c# - Create a dictionary on a list with grouping

Question

I have the following object in a list:

public class DemoClass
{
    public int GroupKey { get; set; }
    public string DemoString { get; set; }
    public object SomeOtherProperty { get; set; }
}

Now, I want to create following dictionary out of it:

Dictionary<int, List<DemoClass>>

I want to group the List<DemoClass> by the property GroupKey, but I don't understand how this is done and some help.

After thinking a bit, I achieved the needed behaviour with:

var groupedDemoClasses = from demoClass in mySepcialVariableWhichIsAListOfDemoClass
                            group demoClass by demoClass.GroupKey
                            into groupedDemoClass
                            select groupedDemoClass;
var neededDictionary = groupedDemoClass.ToDictionary(gdc => gdc.Key, gdc => gdc.ToList());

but, is there a way to make this into a single statement?

Answer 1

Just to make mquander's suggestion concrete:

var groupedDemoClasses = mySpecialVariableWhichIsAListOfDemoClass
                             .GroupBy(x => x.GroupKey)
                             .ToDictionary(gdc => gdc.Key, gdc => gdc.ToList());

You'd make it shorter if you used shorter variable names too, of course :)

However, might I suggest that a Lookup might be more appropriate? A Lookup is basically a dictionary from a key to an IEnumerable<T> - unless you really need the values as a list, it makes the code even shorter (and more efficient) with the ToLookup call:

var groupedDemoClasses = mySpecialVariableWhichIsAListOfDemoClass
                             .ToLookup(x => x.GroupKey);