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c#linqdebugginglinq-to-objects
Thomas
Thomas

Reputation: 34188

How to debug LINQ result

i am new in linq. so sometime just do not understand how linq is working. so there any tool or buil-in mechanism in VS2010 IDE to debug linq execution in detail. suppose i have this linq query

var to_search = new[] { "Geo", "JCB" };

var result = from sr in list
             let w = to_search.FirstOrDefault(ts => sr.Title.ToLower().Contains(ts.ToLower()))
             where w != null
             let a = new {sr=sr, word=w.ToLower()}
             group a by a.word into g
             orderby g.Count() descending
             let sorted = g.OrderByDescending(a=> a.sr.Title.Select((c, i) => a.sr.Title.Substring(i)).Count(sub => sub.ToLower().StartsWith(a.word)))
             from a in sorted 
             select a.sr;

var completeList = result.Concat(list.Except(result));

how to debug this above linq query in detail. please guide me. thanks.

Upvotes: 4

Views: 6727

Answers (3)

Michael Sorens
Michael Sorens

Reputation: 36688

Much can be written in response to this deceptively simple question! In fact, I have written at length in my article LINQ Secrets Revealed: Chaining and Debugging on Simple-Talk.com. Here is a summary of its key points:

  • You can step into some LINQ queries with the debugger, but it depends on the composition of expressions and statements in the query (because you can only step onto statements).
  • A LINQ method chain works because of a fundamental rule: each non-terminal method takes IEnumerable<T> as input and returns IEnumerable<T> as output.
  • You can inject a "no-op" statement to give you a "stepping stone", if you will, as long as it conforms to that fundamental rule. That is, realizing that you could always put .Select(z => z) in a method chain without consequence, use a variation of that, to wit: z => { return z; }.
  • You can similarly inject a diagnostic method to provide output rather than just provide a potential breakpoint. LINQPad, the outstanding scratchpad for not just LINQ but C# in general, provides this in the guise of its powerful Dump() method. Dump is an object visualizer that provides amazing visualizations of complex data structures.
  • Based on work by Bart De Smet in his informative article LINQ to Objects – Debugging you can bring a simplified flavor of Dump back into Visual Studio--I provide code attached to my article mentioned at the top.
  • Thanks to Robert Ivanc's work on LINQPad Visualizer you can even bring the LINQPad visualizer into Visual Studio (though you need to fire it off manually for individual expressions; you cannot hook it up to a Dump() method).

As a brief example, consider this simple method chain:

string[] Words = new string[]
{"   KOOKABURRA", "Frogmouth", "kingfisher   ", "loon", "merganser"};

Words
        .Select(word => word.Trim())
        .Select(word => word.ToLower())
        .Where(word => word.StartsWith("k"))
        .OrderBy(word => word);

Once you include the Dump extension method in your Visual Studio project, you can minimally instrument it like this...

Words
    .Select(word => word.Trim())
    .Dump()
    .Select(word => word.ToLower())
    .Dump()
    .Where(word => word.StartsWith("k"))
    .Dump()
    .OrderBy(word => word)
    .Dump();

... or more elaborately like this...

Words
    .Dump(w => "ORIGINAL: " + w, ConsoleColor.Yellow)
    .Select(word => word.Trim())
    .Dump(w => "TRIMMED: " + w, ConsoleColor.Yellow)
    .Select(word => word.ToLower())
    .Dump(w => "LOWERCASE: " + w, ConsoleColor.Green)
    .Where(word => word.StartsWith("k"))
    .Dump(w => "FILTERED to 'K': " + w, ConsoleColor.Red)
    .OrderBy(word => word)
    .Dump(w => "SORTED: " + w, ConsoleColor.Blue);

... to get output rendered as either the left side or right side of the figure, respectively: Diagnostic output from Dump method in Visual Studio

As a teaser, I will say that while this is indeed useful, you really must see the enhanced visualization that LINQPad can do with the same output (here is the link again for your convenience).

Upvotes: 8

WizardDBA
WizardDBA

Reputation: 46

The way I do is inserting breakpoints in the query, or use a tool called LINQPad. http://www.linqpad.net/

Upvotes: 1

Justin Pihony
Justin Pihony

Reputation: 67065

I believe that you should be able to place break points just as you normally would in any other code. I am not 100% sure as I often write the LINQ syntax out as the matching extension methods.

But, that is another option, you could rewrite the query using extension methods and add break points on each method.

When in doubt, though, use F11 :)

Also, LINQPad should be able to help a good bit here.

Upvotes: 0

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