score:1

you need to materialize the results using tolist() or something similar in order to use your extension method.

var list = (a in db.modelaset
    select a).tolist();

foreach(var item in list)
{
    var query2 = b in list
       where b.code.truncatedcode() == item.code.truncatedcode()   // this doesn't work :(
       select b;
}

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