Debugging Multi-Language Programs

A very cool feature of .NET is the multi-language capability. I still hear discussions about which language is better, but I just don't think it matters that much any more. I don't know anyone (or any company) developing one application with language heterogeneity ”for example, VB .NET and C# ”in the same application, but you can do it. I have to be careful here because I haven't written an enterprise application that tests this presumption , but I have implemented Visual Basic .NET classes that inherit from C# classes and vice versa and traced through the source code in the debugger. Everything seemed to work fine. I have included a sample application in Listing 17.12 for you to play with; it is worth the "cool factor" alone.

TIP

A language must comply with the Common Language Specification to participate in cross-language interoperability.

Listing 17.12 A Multi-Language C# and Visual Basic .NET Demonstration

 1:  // C# base class 2:  namespace TrafficControls 3:  { 4:    using System.Drawing; 5:    using System.Diagnostics; 6: 7:    public abstract class AbstractTrafficLight 8:    { 9:      public abstract void Draw(Graphics graphics); 10: 11:     public abstract class AbstractLamp 12:     { 13:       private Color color; 14: 15:       public AbstractLamp(Color color) 16:       { this.color = color; } 17: 18:       public Color Color 19:       { 20:         get{ return color; } 21:         set{ SetColor(value); } 22:       } 23: 24:       protected virtual void SetColor(Color value) 25:       { 26:         Debug.Assert(value == Color.Red 27:            value == Color.Green 28:            value == Color.Yellow); 29:         color = value; 30:       } 31: 32:       public abstract void Draw(Graphics graphics, _ 33:         Rectangle r, bool isOn); 34:     } 35:   } 36: } 37: // Visual Basic .NET subclass 38: Imports System.Drawing 39: Imports System.Windows.Forms 40: Imports System.Drawing.Drawing2D 41: 42: Public Class TrafficLight 43:   Inherits TrafficControls.AbstractTrafficLight 44: 45:   Public Enum LightState 46:     Green 47:     Yellow 48:     Red 49:     Off 50:   End Enum 51: 52:   Private FState As LightState = State.Green 53: 54:   Public Property State() As LightState 55:   Get 56:     Return FState 57:   End Get 58:   Set(ByVal Value As LightState) 59:     FState = Value 60:   End Set 61:   End Property 62: 63:   Public Sub [Next]() 64:     FState = (FState + 1) Mod LightState.Off 65:     Debug.WriteLine(FState.ToString()) 66:   End Sub 67: 68:   Private Lamps() As Lamp = New Lamp() _ 69:     {New Lamp(Color.Green), New Lamp(Color.Yellow), _ 70:      New Lamp(Color.Red)} 71: 72:   Public Overloads Overrides Sub Draw(ByVal g As Graphics) 73: 74:     g.FillRectangle(Brushes.Brown, New Rectangle(0, 0, 50, 160)) 75: 76:     Dim I As Integer 77:     For I = 0 To Lamps.GetUpperBound(0) 78:       Lamps(I).Draw(g, New Rectangle(5, _ 79:         ((2 - I) * 50) + 3, 40, 45), I = State) 80:     Next 81:     Next 82:   End Sub 83: 84:   Public Class Lamp 85:     Inherits TrafficControls.AbstractTrafficLight.AbstractLamp 86: 87:     Public Sub New(ByVal Color As Color) 88:       MyBase.New(Color) 89:     End Sub 90: 91:     Private Function GetBrush(ByVal isOn As Boolean) As Brush 92:       If (isOn) Then 93:         Return New SolidBrush(Color) 94:       Else 95:         Return New HatchBrush( _ 96:           HatchStyle.DashedHorizontal, Color.Gray) 97:       End If 98:     End Function 99: 100:    Public Overloads Overrides Sub Draw(ByVal g As Graphics, _ 101:      ByVal r As Rectangle, ByVal isOn As Boolean) 102: 103:      g.FillEllipse(GetBrush(isOn), r) 104: 105:    End Sub 106: 107:  End Class 108: 109: End Class

The 109 lines of sample code implement a rudimentary traffic signal with red, yellow, and green lamps. You can run MultiLanguageDemo.sln to see the lamp change colors at regular intervals. The code draws three colored circles inside a rectangle and is itself not very exciting. What is exciting is that these two files are in different assemblies (but in the same solution), and the VB .NET TrafficLight class inherits from the C# AbstractTrafficLight class. I made the code intentionally complex to test the multi-language capability and, in part, because I thought the solution made sense.

AbstractTrafficLight defines the concept of a traffic control comprised of illuminated (colored) lights. Because the light controls which lamps are on and which are off, I also implemented a nested AbstractLamp class. The VB .NET code mirrors the C# code by defining a TrafficLight class and a nested Lamp class. TrafficLight inherits from AbstractTrafficLight , and Lamp inherits from AbstractLamp . The code works perfectly , and debugging in and out of VB .NET and C# works flawlessly in Visual Studio .NET. I can easily use all the techniques we have discussed. (For example, line 65 demonstrates a Debug.WriteLine statement.)