Wednesday, April 14, 2010

Controls collection cannot be modified revisited

A while ago I was writing about the annoying The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>). error and how to fix it by using a PlaceHolder or another container to hold your code blocks you can resolve this.

But what if the ASPX code is not yours and you are only building the control? How can you get around the dreaded code blocks? First, let's try to understand the mechanism that renders the code blocks. When the markup code is read, a CodeDomTreeGenerator class is used to parse it. All DOM tree generators inherit from BaseTemplateCodeDomTreeGenerator which does the following: if the block read is a control, create the control and add it to the control collection, if it is a code block, generate a dynamic render method and use that. From that moment on, you can't change the control collection because (stupid, if you ask me) the render method has already been generated and it only knows about the controls it had then.

You can test if the ControlCollection object cannot be changed via the IsReadOnly property. If it is read only, code blocks have been added. Indeed, in the ControlCollection class, a private field is used to hold an error message and, if not null, it will be used as the exception message when trying to add or remove controls from the collection.

Are you in the mood for some insanity? Ok, let's unset the string via reflection, see what happens! Well, first of all, no error! You can manipulate the control collection at your leisure. The problem is that the render method is still the generated one. If you change the control collection weird stuff will happen, like not getting your control rendered or, if inserted or deleted, seeing controls rendered instead of others or being pushed out of the "rendering queue". So, what is we remove the render method as well? Then the normal Render mechanism will be used. That means that the code blocks will be completely ignored!

So, if you are a mean son of a bitch like myself, instead of begging junior programmers to never use code blocks or to encapsulate them at least, screw the controls so that they ignore that bad code. Not very smart, but oh, so mean :)

Here is a bit of code to remove the "readonlyness" of control collections:


public static class MSOAB
{
private static readonly FieldInfo _readOnlyErrorMsgFieldInfo =
typeof(ControlCollection).GetField("_readOnlyErrorMsg",
BindingFlags.Instance | BindingFlags.NonPublic);
private static readonly PropertyInfo _rareFieldsEnsuredPropertyInfo =
typeof(Control).GetProperty("RareFieldsEnsured",
BindingFlags.Instance | BindingFlags.NonPublic);

private static FieldInfo _renderMethodFieldInfo;

public static void FixReadOnlyControlCollection(Control control)
{
if (control.Controls.IsReadOnly)
{

_readOnlyErrorMsgFieldInfo.SetValue(control.Controls, null);
var rareFieldsEnsured = _rareFieldsEnsuredPropertyInfo.GetValue(control, new object[] { });
if (_renderMethodFieldInfo == null)
{
_renderMethodFieldInfo = rareFieldsEnsured.GetType().GetField("RenderMethod");
}
_renderMethodFieldInfo.SetValue(rareFieldsEnsured, null);
}
}
}

This isn't really tested except the basic functionality and I haven't used it in a production environment, but it was fun as it was. I hope you enjoyed it as well.

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