Thottam R. Sriram

Generic list and powershell

thottams@microsoft.com — Mon, 12 Oct 2009 21:18:00 GMT

I met with an accident recently riding my motorcycle and hence was unable to blog for some time. Here is my post after a brief break.

I wanted to create a generic list of my class in power shell. It was an interesting experience. Here is what I found.

When I tried to create a generic list of say type int it was straight forward ... well sort of ... the syntax was awkward, but still worked.

$list = New-Object "System.Collections.Generic.List``1[System.Int32]"

$list.Add(3)

$list

I wrote a sample assembly as below:

using System;

public class Sample

{

public Sample() {}

public void SampleMethod() { Console.WriteLine("Hello");}

}

Compiled this to sample.dll as csc /target:library sample.cs

[Reflection.Assembly]::LoadFrom("c:\Sample\Sample.dll")

Make sure if the assembly is loaded

[System.AppDomain]::CurrentDomain.GetAssemblies()

Now add the class Sample to the list. It gave me the error below.

$list = New-Object "System.Collections.Generic.List``1[Sample]"

New-Object : Cannot find type [System.Collections.Generic.List`1[Test]]: make sure the assembly containing this type is loaded.

At line:1 char:19 + $list = New-Object <<<< "System.Collections.Generic.List``1[Test]"

I am not sure why this is not working as the syntax is very similar to the one I had above for Int32. After some search and experimenting I figured out another way to get the generic list working.

$si = new-object Sample

$st = [Type] $si.GetType()

$base = [System.Collections.Generic.List``1]

$qt = $base.MakeGenericType(@($st))

$so = [Activator]::CreateInstance($qt)

$so.Add($si)

write-host $so

Sample

$so[0].SampleMethod()

Hello

Debug .NET Framework Source Code and loading SOS in Visual Studio

thottams@microsoft.com — Fri, 08 May 2009 22:32:00 GMT

I found this blog post from Shawn Burke to be very useful. Comes in handy if you have to step through .NET source when debugging.

http://blogs.msdn.com/sburke/archive/2008/01/16/configuring-visual-studio-to-debug-net-framework-source-code.aspx

Another helpful post is to load sos from Visual Studio which is explained here:

http://blogs.msdn.com/vijaysk/archive/2007/11/15/sos-your-visual-studio.aspx

Asynchronous delegates and callback

thottams@microsoft.com — Sat, 11 Apr 2009 21:42:00 GMT

I was debugging some issue related to asynchronous delegates and wanted to share that information with everyone.

When you do a BeginInvoke the delegate is invoked on a thread from the thread pool and on completion the callback is also called on the same thread pool thread. Now, any exceptions that is thrown on this thread cannot be caught by the main thread and so will crash the process. If you want to handle the exception, then you have to catch the expected exception in the callback and store it to some state and rethrow the exception from the main thread so that the callers above you can handle it gracefully.

In the sample below if you remove the try catch block in the callback or replace the exception.Add(e) with a throw, then the exception will crash the process. If you swallow the exception then you will be hiding failures and continuing which could result in other failures.

using System;

using System.Threading;

using System.Runtime.Remoting.Messaging;

using System.Collections.Generic;

namespace Delegate

{

public class SampleClass

{

public static bool SampleMethod()

{

Console.WriteLine("Inside sample method ...");

throw new ArgumentException();

}

public delegate bool SampleMethodCaller();

public class DelegateSample

{

List<Exception> exceptions;

ManualResetEvent waiter;

public DelegateSample()

{

exceptions = new List<Exception>();

}

public void CallBackMethodForDelegate(IAsyncResult result)

{

SampleMethodCaller smd = (SampleMethodCaller)((AsyncResult)result).AsyncDelegate;

try

{

bool returnValue = smd.EndInvoke(result);

}

catch (ArgumentException e)

{

exceptions.Add(e);

}

finally

{

waiter.Set();

}

public void CallDelegateUsingCallBack()

{

try

{

waiter = new ManualResetEvent(false);

SampleMethodCaller smd = new SampleMethodCaller(SampleClass.SampleMethod);

IAsyncResult result = smd.BeginInvoke(CallBackMethodForDelegate, null);

waiter.WaitOne();

if (exceptions.Count != 0)

