Access key after grouping by with dynamic linq

Question

Using System.Dynamic.Linq I have a group by statement that looks like this:

var rowGrouped = data.GroupBy(rGroup, string.Format("new({0})", c));

Where c is just a string array of field names I need to have selected in my grouping. GroupBy in this case returns a IEnumerable (note: not an IEnumerable<T> because we don't know what T is). The regular GroupBy would return a System.Collections.Generic.IEnumerable<IGrouping<TKey, TElement>>

Now my question is, how do I iterate through the groups such that I have access to the key (Key is defined in IGrouping<TKey, TElement> but I don't know TKey and TElement a priori)?

I initially tried this:

foreach (var row in rowGrouped)

Which iterates, but I can't access row.Key (row in this case is type object)

So I did this:

foreach (IGrouping<object,dynamic> row in rowGrouped)

Which, surprisingly, worked...as long as the key was a string. If, however, the key happened to be numeric (for example short), then I get this error:

Unable to cast object of type 'Grouping`2[System.Int16,DynamicClass2]' to type 
'System.Linq.IGrouping`2[System.Object,System.Object]'.

I need to be able to iterate through rowGrouped and get the Key for every row and then iterate through the collection in each group.

Answer 1

The problem is that your key is a short and you're trying to cast to an interface where the key is object. While IGrouping<> has covariant type parameters, it just simply won't work because variance only applies to reference types. Since short is a value type, it will not work.

If you know the key will always be a short, you should cast it as such.

foreach (IGrouping<short, dynamic> row in rowGrouped)
    ...

Otherwise if it's possible that it can be a value type or a reference type, it might just be easier to keep the entire row as dynamic.

foreach (dynamic row in rowGrouped)
    ...

Though I personally have had troubles with this. Perhaps a bug but the runtime cannot figure out that the row has a Key property in this case. I can't tell you why exactly but keep that in mind.