LINQ Select Distinct While Evaluating Multiple Rows

I have an EnumerableRowCollection that looks like the following:

VendorCode | GroupType | Variance
    01165       G          .16
    01165       G          .16

    01165       CH         .16
    01165       CH         .18

    07754       G          .25
    07754       G          .25
    07754       G          .39

Essentially, this is a massive list of vendor codes, their groups, and price variances. I need to compose a query that will create a distinct list vendor codes and group types. The catch, however, is that I need to evaluate all of the variances associated with that particular VendorCode/GroupType to see if they are all the same - it they are not, I need to return some way of signifying that the group has a "custo开发者_运维知识库m" variance, otherwise it needs to return the value (ie: if they are all .16, then return .16, if there are multiple values, return "custom")

The result would look like this, based off of the list I showed above.

VendorCode | GroupType | Variance
   01165        G          .16
   01165        CH         custom
   07754        G          custom

I have no trouble getting a distinct list of VendorCode/GroupType - this is what I have so far:

Dim distinctList = From q In query Select q.VendorCode, q.GroupType, (evaluated q.Variance here?) Distinct

(where "query" is an EnumerableRowCollection(Of (anonymous type)))

I'm at a loss, though, on how to evaluate the variance property to get the result that I need - any ideas?

I can't do the VB, but in C# I think you'd want something like:

var query = from item in source
            group item by new { item.VendorCode, item.GroupType } into g
            select new { g.Key.VendorCode,
                         g.Key.GroupType,
                         Variance = ComputeVariance(g) };

...

string ComputeVariance(IEnumerable<YourItemType> group)
{
    var distinctVariance = group.Select(x => x.Variance)
                                .Distinct();
    using (var iterator = distinctVariance.GetEnumerator())
    {
        iterator.MoveNext(); // Assume this will return true...
        var first = iterator.Current;
        return iterator.MoveNext() ? "custom" : first.ToString();
    }
}

That's assuming you're using LINQ to Objects. If you need to do this in LINQ to SQL etc, I'd take a slightly different approach.