CommandResult()
    End If


    x = gn.Number()
    Dim sx As String = ""
    RhUtil.RhinoFormatNumber(x, sx)


    ' Prompt for a Y coordinate value
    gn.SetCommandPrompt("Value of Y coordinate")
    gn.SetDefaultNumber(y)
    gn.GetNumber()
    If (gn.CommandResult() <> IRhinoCommand.result.success) Then
      Return gn.CommandResult()
    End If


    y = gn.Number()
    Dim sy As String = ""
    RhUtil.RhinoFormatNumber(y, sy)


    ' Now that we have all of the input, we want to intersect
    ' a line curve with the brep. To determine the magnitude of
    ' the line, we will first get the brep's bounding box.
    Dim bbox As New OnBoundingBox()
    If (Not brep.GetTightBoundingBox(bbox)) Then
      Return IRhinoCommand.result.failure
    End If


    ' Calculate the height of the bounding box
    Dim v As On3dVector = bbox.Corner(0, 0, 1) - bbox.Corner(0, 0, 0)


    ' Starting point of line
    Dim p0 As New On3dPoint(x, y, 0.0)


    ' Ending point of line. To make sure the line completely
    ' intersects the brep, we will double the height.
    Dim p1 As New On3dPoint(x, y, v.Length() * 2)


    ' Create the line curve
    Dim line As New OnLineCurve(p0, p1)


    ' Intersect the line with the brep
    Dim curves() As OnCurve = Nothing
    Dim points As On3dPointArray = Nothing
    Dim rc As Boolean = RhUtil.RhinoCurveBrepIntersect(line, brep, _
                        context.m_doc.AbsoluteTolerance(), curves, points)
    If (False = rc OrElse points Is Nothing OrElse points.Count() = 0) Then
      Dim msg As String = String.Format("No maximum surface Z coordinate at {0},{1} found.\n", sx, sy)
      RhUtil.RhinoApp().Print(msg)
      Return IRhinoCommand.result.nothing
    End If


    ' Because it is possible that our line might intersect
    ' the brep in more than one place, find the intersection
    ' point that is farthest from our input coordinate.
    Dim pt As New On3dPoint(p0)
    For i As Integer = 0 To points.Count - 1
      Dim testPt As On3dPoint = points(i)
      If (p0.DistanceTo(testPt) > p0.DistanceTo(pt)) Then
        pt.Set(testPt.x, testPt.y, testPt.z)
      End If
    Next


    ' Print results
    Dim sz As String = ""
    RhUtil.RhinoFormatNumber(pt.z, sz)
    Dim msg2 As String = String.Format("Maximum surface Z coordinate at {0},{1} is {2}.\n", sx, sy, sz)
    RhUtil.RhinoApp().Print(msg2)


    ' Optional, add a point object
    context.m_doc.AddPointObject(pt)
    context.m_doc.Redraw()


    Return IRhinoCommand.result.success
  End Function