Copy key values from NameValueCollection to Generic Dictionary

Question

Trying to copy values from an existing NameValueCollection object to a Dictionary. I have the following code below to do that but seems the Add does not accept that my keys and values are as Strings

IDictionary<TKey, TValue> dict = new Dictionary<TKey, TValue>();
public void copyFromNameValueCollection (NameValueCollection a)
{
    foreach (var k in a.AllKeys)
    { 
        dict.Add(k, a[k]);
    }  
}

Note: NameValueCollection contains String keys and values and so I simply want to provide here a method to allow copying of those to a generic dictionary.

Mcwhorter

It's very confusing, because presumably this is in a generic type declaring TKey, TValue type parameters - and your method declares those type parameters too. Add to that the fact that the keys will always be strings... so what do you expect to happen if TKey is (say) Guid? – Caithness

I need a generic dictionary to work with. Please note that this is in building a serializable dictionary class, that also allows for copying of keys and values from NameValueCollection objects – Mcwhorter

Again, what would this do if TKey isn't string? – Caithness

Answer 1

It doesn't make sense to use generics here since you can't assign strings to some arbitrary generic type:

IDictionary<string, string> dict = new Dictionary<string, string>();

public void copyFrom(NameValueCollection a)
{
            foreach (var k in a.AllKeys)
            { 
                dict.Add(k, a[k]);
            }  
}

although you should probably create a method to create a new dictionary instead:

public static IDictionary<string, string> ToDictionary(this NameValueCollection col)
{
    IDictionary<string, string> dict = new Dictionary<string, string>();
    foreach (var k in col.AllKeys)
    { 
        dict.Add(k, col[k]);
    }  
    return dict;
}

which you can use like:

NameValueCollection nvc = //
var dictionary = nvc.ToDictionary();

If you want a general way of converting the strings in the collection into the required key/value types, you can use type converters:

public static Dictionary<TKey, TValue> ToDictionary<TKey, TValue>(this NameValueCollection col)
{
    var dict = new Dictionary<TKey, TValue>();
    var keyConverter = TypeDescriptor.GetConverter(typeof(TKey));
    var valueConverter = TypeDescriptor.GetConverter(typeof(TValue));

    foreach(string name in col)
    {
        TKey key = (TKey)keyConverter.ConvertFromString(name);
        TValue value = (TValue)valueConverter.ConvertFromString(col[name]);
        dict.Add(key, value);
    }

    return dict;
}

Cyclopean

NameValueCollection has String keys and values. I want to be able to copy those over to a generic dictionary. – Mcwhorter

@Mcwhorter - You can't copy it to a generic dictionary, since the key and value types can be anything. You can only copy it to a Dictionary<string, string> or one with less-derived key or value types e.g. object. – Cyclopean

@Mcwhorter - Of course it's correct - you can't say T obj = "abc" for any type T. It will only compile if T is string or a less-derived type. You could convert your strings to the target type, but that isn't clear from your question. – Cyclopean

@Mcwhorter - I've added a general way of converting the keys/values in the collection into the target types. – Cyclopean

I thought you said earlier that it was not possible? – Mcwhorter

@Mcwhorter - It's not possible without converting the strings. The code in your question is trying to insert strings into a dictionary with unknown key and value types, which is not possible. – Cyclopean

This answer doesn't quite work if NameValueCollection has a null key - which is perfectly acceptable according to the NameValueCollection documentation. Before inserting key into Dictionary check if null and insert empty string I guess? – Coppery

Answer 2

Extension method plus linq:

 public static Dictionary<string, string> ToDictionary(this NameValueCollection nvc) {
    return nvc.AllKeys.ToDictionary(k => k, k => nvc[k]);
 }

 //example
 var dictionary = nvc.ToDictionary();

Heilungkiang

Answer 3

parameters.AllKeys.ToDictionary(t => t, t => parameters[t]);

Nappy

Love this. Thank you! – Parthenope

k makes more sense than t to represent "key" – Forzando

Answer 4

Use LINQ:

public static IDictionary<string, string> ToDictionary(this NameValueCollection collection)
{
    return collection.Cast<string>().ToDictionary(k => k, v => collection[v]);
}

Usage:

IDictionary<string, string> dic = nv.ToDictionary();

Whereupon

Answer 5

Super-Short Version

var dataNvc = HttpUtility.ParseQueryString(data);
var dataCollection = dataNvc.AllKeys.ToDictionary(o => o, o => dataNvc[o]);

Lorolla

Answer 6

If you know that your dictionary is always going to contain strings, specify it to contain strings instead of making your class generic:

IDictionary<string, string> dict = new Dictionary<string, string>();

With this, things will "just work" as written (without the generic method specification).

If you need this to be a generic class, and hold generic data, you need some way to convert from string to TKey and string to TValue. You could provide delegates to your copy method to do this:

public void CopyFrom(NameValueCollection a, Func<string, TKey> keyConvert, Func<string, TValue> valueConvert)
{
    foreach(var k in a.AllKeys)
    {
         dict.Add(keyConvert(k), valueConvert(a[k]));
    }
}

You would then need to pass a delegate in that would perform the conversion from string to TValue and string to TKey.

Interatomic

This answer should mention that NameValueCollections can have null keys, and this mismatch must be accounted for in converting to a dictionary, which has a non null key invariant – Furtek

The Dictionary is not always going to contain strings. I simply want it to also be able to copy values from NameValueCollection object in this method. – Mcwhorter

@Mcwhorter Then you need the second mechanism - You need to convert from string to your types – Interatomic

That is what I am seeking help with here. How do I convert the string keys and values in the NameValueCollection to the generic type in this method so I can load those to my generic dictionary object – Mcwhorter

@Mcwhorter There is no "general purpose" way to do this - you need some type of method to do the conversion. My second option shows how to write the method so this can be passed in as a delegate. – Interatomic

Again, the reason for this question is so I can get help with that "option". I need a way to, in my generic Dictionary class, provide a way for copying of data from NameValueCollection objects as well. That is what I am trying to find here. – Mcwhorter

Answer 7

You should not forget about EqualityComparer. But it is not a public property. So, you should use reflection to get it.

public static IEqualityComparer GetEqualityComparer(this NameObjectCollectionBase nameObjectCollection)
  {
  PropertyInfo propertyInfo = typeof(NameObjectCollectionBase).GetProperty("Comparer", BindingFlags.Instance | BindingFlags.NonPublic);
  return (IEqualityComparer)propertyInfo.GetValue(nameObjectCollection);
  }

public static IEqualityComparer<string> GetEqualityComparer(this NameValueCollection nameValueCollection)
  {
  return (IEqualityComparer<string>)((NameObjectCollectionBase)nameValueCollection).GetEqualityComparer();
  }

public static Dictionary<string, string> ToDictionary(this NameValueCollection nameValueCollection)
  {
  Dictionary<string, string> dictionary =
    nameValueCollection.AllKeys.ToDictionary(x => x, x => nameValueCollection[x], nameValueCollection.GetEqualityComparer());
  return dictionary;
  }

Ebonyeboracum