How to get the TSQL Query from LINQ DataContext.Su

Question

I'm using Linq to SQL. I have a DataContext against which I am .SubmitChanges()'ing. There is an error inserting the identity field, and I'd like to see the query it's using to insert this identity field.

I don't see the query itself within the quickwatch; where can I find it from within the debugger?

Answer 1

FooDataContext dc = new FooDataContext();

StringBuilder sb = new StringBuilder();
dc.Log = new StringWriter(sb);

var result=from r in dc.Tables select d;

.....
string query=sb.ToString();

Answer 2

Further to Portman's answer, if you're a console application it's as simple as:

myDataContext.Log = Console.Out;

Or you could use something like Linq2SQL Profiler which is a rather excellent tool and in fact the right tool for the job:

Linq to SQL Profiler - Real-time visual debugger for Linq to SQL

Answer 3

I agree that Linq to SQL Profiler is the right tool for this job. But if you don't want to spend the money or just need to do something simple, I like the DebugTextWriter approach.

After reading this question I went off looking for something more robust. It turns out Damien Guard also wrote a very nice article about building different writers to deal with different things like outputting to Memory, Debug, a File, Multiple Destinations, or even using simple Delegates.

I wound up using a couple of his ideas and writing an ActionTextWriter that can handle more than one delegate, and I thought I would share it here:

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;

namespace Writers
{
    public class ActionTextWriter : TextWriter
    {
        protected readonly List<Action<string>> Actions = new List<Action<string>>();

        public ActionTextWriter(Action<string> action)
        {
            Actions.Add(action);
        }

        public ActionTextWriter(IEnumerable<Action<string>> actions)
        {
            Actions.AddRange(actions);
        }

        public ActionTextWriter(params Action<string>[] actions)
        {
            Actions.AddRange(actions);
        }

        public override Encoding Encoding
        {
            get { return Encoding.Default; }
        }

        public override void Write(char[] buffer, int index, int count)
        {
            Write(new string(buffer, index, count));
        }

        public override void Write(char value)
        {
            Write(value.ToString());
        }

        public override void Write(string value)
        {
            if (value == null)
            {
                return;
            }

            foreach (var action in Actions)
            {
                action.Invoke(value);
            }
        }
    }
}

You can add as many actions as you like. This example writes to a log file and the Console in Visual Studio via Debug.Write:

// Create data context
var fooDc = new FooDataContext();

// Create writer for log file.
var sw = new StreamWriter(@"C:\DataContext.log") {AutoFlush = true};

// Create write actions.
Action<string> writeToDebug = s => Debug.Write(s);
Action<string> writeToLog = s => sw.Write(s);

// Wire up log writers.
fooDc.Log = new ActionTextWriter(writeToDebug, writeToLog);

And of course if you want to make simpler ones to use off the cuff, you can always extend ActionTextWriter... write the generic approach and reuse, right?

using System.Diagnostics;
using System.IO;

namespace Writers
{
    public class TraceTextWriter : ActionTextWriter
    {
        public TraceTextWriter()
        {
            Actions.Add(s => Trace.Write(s));
        }
    }

    public class FileTextWriter : ActionTextWriter
    {
        public FileTextWriter(string path, bool append = false)
        {
            var sw = new StreamWriter(path, append) {AutoFlush = true};
            Actions.Add(sw.Write);
        }
    }
}

Answer 4

Run SQL Profiler if you have it. It'll show all traffic to your database, including SQL command text.

Answer 5

There is actually a very simple answer to your question

Just paste this in your watch window

((System.Data.Objects.ObjectQuery)myLinqQueryVar).ToTraceString()

Answer 6

Lots of people have been writing their own "DebugWriter" and attaching it like so:

// Add this class somewhere in your project...
class DebugTextWriter : System.IO.TextWriter {
   public override void Write(char[] buffer, int index, int count) {
       System.Diagnostics.Debug.Write(new String(buffer, index, count));
   }

   public override void Write(string value) {
       System.Diagnostics.Debug.Write(value);
   }

   public override Encoding Encoding {
       get { return System.Text.Encoding.Default; }
   }
}

// Then attach it to the Log property of your DataContext...
myDataContext.Log = new DebugTextWriter()

This will output everything that Linq-to-Sql is doing into Visual Studio's debug window.