C# – Why does ReSharper tell me “implicitly captured closure”


I have the following code:

public double CalculateDailyProjectPullForceMax(DateTime date, string start = null, string end = null)
    Log("Calculating Daily Pull Force Max...");

    var pullForceList = start == null
                             ? _pullForce.Where((t, i) => _date[i] == date).ToList() // implicitly captured closure: end, start
                             : _pullForce.Where(
                                 (t, i) => _date[i] == date && DateTime.Compare(_time[i], DateTime.Parse(start)) > 0 && 
                                           DateTime.Compare(_time[i], DateTime.Parse(end)) < 0).ToList();

    _pullForceDailyMax = Math.Round(pullForceList.Max(), 2, MidpointRounding.AwayFromZero);

    return _pullForceDailyMax;

Now, I've added a comment on the line that ReSharper is suggesting a change. What does it mean, or why would it need to be changed? implicitly captured closure: end, start

Best Solution

The warning tells you that the variables end and start stay alive as any of the lambdas inside this method stay alive.

Take a look at the short example

protected override void OnLoad(EventArgs e)

    int i = 0;
    Random g = new Random();
    this.button1.Click += (sender, args) => this.label1.Text = i++.ToString();
    this.button2.Click += (sender, args) => this.label1.Text = (g.Next() + i).ToString();

I get an "Implicitly captured closure: g" warning at the first lambda. It is telling me that g cannot be garbage collected as long as the first lambda is in use.

The compiler generates a class for both lambda expressions and puts all variables in that class which are used in the lambda expressions.

So in my example g and i are held in the same class for execution of my delegates. If g is a heavy object with a lot of resources left behind, the garbage collector couldn't reclaim it, because the reference in this class is still alive as long as any of the lambda expressions is in use. So this is a potential memory leak, and that is the reason for the R# warning.

@splintor As in C# the anonymous methods are always stored in one class per method there are two ways to avoid this:

  1. Use an instance method instead of an anonymous one.

  2. Split the creation of the lambda expressions into two methods.

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