C# Iterate through NameValueCollection

Question

I have a NameValueCollection, and want to iterate through the values. Currently, I’m doing this, but it seems like there should be a neater way to do it:

NameValueCollection nvc = new NameValueCollection();
nvc.Add("Test", "Val1");
nvc.Add("Test2", "Val1");
nvc.Add("Test2", "Val1");
nvc.Add("Test2", "Val2");
nvc.Add("Test3", "Val1");
nvc.Add("Test4", "Val4");

foreach (string s in nvc)
    foreach (string v in nvc.GetValues(s))
        Console.WriteLine("{0} {1}", s, v);

Console.ReadLine();

Is there?

Wry

What's wrong with what you have? – Arad

There's nothing wrong with it per se - just that I thought I should be able to iterate using a single loop. Looking at the answers so far, this doesn't seem to be possible if there may be duplicate key values. – Wry

that's correct, but you can use different collection e.g. Dictionary<string, List<string>> – Faceoff

see also #391523 – Schilt

Answer 1

You can flatten the collection with Linq, but it's still a foreach loop but now more implicit.

var items = nvc.AllKeys.SelectMany(nvc.GetValues, (k, v) => new {key = k, value = v});
foreach (var item in items)
    Console.WriteLine("{0} {1}", item.key, item.value);

The first line, converts the nested collection to a (non-nested) collection of anonymous objects with the properties key and value.

It's flatten in the way that it's now a mapping key -> value instead of key -> collection of values. The example data:

Before:

Test -> [Val],

Test2 -> [Val1, Val1, Val2],

Test3 -> [Val1],

Test4 -> [Val4]

After:

Test -> Val,

Test2 -> Val1,

Test2 -> Val1,

Test2 -> Val2,

Test3 -> Val1,

Test4 -> Val4

Disuse

Tested and using using System.Linq; – Disuse

In case someone wants to know the vb.net syntax: Dim items = nvc.AllKeys.SelectMany(AddressOf col.GetValues, Function(k, v) New With {.key = k, .value = v}) – Whitehorse

@EndyTjahjono slight typo, change "col" to "nvp" so it becomes Dim items = nvc.AllKeys.SelectMany(AddressOf nvp.GetValues, Function(k, v) New With {.key = k, .value = v}) – Brumbaugh

Wouldn't v simply refer to the index of the current key? – Borrell

What if it's a blank value, how do I just get add string.empty instead? – Piton

what is a blank value? null? – Disuse

Alternatively: nvc.AllKeys.Select(k => $"{k}={nvc[k]}"). Formats as a key and its comma-separated values. – Savoy

This way is too inefficient! using AllKeys, SelectMany, GetValues, Creating new anonym/dynamic object.... too much to just enumerate a simple collection! – Cobble

Answer 2

You can use the key for lookup instead of having two loops:

foreach (string key in nvc)
{
    Console.WriteLine("{0} {1}", key, nvc[key]);
}

Nonplus

Note that this will behave differently to the OP's code if there are any keys with multiple values. Your code will output "key value1,value2,value3", whereas the OP's code would output "key value1" then "key value2" then "key value3". – Basinet

Answer 3

Nothing new to see here (@Julian's +1'd by me answer is functionally equivalent), y'all move along y'all please.

---

I have an [overkill for this case but possibly relevant] set of extension methods in an answer to a related question, which would let you do:

foreach ( KeyValuePair<string,string> item in nvc.AsEnumerable().AsKeyValuePairs() )
    Console.WriteLine("{0} {1}", item.key, item.value);

Schilt

NameValueCollection does not have AsEnumerable or as keyValuePairs – Compressibility

@Luis Tellez. I know. The text says I have a set of extension methods. And it links to it. And the body text is flagging that there is nothing new to see here please. Does this clarify or do I need to re-explain? – Schilt

You can make an enumerable as IEnumerable<KeyValuePair<string,string>> e = nvc.AllKeys.Select(k => new KeyValuePair<string, string>(k, nvc[k])). Another stumbling point brought to you by Microsoft. – Savoy

Answer 4

I think this is simpler: VB.NET

For i As Integer = 0 To nvc.Count - 1
   Console.Write("No", "Key", "Value")
   Console.Write(i, nvc.GetKey(i), nvc.Get(i))
Next

C#:

for (int i = 0; i <= nvc.Count - 1; i++)
{
    Console.Write("No", "Key", "Value");
    Console.Write(i, nvc.GetKey(i), nvc.Get(i));
}

Dail

This is the worst C# I've ever seen. – Biosphere

Sorry, I think in vb.net! Updated – Dail

Answer 5

var enu = myNameValueCollection.GetEnumerator();
while (enu.MoveNext())
{
    string key = (string)enu.Current;
    string value = myNameValueCollection[key];
}

OR when keys nullable:

for (int i = 0; i < myNameValueCollection.Count; i++)
{
    string key = myNameValueCollection.GetKey(i);
    string value = myNameValueCollection.Get(i);
}

Kentledge

Answer 6

The only way I found to avoid the nested loops is using additional List to store the values:

List<string> arrValues = new List<string>();
for (int i = 0; i < nvc.Count; i++)
    arrValues.AddRange(nvc.GetValues(i));
foreach (string value in arrValues)
    Console.WriteLine(value);

(Requires [only] .NET 2.0 or later)

Faceoff

(obviously superseded by using SelectMany like the other answer) – Schilt

@Ruben true, but I prefer to leave this one even if just for historical value showing how to do that with old .NET version. :) – Faceoff

The thing with SelectMany i came across is if the key has a value that's blank it fails. Any way around it? Thanks. – Piton

@Piton what exactly fails? In theory, just throw an if statement and check for null, though if your code is one-liner, it will make it much less readable. – Faceoff

Your method works with the null but the LINQ doesn't is what I was stating... sorry for not clear. – Piton

Answer 7

for (int i = 0; i < nvc.Count; i++) {
    System.Console.WriteLine(

        nvc.AllKeys[i] + " = " + nvc[i]

    );
}

Iniquity

Answer 8

foreach ( string key in nvc.Keys )
   Console.WriteLine("{0} {1}", key, nvc[key]);

This will return you all keys and corresponding values.

Bushhammer

No; it won't return duplicate values. – Staghound

Isnt this a dup of https://mcmap.net/q/237565/-c-iterate-through-namevaluecollection which is milliseconds earlier (wow!) - personally I'd delete. @Slaks to be pedantic, it will give the duplicate values, just with commas separating them. – Schilt