{

throw exceptions[0];

}

catch (ArgumentException)

{

Console.WriteLine("Catch exceptions here ...");

}

public static void Main()

{

DelegateSample ds = new DelegateSample();

ds.CallDelegateUsingCallBack();

Console.WriteLine(" -----------------");

}

System.Data.DataException and System.Data.DataSet.RaiseMergeFailed exception

thottams@microsoft.com — Wed, 25 Mar 2009 03:06:00 GMT

This was another interesting issue I encountered recently. The error presented itself as an event log entry from our logging as below:

Unhandled Exception: System.Data.DataException: <target>.Column2 and <source>.Column2 have conflicting properties: DataType property mismatch.

at System.Data.DataSet.RaiseMergeFailed(DataTable table, String conflict, MissingSchemaAction missingSchemaAction)

at System.Data.Merger.MergeSchema(DataTable table)

at System.Data.Merger.MergeTableData(DataTable src)

at System.Data.Merger.MergeDataSet(DataSet source)

at System.Data.DataSet.Merge(DataSet dataSet, Boolean preserveChanges, MissingSchemaAction missingSchemaAction)

at System.Data.DataSet.Merge(DataSet dataSet)

at ConsoleApplication1.Program.TryMerge(DataSet original)

at ConsoleApplication1.Program.DemonstrateMerge()

at ConsoleApplication1.Program.Main(String[] args)

If you look deeper in to the exception it tells you the column that Merge is complaining about. You get into this problem if the data type of the columns that you are merging are different. Now, you might wonder how you could run into an issue like this.

The scenario in which I ran into this issue was interesting. If one of your stored procedures returns a nvarchar in one case and returns a null in another, you will run into this error. One the merge when C# tries to merge the nvarchar and null it does not know the type of null and fails with a DataException. Another interesting observation is that the same error does not happen when you return a variable of a defined type and assign null to it.

Hope it is useful to others who have run into a similar stack and are trying to debug it.

I have tweaked a sample from MSDN to expose the same stack.

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Data;

namespace ConsoleApplication1

{

class Program

{

static void Main(string[] args)

{

Program p = new Program();

p.DemonstrateMerge();

}

private void DemonstrateMerge()

{

DataSet dataSet = new DataSet("dataSet");

DataTable table = new DataTable("Table1");

DataColumn idColumn = new DataColumn("Column1", Type.GetType("System.Int32"));

idColumn.AutoIncrement = true;

DataColumn itemColumn = new DataColumn("Column2", Type.GetType("System.Int32"));

itemColumn.Unique = true;

DataColumn[] keyColumn = new DataColumn[1];

DataRow row;

table.Columns.Add(idColumn);

table.Columns.Add(itemColumn);

dataSet.Tables.Add(table);

keyColumn[0] = idColumn;

table.PrimaryKey = keyColumn;

for (int i = 0; i < 3; i++)

{

row = table.NewRow();

row["Column2"] = i+100;

table.Rows.Add(row);

}

dataSet.AcceptChanges();

PrintValues(dataSet, "Original values");

dataSet.EnforceConstraints = true;

TryMerge(dataSet);

}

private void TryMerge(DataSet original)

{

DataSet dataSet = new DataSet("dataSet");

DataTable table = new DataTable("Table1");

DataColumn idColumn = new DataColumn("Column1", Type.GetType("System.Int32"));

idColumn.AutoIncrement = true;

DataColumn itemColumn = new DataColumn("Column2", Type.GetType("System.Int64"));

itemColumn.Unique = false;

DataColumn[] keyColumn = new DataColumn[1];

DataRow row;

table.Columns.Add(idColumn);

table.Columns.Add(itemColumn);

dataSet.Tables.Add(table);

keyColumn[0] = idColumn;

table.PrimaryKey = keyColumn;

for (int i = 0; i < 3; i++)

{

row = table.NewRow();

row["Column2"] = 100;

table.Rows.Add(row);

}

dataSet.AcceptChanges();

original.Merge(dataSet);

PrintValues(original, "Merged values");

}

private void PrintValues(DataSet dataSet, string label)

{

Console.WriteLine("\n" + label);

foreach (DataTable table in dataSet.Tables)

{

Console.WriteLine("TableName: " + table.TableName);

foreach (DataRow row in table.Rows)

{

foreach (DataColumn column in table.Columns)

{

Console.Write("\t table " + row[column]);

}

Console.WriteLine();

}

CString and GetLength

thottams@microsoft.com — Sat, 07 Feb 2009 06:52:00 GMT

I ran into this problem recently when debugging some native code and thought that it will be good to share this with everyone.

