Using PredicateBuilder and make a predicate on many ID from a table , we have result of many list with filterd ID like this:
predicate builder make a predicate with OR for {aaa,bbb,ccc,ddd} . i want to have first item of each group. is it possible with linq query ?
var predicate = PredicateBuilder.False<someobject>();
foreach (var item in items)
{
predicate = predicate.Or(p => p.id == item.id );
}
...
Unless this is part of a bigger query requiring predicate builder, this simple LINQ
should work:
var result = items.GroupBy(x => x.Id)
.Select(x => x.First());
Or if you want to order the elements first, then choose the first from each group:
var result = items.OrderBy(x => x.Id)
.ThenBy(x => x.Quantity) // not sure what 2nd column is called
.GroupBy(x => x.Id)
.Select(x => x.First());
If you want to build a predicate based on you rule (to get a first item from a group), you need to build a predicate based on this rule:
Example class:
public class Person
{
public int Id { get; set; } //example: "aaa"
public string Name { get; set; } //example: 1
}
Query:
var predicate2 = PredicateBuilder.False<Person>();
foreach (var item in items.GroupBy(x => x.Name).SelectMany(x => x.OrderBy(y => y.Id).Take(1)))
{
predicate2 = predicate2.Or(p => p.Id == item.Id);
}
var fnc2 = predicate2.Compile();
var filtered2 = allItems.Where(fnc2).ToList();
As a result it will build a predicate like this (for data you have provided):
p => p.Id == 1 || p.Id == 10 || p.Id == 15 || p.Id == 17
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