The question in general is: is there a more effective way of implementation of table with structure like Dictionary<int, Dictionary<string, string>>?

The reason I am asking this is because I have made few performance tests and it does not perform well for data with > 5M rows. Now, I dont really need this amount of data but I was wandering if there is a more effective way. It could also help performance for smaller tables with thousands of rows. Last but not least I am interested in what COULD be done to improve it.

What I have thought of is to use string[][] and have some method transform string rows/column to numbers. That would however require a quite significant rewrite of my work so far. Is there something simpler? I need rows to be able to handle gaps.

Background on my project:

I have a home brewed structure of objects that represent a table along with some additional functionality that I need. I have table called T, and it stores data (rows) in Dictionary<int, TRow>. Each TRow has another dictionary<string, TCell> that represents the row data, in which TCells are indexed by column names. TCells is basically a wrapper around simple string. Table and each row has a Schema definition (column -> {INT, DOUBLE, STRING, BOOL, ...} that is parsed when getting data from the table by methods like .getBool( int row, string column ) etc. Each object (T,TRow,TCell) has quite a lot of helper methods that I use, so they are not a simple wrapper with get methods.

EDIT TO ANSWER FOLLOW-UP QUESTIONS: The table is meant for general purpose. No special focus on reading/writing only. The table is often initially loaded from result-set produced by stored procedure in database and then only read from - but not exclusively. The composite key is an interesting idea, but that would break my T, TRow, TCell structure I am afraid. The Dictionary INT X STRING -> STRING is only a simplification, as written in my last paragraph the table T has Dictionary< int, TRow> and TRow has Dictionary< string, string >. The reason I need to keep Table, Row and Cell broken up is that sometimes I work directly with rows, e.g. some method can return a single row etc. Any ideas please? Or there is nothing better :/.

  • 1
    I see 2 options, use a Pair<int, String> as key or cache the Dictionary<string, string> when you need multiple values of the same row Commented Jan 11, 2013 at 11:29

2 Answers 2


It all depends on what operations are you doing often. Writing? Reading? Do you work separately with Dictionary<string, string> or do you always work with Dictionary<int, Dictionary<string, string>> as a whole? Also, you are saying you have rows and columns. Do you add/remove them, or are they constant for each instance of this dictionary?

First thing that comes to mind is to create a composite key from int and string, and use that as a key for dictionary.

public struct IntStringKey
    public int A {get; private set;}
    public string B {get; private set;}

    public IntStringKey(int a, string b)
        A = a;
        B = b;

    // override GetHash and Equals here

And then use it :

Dictionary<IntStringKey, string> data;

This will make it easier on GC, because you are not creating so many internal dictionaries and also you call the get/set only once.

  • Thank you for the idea of composite keys, that sure is interesting approach. For clarifications please see my edit in the question area. Commented Jan 11, 2013 at 13:14
  • @NeverStopLearning I still think you need to optimize according to how you use the data structure. There is not much we can do if you are after generic solution for "all your problems".
    – Euphoric
    Commented Jan 11, 2013 at 13:51
  • Well as I have said the generic workflow is to create this table using database result set and then use this table to display a grid in ASP.NET (i.e. traverse all the cells one by one). But when simplified to this level I might just as well use a DataGridView with bound DataSource. Anyway, I think its fairly obvious that there is no hands-down better option available. I might use your suggestion to refactor the table into base class with common API and children that each implement the storage differently. Commented Jan 11, 2013 at 15:05

I'm not sure this will help speed, but memory-wise, if the keys in TRow are the column ids (ie. the same for every row), then you could do something like this:

class TColumnData
    private readonly IDictionary<string, int> nameToIndex;
    public int IndexFor(string id) => nameToIndex[id];

class TRow
    private IList<TCell> cells;
    private TColumnData columnData;
    public TCell this[string id] => cells[columnData.IndexFor(id)];

With constructors and such as appropriate.

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