LINQ MAX懒惰评估

Question

I'm having a bit of trouble understanding how lazy evaluation relates to the linq Max operator.

This is my code:

        var someList = new List<someType>
        {
            new someType { decimalValue = 32 }
        };

line1: var valA = someList.Any() ? someList.Max(s => s.decimalValue) : 0.0M;

line2: var valB = someList.Any() ? someList.Max(s => s.decimalValue) : 0.0M;

line3: var valC = valB;

When stepping through the code I see the following behaviour:

After line 1 has executed, valA = 0, valB = 0, valC = 0

After line 2 has executed, valA = 32, valB = 0, valC = 0

After line 3 has executed, valA = 32, valB = 32, valC = 32

Why does valA only show a value after valB is assigned, and why does valB only show a value once valC is assigned?

Below is a screenshot of my debugger. I guess there must be something non-standard about my IDE.

_{(source: toastermagic.co.uk )}

Answer 1

The Max operator executes immediately, it is not deferred at all. What you're seeing is just the normal behavior of the debugger. When it has a line highlighted it means, "I'm about to run this line" rather than, "I just ran this line". You'll always need to be on the 'next' line to see the results of a given line of code.

It seems that you have accounted for this. The fact that you are see all changes (this shouldn't be just LINQ specific) a line late probably means your source code and the executable are a bit out of sync. You should do a clean and re-bulid of the solution and then everything should be fine. I'm guessing that you made some small modification to the source code that didn't get picked up properly in a build and so the line numbers in the executable vs. the source code are each off by one.

If something is really, really wrong, you can try moving the code to a new project entirely (if your code base is small enough) in an attempt to get rid of whatever corrupted or broken file/setting is causing this problem.

Answer 2

Cannot Replicate:

    static void Main(string[] args)
    {
        var data = (from n in Enumerable.Range(0, 10)
                    select new { value = (decimal)n })
                   .ToList();

        var valA = data.Any() ? data.Max(s => s.value) : 0.0M;
        Console.WriteLine("A = {0}, B = {1}, C = {2}", valA, 0, 0);

        var valB = data.Any() ? data.Max(s => s.value) : 0.0M;
        Console.WriteLine("A = {0}, B = {1}, C = {2}", valA, valB, 0);

        var valC = valB;
        Console.WriteLine("A = {0}, B = {1}, C = {2}", valA, valB, valC);

        Console.ReadLine();
    }

Works as expected.

Looks to be a misread of the debugger.