How do I use foreach to write each item when inheriting from NameObjectCollectionBase

  • Thread starter Thread starter Tony Johansson
  • Start date Start date
T

Tony Johansson

Hi!

The complete source code is included.
Here in main I first fill a ListDictionary with some items then I
instansiate a new object of type
MyCollection which is derived from NameObjectCollectionBase.
In the C-tor of MyCollection I add all the items in the ListDirectory to the
baseclass using
this.BaseAdd((String)de.Key, de.Value );

Then in main a pass the object of type MyCollection to a static method
called PrintKeysAndValues
where I want to use foreach to write both the key and the value.
In the original code they use an indexer in this way
public DictionaryEntry this[ int index ]
{
get
{
return ( new DictionaryEntry( this.BaseGetKey(index),
this.BaseGet(index) ) );
}
}
which is called from PrintKeysAndValues and works as expected.
I can use the original way to write key and value and be happy with that but
I want to understand why I can't use the foreach because the baseclass
NameObjectCollectionBase do supply one which I have hopped to use in my
derived class MyCollection but it doesnt't work because the element that I
think is a DictioneryEntry is string so that give runtime error.
I have also tried to use this way but this will only write the value. I have
hopped that this current here would have been
a DictionaryEntry but it's a string.
IEnumerator e = myCol.GetEnumerator();
while (e.MoveNext())
Console.WriteLine(e.Current);

So can somebody explain why it's not possible to a foreach when the
baseclass have one that that derived class can inherit and use ??

using System;
using System.Collections.Generic;
using System.Text;
using System.Collections.Specialized;
using System.Collections;


public class SamplesNameObjectCollectionBase
{
public static void Main()
{
IDictionary d = new ListDictionary();
d.Add("red", "apple");
d.Add("yellow", "banana");
d.Add("green", "pear");
MyCollection myROCol = new MyCollection(d, true);
PrintKeysAndValues(myROCol);
}


public static void PrintKeysAndValues(MyCollection myCol)
{
IEnumerator e = myCol.GetEnumerator();
while (e.MoveNext())
Console.WriteLine(e.Current);

foreach (DictionaryEntry item in myCol)
{
Console.WriteLine("Key={0}, Value = {1}", item.Key, item.Value);
}

//This code work
//for (int i = 0; i < myCol.Count; i++)
//{
// Console.WriteLine("[{0}] : {1}, {2}", i, myCol.Key,
myCol.Value);
//}
}
}


public class MyCollection : NameObjectCollectionBase
{
public MyCollection() {}

public MyCollection( IDictionary d, Boolean bReadOnly )
{
foreach ( DictionaryEntry de in d )
{
this.BaseAdd((String)de.Key, de.Value );
}
this.IsReadOnly = bReadOnly;
}
}


public DictionaryEntry this[ int index ]
{
get { return ( new DictionaryEntry( this.BaseGetKey(index),
this.BaseGet(index) ) ); }
}
}

//Tony
 
Tony said:
[...]
I can use the original way to write key and value and be happy with that but
I want to understand why I can't use the foreach because the baseclass
NameObjectCollectionBase do supply one which I have hopped to use in my
derived class MyCollection but it doesnt't work because the element that I
think is a DictioneryEntry is string so that give runtime error.
I have also tried to use this way but this will only write the value. I have
hopped that this current here would have been
a DictionaryEntry but it's a string.
IEnumerator e = myCol.GetEnumerator();
while (e.MoveNext())
Console.WriteLine(e.Current);

So can somebody explain why it's not possible to a foreach when the
baseclass have one that that derived class can inherit and use ??
Click to expand...

You've already answered the question. It's because the enumerator for
NameObjectCollectionBase doesn't enumerate key/value pairs, but rather
just the keys. This is documented:
http://msdn.microsoft.com/en-us/lib...d.nameobjectcollectionbase.getenumerator.aspx

NameObjectCollectionBase doesn't implement any dictionary interface that
would allow paired enumeration. If you want to retrieve keys and values
in pairs, you can:

– Use the indexer as you've shown in your own code
– Enumerate the keys, and use the BaseGet(String) method overload to
get each value for each key
– Call the BaseGetAllKeys() and BaseGetAllValues() methods to get a
pair of arrays you can enumerate in parallel

Pete
 
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