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

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;
}

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;
}

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