Wensveen's Blog

November 12, 2012

Synchronous .NET event handling with PowerShell

Filed under: Uncategorized — Tags: , , , — wensveen @ 11:12 pm

There are multiple ways in Windows PowerShell to handle events generated by objects in the .NET framework. The most common, or at least, the most documented way to register an event handler to a .NET object is with Register-ObjectEvent. Usage is fairly straightforward: Register-ObjectEvent -InputObject $obj -EventName "BeforeProcess" -Action { Write-Host "BeforeProcess called" }.

The thing is, that the action you registered with Register-ObjectEvent is called asynchronously. This means that the code that generates the BeforeProcess event doesn’t have to wait for the action to end. Consider, for example, a .NET class:

namespace Echo
{
	public class Echo
	{
		public event EventHandler<EchoEventArgs> BeforeEcho;

		public void EchoMessages(string messages)
		{
			foreach (string message in messages.Split(','))
			{
				EchoEventArgs args = new EchoEventArgs { Message = message };
				if (BeforeEcho != null)
				{
					BeforeEcho(this, args);
				}

				Console.WriteLine(message);
			}
		}
	}

	public class EchoEventArgs : EventArgs
	{
		public string Message { get; set; }
	}
}

Now, create an Echo object in PowerShell:

[void] [System.Reflection.Assembly]::LoadFrom('echo.dll')
$echoObject = New-Object Echo.Echo

Running it without event handlers:

$echoObject.EchoMessages('Mary,had,a,little,lamb')

should yield:
Mary
had
a
little
lamb

Now, when you attach an event handler with Register-ObjectEvent:

Register-ObjectEvent -InputObject $echoObject -EventName "BeforeEcho" -Action { Write-Host ("ObjectEvent: " + $EventArgs.Message) }
$echoObject.EchoMessages('Foo,bar')

Result:
Foo
bar
ObjectEvent: Foo
ObjectEvent: bar

As you can see, the foreach loop in EchoMessages doesn’t wait for the Action to finish, before going on to the next message.

Very often, this is exactly what you want. A lot of examples use System.Timers.Timer, where you just want to execute something every so-and-so hours. But once in a while, you may need the event handler to finish before the next line of code is executed.

Let’s say we want the event handler to be able to skip certain messages, or cancel the rest of the messages altogether. Or maybe modify the message before it is written.

namespace Echo
{
	public class Echo
	{
		public event EventHandler<EchoEventArgs> BeforeEcho;

		public void EchoMessages(string messages)
		{
			foreach (string message in messages.Split(','))
			{
				EchoEventArgs args = new EchoEventArgs { Message = message };
				if (BeforeEcho != null)
				{
					BeforeEcho(this, args);
				}

				if (args.Cancel)
				{
					break;
				}
				else if (!args.Skip)
				{
					Console.WriteLine(args.Message);
				}
			}
		}
	}

	public class EchoEventArgs : EventArgs
	{
		public string Message { get; set; }
		public bool Skip { get; set; }
		public bool Cancel { get; set; }
	}
}

Now, let’s say I want to cancel all messages straight away:

Register-ObjectEvent -InputObject $echoObject -EventName "BeforeEcho" -Action { $EventArgs.Cancel = $true }
$echoObject.EchoMessages('1,2,3,4,5')

Result:
1
2
3
4
5

Why? Because you’re already too late. When you decide to cancel, the number is already outputted. Next invocation is with a new EchoEventArgs object. So we need to register the event handler for synchronous execution. Unfortunately, it isn’t documented at all how to do this. Looking with Reflector, we can see the compiler generated two methods for us: add_BeforeEcho and remove_BeforeEcho:
Echo class in Reflector

So, when we register the event handler with add_BeforeEcho, it all works as expected:

$echoObject.add_BeforeEcho({
  param($Source, $EventArgs)
  if ($EventArgs.Message -eq '2') {
    $EventArgs.Skip = $true
  } elseif ($EventArgs.Message -eq '7') {
    $EventArgs.Cancel = $true
  } else {
    $EventArgs.Message = ($EventArgs.Message + ".00")
  }
})
$echoObject.EchoMessages('1,2,3,4,5,6,7,8,9,10')

Result:
1.00
3.00
4.00
5.00
6.00

Hurray!

May 27, 2011

Toggling boilerplate HTML visibility in ASP.NET

Filed under: Uncategorized — Tags: , — wensveen @ 10:23 pm

The problem

When you are writing an aspx page, you write HTML mixed with ASP.NET controls. Sometimes, when some control is not visible, or has no visible (html) output, you may need to hide the html that is containing the control as well.

For example, suppose a document from some CMS may or may not have a Title, or said Title may be empty. Your aspx page might look like this:

..
<h1><cms:FieldControl runat="server" Field="Title" /></h1>
..

Suppose FieldControl is a control that renders the a field of the current document when that field exists and isn’t empty. When Title is empty, you don’t want the <h1> tag to be rendered either.

Solution 1: Placeholders and code-behind

The most straightforward solution to this problem is to place an <asp:Placeholder> control around the <h1>. Then in code-behind, test the FieldControl for output and set the Placeholder visibility accordingly. For example:

<asp:Placeholder runat="server" ID="phTitle">
	<h1><cms:FieldControl runat="server" ID="fcTitle" Field="Title" /></h1>
</asp:Placeholder>

code behind:

protected override void OnPreRender(EventArgs e)
{
	base.OnPreRender(e);
	phTitle.Visible = fcTitle.Visible && !String.IsNullOrEmpty(fcTitle.Text)
}

While there is nothing wrong with this code, it could become cumbersome if you have more than a few of these snippets. There may even be controls that don’t expose a Text property or a similar way to test the control for output.

Solution 2: Introducing the Enclosure control

(more…)

April 12, 2011

A Fluent interface for creating Control hierarchies (method chaining)

Filed under: Uncategorized — Tags: , — wensveen @ 8:34 am

I was creating a custom ASP.NET control for a project and I noticed that creating Control hierarchies was a lot of work, and a bit clunky. For instance, create a table with a single row, with a single cell with just two Panels and some literals in them.

Control structure:

  • (this)
    • Table
      • TableRow
        • TableCell
          • Panel
            • Literal
          • Panel
            • Literal
Table table = new Table { BorderStyle = BorderStyle.None, Width = Unit.Percentage(100) };
TableRow tr = new TableRow();
TableCell td = new TableCell();

Panel ush = new Panel { CssClass = "UserSectionHead" };
ush.Controls.Add(new Literal { Text = "Title" });
td.Controls.Add(ush);

Panel usb = new Panel { CssClass = "UserSectionBody" };
usb.Controls.Add(new TextField { Text = "Enter title" });
td.Controls.Add(usb);

tr.Cells.Add(td);
table.Rows.Add(tr);
this.Controls.Add(table);

As you can see, there’s a lot of variables you need to create just to be able to add (nested) child controls to the control hierarchy. And this is just a very small example. Fortunately, most properties can be set in the initializer, otherwise the boilerplate would be even larger!

(more…)

March 1, 2010

Currying in C#

Filed under: Uncategorized — Tags: , — wensveen @ 4:06 pm

A few days ago, I suddenly felt like experimenting with currying in C#. For those of you that don’t know what currying is, from Wikipedia currying: “[..] currying [..] is the technique of transforming a function that takes multiple arguments [..] in such a way that it can be called as a chain of functions each with a single argument.”

A simple example:

int Add(int x, int y) { return x + y; }
add3 = Add(3);

r1 = add3(6);
r2 = add3(9);

(more…)

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