Question

I learning LINQ, and i want to use pending request, but i have this problem

            List<string> _strs = new List<string> { "1", "2", "1", "1", "0" };
            var selind = _strs.Select((name, ind) => new { indexname = name, index = ind }).Where(o => o.indexname == "1");
            string sind = "";
            foreach (var item in selind)
                sind += item.index.ToString() + " ";
            //i get 0 2 3 
            _strs.Add("2");
            _strs.Add("1");
            sind = "";
            foreach (var item in selind)
                sind += item.index.ToString() + " ";
            //Good, i get 0 2 3 6
            _strs = new List<string>() { "1" };
            sind = "";
            foreach (var item in selind)
                sind += item.index.ToString() + " ";
            //Why i get again 0 2 3 6

Ok i understand why, but I wolud like to know two things:

• How should i clear memory?

  selind = null;
  Or you can tell me more nice way?

• For work with selind after total rebuild of _strs i find two way

          _strs.Clear();
          _strs.Add();
  

or call again

selind = _strs.Select((name, ind) => new { indexname = name, index = ind }).Where(o => o.indexname == "1");

Can you advice me another way?

Thanks in advance!

Answer 1

Your query:

_strs.Select((name, ind) => new { indexname = name, index = ind }).Where(o => o.indexname == "1");

Is tied to a particular list reference in memory (whatever _strs is at that time), not to a particular variable name. They are not the same thing. When you do this:

_strs = new List<string>() { "1" };

You’re not clearing the memory reference of where _strs originally pointed. You are instead having that variable name point to a new memory location. Whereas _strs.Clear() does clear the original list.

The best solution for your problem is to wrap the LINQ query in a function that accepts a list, so you can call it again on new lists without typing it again. Alternatively, depending on your use case, just call .Clear() when you need to start again.

(in case this wasn’t clear, _strs = null does nothing to the list that _strs used to point to, it just makes that particular variable name invalid)