C# Dynamic LINQ: Select with Sum on dictionary indexer

I'm using dynamic LINQ to create a groupby and select on the fly. My items are key/value collections (dictionaries) so they contain no properties (it's a design requirement and can't be changed). I was able to solve the groupby part in another question, but it doesn't seem to work in the select method.

My code below:

    private void GetValuesGroupedBy(List<Dictionary<string, object>> list, List<string> groupbyNames, List<string> summableNames)
    {
        // build the groupby string
        StringBuilder groupBySB = new StringBuilder();
        groupBySB.Append("new ( ");
        bool useComma = false;
        foreach (var name in groupbyNames)
        {
            if (useComma)
                groupBySB.Append(", ");
            else
                useComma = true;

            groupBySB.Append("it[\"");
            groupBySB.Append(name);
            groupBySB.Append("\"]");
            groupBySB.Append(" as ");
            groupBySB.Append(name);
        }
        groupBySB.Append(" )");

        // and now the select string
        StringBuilder selectSB = new StringBuilder();
        selectSB.Append("new ( ");
        useComma = false;
        foreach (var name in groupbyNames)
        {
            if (useComma)
                selectSB.Append(", ");
            else
                useComma = true;

            selectSB.Append("Key.")
                //.Append(name)
                //.Append(" as ")
                .Append(name);
        }
        foreach (var name in summableNames)
        {
            if (useComma)
                selectSB.Append(", ");
            else
                useComma = true;

            selectSB.Append("Sum(")
                .Append("it[\"")
                .Append(name)
                .Append("\"]")
                .Append(") as ")
                .Append(name);
        }
        selectSB.Append(" )");

        var groupby = list.GroupBy(groupBySB.ToString(), "it");
        var select = groupby.Select(selectSB.ToString());
    }

The Key part of the select string is ok but the Sum part doesn't work. Assuming the key I want is called value, I've tried:

• "Sum(value)" : ParseException: Expression expected

• "Sum(\"value\")" : ParseException: Expression expected

• "Sum(it[\"value\"])" : ParseException : No applicable aggregate method 'Sum' exists

• "Sum(it[value])" : ParseException : No property or field 'value' exists in type 'Dictionary'

• "Sum([\"value\"])" : ParseException: Expression expected

But all have failed. Any ideas?

Thanks! Sean