try this

CurrentModel.DescendantsOrSelf().Items.SingleOrDefault(x => x.Name == collect && x.GetPropertyValue<string>(propertyName) == type);

Thanks Ali - I'm getting this error now:

 The non-generic method 'umbraco.MacroEngines.DynamicNode.GetPropertyValue(string)' cannot be used with type arguments

Here's a little bit more code:

string collect = String.Format("{0}", Request.QueryString["Bottle"]);
string type = String.Format("{0}", Request.QueryString["Type"]);
DynamicNode rep = null;

rep = CurrentModel.DescendantsOrSelf().Items.SingleOrDefault(x => x.Name == collect && x.GetPropertyValue<string>(recipeType) == type);

I see you are using umbraco 4.

can you please try this (removed <string>)

CurrentModel.DescendantsOrSelf().Items.SingleOrDefault(x => x.Name == collect && x.GetPropertyValue(propertyName) == type);

oo, getting close!

I'm now getting an error

Exception: System.NullReferenceException: Object reference not set to an instance of an object.

from:

for (i = 0; i < rep.Children.Count() / numInRow; i++) { 

a loop to do something to each of the collected items in rep. Can I not use .Count() anymore?

I am running Umbraco v6.1.3 (Assembly version: 1.0.4954.19342)

it seems that rep is null and you are trying to get the children of it.

so you should check for null before loop

if(rep != null)
{
    for (i = 0; i < rep.Children.Count() / numInRow; i++) 
    { 
     // your logic 
    }
}

rep =CurrentModel.DescendantsOrSelf().Items.SingleOrDefault(x => x.Name== collect && x.GetPropertyValue<string>(recipeType)== type);

Shouldn't this populate rep so that it's no longer null?

it depends on your data to be honest.

sorry but I'm completetly confused now. 

If I define DynamicNode rep = null;

then define rep = a collection of items

Why would it matter what type of data it is?

the problem here is that the "a collection of items" are null.

to be frank it is really difficult as I am not sure what you would like to do and what data structure you have in Umbraco

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