Question

I am joining 2 in memory collections

var items = 
    from el in elements 
    join def in Cached.Elements() 
        on el.Value equals def.Name into temp1
    from res in temp1.DefaultIfEmpty()                        
    select new
    {
        el.NodeType, 
        res.DefKey, 
        res.DefType, 
        res.BaseKey, 
        el.Value 
    };

However, ideally if one of the elements can’t be found, I’d like to raise an exception, something akin to

throw new System.Exception(el.Value + " cannot be found in cache!");

I was looking at the System.Interactive which offers a Catch extension method but I am unsure how to reference the current ‘el’ in that context. So for example I was wondering about something like

    var items = (
        from el in elements 
        join def in Cached.Elements() 
            on el.Value equals def.Name into temp1
        from res in temp1.DefaultIfEmpty()                        
        select new 
        { 
            el.NodeType, 
            res.DefKey, 
            res.DefType, 
            res.BaseKey, 
            el.Value 
        })
        .ThrowIfEmpty();

but, istm, that that would entail passing the whole set into the extension method rather than raising the exception when the missing value is encountered.

Alternatively, I could replace the DefaultIfEmpty with a ThrowIfEmpty

    var items = (
        from el in elements 
        join def in Cached.Elements() 
            on el.Value equals def.Name into temp1
        from res in temp1.ThrowIfEmpty()                        
        select new 
        { 
            el.NodeType, 
            res.DefKey, 
            res.DefType, 
            res.BaseKey, 
            el.Value 
        });

Is there a ‘proper’/better way to do this?

Answer 1

You can use GroupJoin. Something like this should work for you:

elements.GroupJoin(Cached.Elements(), e => e.Value, d => d.Name, (e, dSeq) => {
    var d = dSeq.Single();
    return new { e, d };
});

The GroupJoin resultSelector accepts two arguments: the left key, and the sequence of matching right keys. You can raise an exception if the sequence is empty; one way to achieve that would be to use the Single operator.