LINQ Exchange

Debugging LINQ queries can be problematic.  One
of the reasons is that quite often, you write a large query as a single
expression, and you can’t set a breakpoint mid-expression.  Writing
large queries in expression context is particularly powerful when using
functional construction to form XML (or using the strongly typed DOM in
Open XML SDK V2).  This post presents a little
trick that makes it easier to use the debugger with LINQ queries that
are written using ‘method syntax’.

The gist of the technique is to insert a call to the Select extension method in the middle of your query.  You code the Select so that it projects exactly the same results as its source, but using a statement lambda expression  If
you place the return statement of the lambda expression on its own
line, you can set a breakpoint and examine values as they make their
way through the query.

The
following is a query to split a string into words, convert the words to
lower case, count the number of occurances of each word, and return the
ten most used words:

var uniqueWords = text

    .Split(' ', '.', ',')

    .Where(i => i != "")

    .Select(i => i.ToLower())

    .GroupBy(i => i)

    .OrderByDescending(i => i.Count())

    .Select(i => new { Word = i.Key, Count = i.Count() })

    .Take(10);

 

But if you set a breakpoint anywhere on the query, you see this:

Let’s say that you want to examine each group and see the group key and the count of items for each group.  You can insert a Select statement, as follows:

var uniqueWords = text

    .Split(' ', '.', ',')

    .Where(i => i != "")

    .Select(i => i.ToLower())

    .GroupBy(i => i)

    .Select(z => {

        return z;

    })

    .OrderByDescending(i => i.Count())

    .Select(i => new { Word = i.Key, Count = i.Count() })

    .Take(10);

 

you
can now set a breakpoint on the ‘return z’ statement, and examine each
group in turn as they are yielded up by the GroupBy extension method:

You can see that the key is “on”, and that there are four items in the group.

After you are done debugging, you can remove the added call to Select.

Alternatively,
you could convert any of the other lambda expressions to a statement
lambda expression, format code so that a statement is on its own line,
and then set a breakpoint.

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