FindIndex on list by linq

Question

How may i get index using linq ? I want to find by FieldNo and get back to index. let say if i search 2 it should be return index 1.

Regards,

Answer 1

With LINQ:

int index = fields.Select((f, i) => new { Field = f, Index = i})
    .Where(x => x.Field.FieldNo == 2)
    .Select(x => x.Index)
    .DefaultIfEmpty(-1)
    .First();

without LINQ using List.FindIndex, more readable, efficient and works even on .NET 2:

int index = fields.FindIndex(f => f.FieldNo == 2);

---

The above code used anonymous-types which are GC heap-allocated, which may cause performance issues - so if you're using .NET Core or .NET Framework 4.7 or later we should use ValueTuple instead which avoids using the heap at all for maximum performance:

int index = fields
    .Select((f, idx) => (f, idx))
    .Where(t => t.f.FieldNo == 2)
    .Select(t => t.idx)
    .DefaultIfEmpty(-1)
    .First();

Or slightly more succinctly by using FirstOrDefault and the Elvis operator:

int index = fields
    .Select((f, idx) => (f, idx))
    .FirstOrDefault(t => t.f.FieldNo == 2)
    ?.idx ?? -1;

Answer 2

If i understood your question right, this is what you need:

Field field = Field.Where(x => x.FieldNo == 2).FirstOrDefault();
if (field != null)
    {
        Field.IndexOf(field);
    }