Question

I have a generic Dictionary where TValue is of type String (Dictionary<int, string>). I chose to use string as the value type because the data was loaded from an Xml file where the source values can be character or numeric data types (I suppose Object would’ve been an acceptable TValue type too, but even then this question would be equally applicable).

The character data types also have importance, so excluding them outright isn’t an option.

I’d like to extract a subset of this Dictionary<int, double>. In other words, I’d like the subset of the dictionary where the values are numeric.

Right now I’m doing it like this:

Dictionary<int, string> myDictionary;
// Do some loading.
var numericData = myDictionary.Where(kvp => Double.TryParse(kvp.Value, out temp)       

This approach is awfully ugly and doesn’t get me the result as a Dictionary<int, double> Can anyone offer other ways to improve this?

Thanks!

Answer 1

The code you’ve given is not only ugly – it will fail with an InvalidCastException at execution time. I suspect you actually want:

var numericData = myDictionary
        .Select(kvp => {
                    double value;
                    return new { kvp.Key,
                        Value = double.TryParse(kvp.Value, out value) 
                                   ? value : (double?) null
                    };
               })
        .Where(pair => pair.Value != null)
        .ToDictionary(pair => pair.Key, pair => pair.Value.Value);

And yes, that’s ugly – but:

• It avoids parsing the value more than once
• It avoids putting side-effects in your query

You can make it slightly cleaner but less efficient if you’re happy to parse twice:

var numericData = myDictionary
       .Where(kvp => { double tmp; return double.TryParse(kvp.Value, out tmp); })
       .ToDictionary(pair => pair.Key, pair => double.Parse(pair.Value));

Or (more cleanly) you could create a separate method:

public static double? TryParseNullableDouble(string text)
{
    double value;
    return double.TryParse(text, out value) ? value : (double?) null;
}

Then the first version becomes:

var numericData = myDictionary
        .Select(kvp => new { kvp.Key, TryParseNullableDouble(kvp.Value) })
        .Where(pair => pair.Value != null)
        .ToDictionary(pair => pair.Key, pair => pair.Value.Value);