CString sampleString = CString(_T("Sample\0String"), 14);

int len = sampleString.GetLength(); // len is 14

CString trimmedString = sampleString.Trim(); // trimmedString = "Sample"

CString newstring = CString(sampleString); // newString = "Sample"

len = newstring.GetLength(); // len = 14

CString GetLength returns the length that you passed to it in the constructor and not the length of the string. This can be confusing if you copy the string and loop through the length. It can also be the cause of bugs is you get the length and use CString.GetBuffer() and loop through the buffer for the length.

It looks like CString.GetLength() is the size of the internal buffer and nothing more.

MSDN documentation for CString - http://msdn.microsoft.com/en-us/library/aa300471(VS.60).aspx

NTSD and SOS: StopOnException

thottams@microsoft.com — Thu, 15 Jan 2009 22:30:00 GMT

using System;

class Program

{

static void Main(string[] args)

{

Program p = new Program();

p.ExceptionSample();

}

private void ExceptionSample()

{

int i=0;

while (i < 100)

{

if (i == 60)

{

try

{

throw new ArgumentException();

}

catch (Exception)

{

}

else

{

try

{

throw new OutOfMemoryException();

}

catch (Exception)

{

}

i++;

}

!StopOnException comes in handy when you want if you want to break when a specific exception occurs.

csc /debug Program.cs

ntsd program.exe

.sympath+ .

sxe ld mscorwks

.loadby sos msorwks

!StopOnException -Create System.ArgumentException 1

!ClrStack -a

OS Thread Id: 0x103c (0)

Child-SP RetAddr Call Site

000000000019ee50 000007ff00180182 Program.ExceptionSample()

PARAMETERS:

this = 0x0000000002603358

LOCALS:

0x000000000019ee78 = 0x000000000000003c

0x000000000019ee7c = 0x0000000000000000

000000000019eec0 000007fefa3f2672 Program.Main(System.String[])

PARAMETERS:

args = 0x0000000002603338

LOCALS:

0x000000000019eee0 = 0x0000000002603358

!pe

Exception object: 00000000026087d0

Exception type: System.ArgumentException

Message: Value does not fall within the expected range.

InnerException: <none>

StackTrace (generated):

<none>

StackTraceString: <none>

HResult: 80070057

This will stop when the exception occurs. You can print the details of the exception using pe and look into locals using the clrstack -a. This comes in handy if you want to debug some problem and want to break when the exception occurs.

XMLSerializer, XDocument and LINQ

thottams@microsoft.com — Wed, 01 Oct 2008 07:19:00 GMT

I took the sample from my previous post and wanted to use LINQ to query the XML. You can pretty much use the power of SQL on the document object to richly query for things you are looking for.

using System;

using System.Linq;

using System.Xml.Linq;

using System.Collections.Generic;

using System.Xml;

using System.Xml.Serialization;

using System.IO;

namespace Sample

{

public class Company

{

List<Book> books;

[XmlElement(ElementName = "Book")]

public List<Book> Books

{

get

{

if (this.books == null)

{

this.books = new List<Book>();

}

return books;

}

set { }

}

public class Book

{

[XmlAttribute(AttributeName = "Title")]

public string Name { get; set; }

[XmlAttribute(AttributeName = "Author")]

public string Author { get; set; }

[XmlAttribute(AttributeName = "Year")]

public string Year { get; set; }

}

public class Demo

{

static void Main(string[] args)

{

Company c = new Company

{

Books =

{

new Book

{

Name = "First Book",

Author = "First Author",

Year = "First Year",

new Book

{

Name = "Second Book",

Author = "Second Author",

Year = "Second Year",

new Book

{

Name = "Third Book",

Author = "Third Author",

Year = "Third Year",

new Book

{

Name = "Fourth Book",

Author = "Fourth Author",

Year = "Fourth Year",

new Book

{

Name = "Fifth Book",

Author = "Fifth Author",

Year = "Fifth Year",

}

};

