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How to: Iterate Over a User-Defined Collection with for each

 

The latest version of this topic can be found at How to: Iterate Over a User-Defined Collection with for each.

For a class to be a managed collection, it needs a non-private GetEnumerator function that returns a handle to an Enumerator class or an interface. An enumerator class must contain the declaration for non-static MoveNext function and Current property.

Simple user defined collection with reference types.

// for_each_user_defined_collections.cpp  
// compile with: /clr  
using namespace System;  
public interface struct IMyEnumerator {  
   bool MoveNext();  
   property Object^ Current {  
      Object^ get();  
   }  
   void Reset();  
};  
  
public ref struct MyArray {     
  
   MyArray( array<int>^ d ) {  
      data = d;  
   }  
  
   ref struct enumerator : IMyEnumerator {  
      enumerator( MyArray^ myArr ) {  
         colInst = myArr;  
         currentIndex = -1;  
      }  
  
      virtual bool MoveNext() {  
         if( currentIndex < colInst->data->Length - 1 ) {  
            currentIndex++;  
            return true;  
         }  
         return false;  
      }  
  
      property Object^ Current {  
         virtual Object^ get() {  
            return colInst->data[currentIndex];  
         }  
      };  
  
      virtual void Reset() {}  
      ~enumerator() {}  
  
      MyArray^ colInst;  
      int currentIndex;  
   };  
  
   array<int>^ data;  
  
   IMyEnumerator^ GetEnumerator() {  
      return gcnew enumerator(this);  
   }  
};  
  
int main() {  
   int retval = 0;  
  
   MyArray^ col = gcnew MyArray( gcnew array<int>{10, 20, 30 } );  
  
   for each ( Object^ c in col )  
      retval += (int)c;  
  
   retval -= 10 + 20 + 30;  
  
   for each ( int c in gcnew MyArray( gcnew array<int>{10, 20, 30 } ) )  
      retval += c;  
  
   retval -= 10 + 20 + 30;  
  
   Console::WriteLine("Return Code: {0}", retval );  
   return retval;  
}  

Return Code: 0  

for each, in

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