using foreach to enumerate objects 1

[Pumzy]

hello people!
i want to find a way to enumerate my objects (such as textboxes, buttons, etc) in my form using foreach statement.
i don't know how to do this in C#. can anyone help? tnx. happy new year!!!

KDjLogan

 

[obislavu]

System.Windows.Forms.Form frm = new System.Windows.Forms.Form ();
//... Add controls in the frm
// Enumerate the controls

foreach (Control ctrl in frm.Controls)
{
// ... ctrl
Type t = ctrl.GetType();
// ...
}

-obislavu-

 

[Pumzy]

THANKS OBISLAVU DUDE! TIL NEXT TIP...

KDjLogan

 

[chiph]

Pumzy -
FYI, this can be done to any object that implements the IEnumerable interface. If you look in the help, you'll find a list of objects on which you can do foreach(){}. It goes from Array to XmlSchemaCollection.

You can implement this interface in your classes too:

using System.Collections;
using System.Drawing;

class Triangle : IEnumerable
{
   public Point A, B, C;

   // declare method required by IEnumerable
   public IEnumberator GetEnumerator()
   {
      return new PointEnumerator(this);
   }

   // Inner class implements IEnumerator interface
   private class PointEnumerator : IEnumerator
   {
      private int position = -1;
      private Triangle t;

      // constructor
      public PointEnumerator(Triangle t)
      {
         this.t = t;  // make a local copy of the Triangle object
      }

      // Declare the public method required by IEnumerator
      public bool MoveNext()
      {
         if (position < (3 - 1) // three points in triangle, 
                                // subtract one for starting at 0
            position++;
            return true;
         }
         else
         {
            return false;
         }
      }

      // declare the public method required by IEnumerator
      public void Reset()
      {
         position = -1;
      }

      // declare the public property required by IEnumerator
      public object Current
      {
         get
         {
            switch (position)
            {
               case 0:
                  return t.A.ToObject();
                  break;
               case 1:
                  return t.B.ToObject();
                  break;
               case 2:
                  return t.C.ToObject();
                  break;
               default:
                  return null;
                  break;
            }
         }
      }
   }      
}

What this code does is declare a class named Triangle that implements the IEnumerable interface. It will allow you to go through the points of the triangle using the foreach(){} operator.

IEnumerable uses another interface named IEnumerator, which does all the dirty work -- it requires you to implement two methods and a property. The methods reset the position to the beginning, and move the position to the next point. The property retrives whatever Point is current.

This is actually a good example for someone new to C# and .NET, as it illustrates some cool features of the language -- private classes (classes which are only visible inside their containing class), interfaces (programming by contract -- you promise to write code which will implement the interface), and passing a reference to an object to another object via it's constructor method.

Some notes about doing this: The Point object is a value type, not a reference type (meaning it's a struct, not a class). When the Current property returns one, it has to call ToObject (since Current demands that it be an object). This involves what's known as "boxing" the Point object, which is converting the value type to a reference type. This is inherently slow, as the calling function would have to cast it (it's now an Object) back into a Point datatype before accessing the .x and .y properties of a Point. Which brings up another point (pun intended) -- IEnumerator only knows about Objects. Which means you could (if you were clever or dumb, depending on your viewpoint) cast into another datatype other than a Point. When the next version of .NET comes out (Whidbey) you will be able to use Generics, which prevent this feature/problem.

Chip H.


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