IDictionaryEnumerator Interface

Enumerates the elements of a nongeneric dictionary.

Namespace: System.Collections
Assembly: mscorlib (in mscorlib.dll)

public interface IDictionaryEnumerator : IEnumerator
/** @attribute ComVisibleAttribute(true) */ 
public interface IDictionaryEnumerator extends IEnumerator
public interface IDictionaryEnumerator extends IEnumerator
Not applicable.

The foreach statement of the C# language (for each in Visual Basic) hides the complexity of the enumerators. Therefore, using foreach is recommended instead of directly manipulating the enumerator.

Enumerators can be used to read the data in the collection, but they cannot be used to modify the underlying collection.

Initially, the enumerator is positioned before the first element in the collection. The Reset method also brings the enumerator back to this position. At this position, calling the Current property throws an exception. Therefore, you must call the MoveNext method to advance the enumerator to the first element of the collection before reading the value of Current.

Current returns the same object until either MoveNext or Reset is called. MoveNext sets Current to the next element.

If MoveNext passes the end of the collection, the enumerator is positioned after the last element in the collection and MoveNext returns false. When the enumerator is at this position, subsequent calls to MoveNext also return false. If the last call to MoveNext returned false, calling Current throws an exception. To set Current to the first element of the collection again, you can call Reset followed by MoveNext.

An enumerator remains valid as long as the collection remains unchanged. If changes are made to the collection, such as adding, modifying, or deleting elements, the enumerator is irrecoverably invalidated and the next call to MoveNext or Reset throws an InvalidOperationException. If the collection is modified between MoveNext and Current, Current returns the element that it is set to, even if the enumerator is already invalidated.

The enumerator does not have exclusive access to the collection; therefore, enumerating through a collection is intrinsically not a thread-safe procedure. Even when a collection is synchronized, other threads can still modify the collection, which causes the enumerator to throw an exception. To guarantee thread safety during enumeration, you can either lock the collection during the entire enumeration or catch the exceptions resulting from changes made by other threads.

Notes to Implementers: The Current property that is inherited from IEnumerator returns an Object that is a boxed DictionaryEntry, similar to the return value of the Entry property.

This code example shows how to define a dictionary enumerator that implements the IDictionaryEnumerator interface.

    private class SimpleDictionaryEnumerator : IDictionaryEnumerator
        // A copy of the SimpleDictionary object's key/value pairs.
        DictionaryEntry[] items;
        Int32 index = -1;

        public SimpleDictionaryEnumerator(SimpleDictionary sd)
            // Make a copy of the dictionary entries currently in the SimpleDictionary object.
            items = new DictionaryEntry[sd.Count];
            Array.Copy(sd.items, 0, items, 0, sd.Count);

        // Return the current item.
        public Object Current { get { ValidateIndex(); return items[index]; } }

        // Return the current dictionary entry.
        public DictionaryEntry Entry
            get { return (DictionaryEntry) Current; }

        // Return the key of the current item.
        public Object Key { get { ValidateIndex();  return items[index].Key; } }

        // Return the value of the current item.
        public Object Value { get { ValidateIndex();  return items[index].Value; } }

        // Advance to the next item.
        public Boolean MoveNext()
            if (index < items.Length - 1) { index++; return true; }
            return false;

        // Validate the enumeration index and throw an exception if the index is out of range.
        private void ValidateIndex()
            if (index < 0 || index >= items.Length)
            throw new InvalidOperationException("Enumerator is before or after the collection.");

        // Reset the index to restart the enumeration.
        public void Reset()
            index = -1;
    public IDictionaryEnumerator GetEnumerator()
        // Construct and return an enumerator.
        return new SimpleDictionaryEnumerator(this);

    #region ICollection Members
    public bool IsSynchronized { get { return false; } }
    public object SyncRoot { get { throw new NotImplementedException(); } }
    public int Count { get { return ItemsInUse; } }
    public void CopyTo(Array array, int index) { throw new NotImplementedException(); }

    #region IEnumerable Members
    IEnumerator IEnumerable.GetEnumerator() 
        // Construct and return an enumerator.
        return ((IDictionary)this).GetEnumerator();

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0, 1.0

XNA Framework

Supported in: 1.0

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