XmlSerializer xmlSerializer = new XmlSerializer(typeof(Company));

FileStream fs = new FileStream("test.xml", FileMode.Create);

xmlSerializer.Serialize(fs, c);

fs.Close();

XDocument doc = XDocument.Load("test.xml");

var bks = from books in doc.Elements("Company").Elements("Book") where (books.Attribute("Title").Value.Equals("Fourth Book")) select books;

foreach (var book in bks)

{

Console.WriteLine(book);

}

XmlSerialization to generate and consume XML

thottams@microsoft.com — Fri, 29 Aug 2008 23:17:00 GMT

I want to talk about Xml Serialization this month. One of the work I was involved in recently required generation of XML and a I came across this wonderful technology which helps you generate and consume XML seamlessly.

If you are generating XML in a clumsy way using printf containing the XML tags, I would strongly suggest you to look into this technology. Incidetaly this plays well with Linq too. The XML after it is loaded into the object can be parsed with Linq queries. I will write a sample for that soon.

using System;

using System.Collections.Generic;

using System.Xml;

using System.Xml.Serialization;

namespace Sample

{

public class Company

{

List<Book> books;

[XmlElement(ElementName = "Book")]

public List<Book> Books

{

get

{

if (this.books == null)

{

this.books = new List<Book>();

}

return books;

}

set { }

}

public class Book

{

[XmlAttribute(AttributeName = "Title")]

public string Name { get; set; }

[XmlAttribute(AttributeName = "Author")]

public string Author { get; set; }

[XmlAttribute(AttributeName = "Year")]

public string Year { get; set; }

}

public class Demo

{

static void Main(string[] args)

{

Company c = new Company

{

Books =

{

new Book

{

Name = "First Book",

Author = "First Author",

Year = "First Year",

new Book

{

Name = "Second Book",

Author = "Second Author",

Year = "Second Year",

}

};

XmlSerializer xmlSerializer = new XmlSerializer(typeof(Company));

xmlSerializer.Serialize(Console.Out, c);

}

I find that there are too many ways to load and parse XML such as XDocument, XPath, XMLSerialize, XDocument and so on. I feel it to be hard to make the right choice of technology for the scenario on hand sometimes. Yet again, documentation leaves you stranded in this space!

Simple sample for Events and Delegates

thottams@microsoft.com — Fri, 01 Aug 2008 09:44:00 GMT

I wanted to write a small sample to illustrate events and delegates. Here is a compact sample that illustrates it.

using System;

public class EventSample

{

public delegate void EventHandler();

public event EventHandler myeh;

public void Method()

{

Console.WriteLine("Inside Sample Method ... ");

}

public void OnChange()

{

myeh();

}

public static void Main()

{

EventSample es = new EventSample();

es.myeh += new EventHandler(es.Method);

es.OnChange();

}

DBNull and Nullable types

thottams@microsoft.com — Mon, 30 Jun 2008 21:29:00 GMT

When we have C# code interacting with the data base you run into this problem. Let me try and explain it. The database has nullable columns and C# has nullable types. For example, let us take a table Employee which has Id (int), name (string) and age (int). Let us assume that age will be null if no values are provided.

When you query the database for employee, the dataset or datareader will have DBNull for age. Now, this DBNull is not the same as C# null. This is the beginning of the problem. In order to make sure your code works properly, it has to expect that the reader can return a DBNull. You have to do the following in your code:

int? age = null;

if (!DBNull.Value.Equals(reader["Age"]))

{

age = (int)reader["Age"];

}

This code converts the DBNull to a C# null. The lack of common null types between the two makes code have two sets of defaults for data, one in the DB and the other in C#. Which can get confusing.

If you don't use nullable types due to performance, you have to invent your own null value:

int age = -1;

if (!DBNull.Value.Equals(reader["Age"]))

{

age = (int)reader["Age"];

}

This conversion seems ugly and it is all over the code. Is there any other elegant solution to this?