How to iterate through List of Dictionaries?

Question

I have the following code :

List<Dictionary<string, string>> allMonthsList = new List<Dictionary<string, string>>(); while (getAllMonthsReader.Read()) { Dictionary<string, string> month = new Dictionary<string, string>(); month.Add(getAllMonthsReader["year"].ToString(), getAllMonthsReader["month"].ToString()); allMonthsList.Add(month); } getAllMonthsReader.Close();

Now I'm trying to loop through all of the months, like this :

foreach (Dictionary<string, string> allMonths in allMonthsList)

How do I access the key values? Am I doing something wrong?

Sergey Berezovskiy · Accepted Answer · 2012-12-05 08:21:23Z

foreach (Dictionary<string, string> allMonths in allMonthsList) { foreach(KeyValuePair<string, string> kvp in allMonths) { string year = kvp.Key; string month = kvp.Value; } }

BTW year usually has more than one month. Looks like you need a lookup here, or Dictionary<string, List<string>> for storing all months of year.

Explanation generic dictionary Dictionary<TKey, TValue> implements IEnumerable interface, which returns an enumerator that iterates through the collection. From msdn:

For purposes of enumeration, each item in the dictionary is treated as a KeyValuePair<TKey, TValue> structure representing a value and its key. The order in which the items are returned is undefined.

The foreach statement of the C# language requires the type of each element in the collection. Since the Dictionary<TKey, TValue> is a collection of keys and values, the element type is not the type of the key or the type of the value. Instead, the element type is a KeyValuePair<TKey, TValue> of the key type and the value type.

Ya i know i need 1 month from it :) but this returns Error 1 'System.Collections.Generic.Dictionary<string,string>' does not contain a definition for 'Key' and no extension method 'Key' accepting a first argument of type 'System.Collections.Generic.Dictionary<string,string>' could be found (are you missing a using directive or an assembly reference?) when trying to use the string.
@lazyberezovsky i think he ment you have a syntax error there.
ya i have. this returns this only for using year and not month, i tried ToString() it but didn't work.

Levi Botelho · Accepted Answer · 2012-12-05 08:13:33Z

3

var months = allMonthsList.SelectMany(x => x.Keys);

You can then iterate through the IEnumerable<string> as you please which is a simple enumeration of all your keys.

edited Dec 5, 2012 at 8:13

answered Dec 5, 2012 at 8:07

Levi Botelho

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1 Comment

Dan Field Over a year ago

Alternatively, if you want to iterate directly through the KeyValuePairs, you can do allMonthsList.SelectMany(x => x)

Hamlet Hakobyan · Accepted Answer · 2012-12-05 08:32:36Z

Your design is wrong. Using one pair in dictionary is meaningless. You don't need to use list of dictionary.

Try this:

class YearMonth { public string Year { get; set; } public string Month { get; set; } } List<YearMonth> allMonths = List<YearMonth>(); while (getAllMonthsReader.Read()) { allMonths.Add(new List<YearMonth> { Year = getAllMonthsReader["year"].ToString(), Month = getAllMonthsReader["month"].ToString() }); } getAllMonthsReader.Close();

Use as:

foreach (var yearMonth in allMonths) { Console.WriteLine("Year is {0}, Month is {1}", yearMonth.Year, yearMonth.Month); }

or, if you use .Net framework 4.0 or above, you can use Tuple

List<Tuple<string, string>> allMonths = List<Tuple<string, string>>(); while (getAllMonthsReader.Read()) { allMonths.Add(Tuple.Create( getAllMonthsReader["year"].ToString(), getAllMonthsReader["month"].ToString()) ); }

getAllMonthsReader.Close();

Then use:

foreach (var yearMonth in allMonths) { Console.WriteLine("Year is {0}, Month is {1}", yearMonth.Item1, yearMonth.Item2); }

It's not necessarily meaningless - what if the number of fields isn't known until runtime? All your suggestions require that knowledge at compile time (which would work if the months list is just Jan-Dec simply, but not if it's a list of repeating months with other data that is read in from input).

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