How to debug a LINQ Statement

Question

I have a Linq to objects statement

 var confirm = from l in lines.Lines 
 where (l.LineNumber == startline.LineNumber) || (l.LineNumber == endline.LineNumber         


        

Answer 1

It is possible to step inside the LINQ expression without setting any temporary breakpoints. You need to step into the function which evaluates the LINQ expression, e.g.:

var confirm = from l in lines.Lines 
              where (l.LineNumber == startline.LineNumber)
                    || (l.LineNumber == endline.LineNumber) 
              select l;

 confirm.ToArray(); // Press F11 ("Step into") when you reach this statement

 foreach(var o in q) // Press F11 when "in" keyword is highlighted as "next statement"
    // ...

Answer 2

From the looks of the error I would suggest you take a look at line.Lines and make sure its enumerator is implemented properly. I think it's returning a null when it shouldn't.

Oh and just make sure the line and line.Lines objects aren't null or returning nulls as well.

Answer 3

Yes it is indeed possible to pause execution midway through a linq query.

Convert your linq to query style using lambda expressions and insert a Select statement that returns itself somewhere after the point in the linq that you want to debug. Some sample code will make it clearer -

        var query = dataset.Tables[0].AsEnumerable()
            .Where (i=> i.Field<string>("Project").Contains("070932.01"))
 //         .Select(i =>
 //         {return i;}
 //           )
            .Select (i=>i.Field<string>("City"));

Then uncomment the commented lines. Make sure the {return i;} is on its own line and insert a debug point there. You can put this select at any point in your long, complicated linq query.