Solution to error when rendering controls in VS2010

When making controls, it is easy to encounter errors such as "Error while rendering the control" and "An unhandled exception occurred. The object reference was not set to an instance of the object.", as shown below: (It may also be just because the object is not referenced to an instance of the object.) Setting the runat="server" tag causes this error, please check this item first)
But there is no error at compile time/runtime.

Analysis
The situation shown in the figure is called "design time" to distinguish it from "compile time"/"run time".

Design time: In the ASP.NET environment, that is when we use the web designer to edit in Visual Studio. The direct understanding is when the ASPX page switches to "Design".

Compile time: The direct understanding is that when you compile, usually the errors at this time are errors caused by type checking, parameter matching, etc. that can be directly limited by syntax constraints.

Runtime: The direct understanding is when you preview/run. Usually the errors at this time are composed of specific exceptions and logical errors.
Let us analyze the performance of the control during design. When our control is designed, VS intelligently simulates the appearance of the output control at runtime. The presentation of the control passes a certain sequence of methods, and finally forms the current output. According to the standard, we should perform the output operation on the control in Render or RenderContents (in fact, others are also possible, but we usually do not do that, or more "error when rendering the control" exceptions mainly come from Render or RenderContents).
From the error message "The object reference is not set to an instance of the object." Judging from this sentence, that is to say, one or more instances of the object are used without assigning an initial value. .

Let us take a peek at our code:

protected override void RenderContents(HtmlTextWriter writer)
{
 UpButton.Text = Page.Server.HtmlDecode(UpButton.Text);
 DownButton.Text = Page.Server.HtmlDecode(DownButton.Text);
 base.RenderContents(writer);
}

Because the control needs to have an up button and a down button when designing, two special The punctuation marks are up and down, and these two symbols need to be correctly rendered by the browser by setting the code numbers as shown below:

private string upButtonText = "∧";
private string downButtonText = "∨";

However, these two symbols will be displayed by the page before being rendered. Compile an HtmlEncode method and then output it, but these two special signs can only be presented through direct output. That is to say, after HtmlEncode, the special signs can only be output in the form of text ∧ ∨ and will not be output. Up and down arrows. At this time we need to introduce its reverse method Page.Server.HtmlDecode for decoding. Notice that we use the Page instance here. This instance is only non-null when the page actually exists. Otherwise, subsequent operations will be null. operation, and such an operation will display an error such as "Object reference not set to an instance of the object."
Design time: As we just mentioned, design time only simulates the process of page rendering, and the page does not actually exist. Therefore, the instance of the Page object will be empty at this time, and subsequent calls will throw an exception.

Assuming that we only have this method to handle the currently required behavior, then when we call Page, it will definitely result in a null object and cause exceptions in subsequent operations. At this time we introduce the concept of "design mode" (not DesignPattern but DesignMode). DesignMode is a protected property of the Control class, which obtains a value to indicate whether the component is currently in design mode. The design pattern here is equivalent to the concept of design.

So we can transform the code into the following form:

protected override void RenderContents(HtmlTextWriter writer)
{
 if (!DesignMode)
 {
 UpButton.Text = Page.Server.HtmlDecode(UpButton.Text);
 DownButton.Text = Page.Server.HtmlDecode(DownButton.Text);
 }
 base.RenderContents(writer);
}

In this way, we only introduce instances of the Page object when it is not at design time, so design-time exceptions will be easily solved .

Summary

Therefore, when designing a control, especially when considering the presentation of the control, in order to avoid the occurrence of similar exceptions, we should consider that the control can obtain enough Resources, for items that fail to obtain resources, we should explicitly distinguish them (as in the above code, DesignMode is used to determine whether it is code to be executed at design time).

Extension

The situation we just saw can be attributed to the exception caused by the inability to reference specific instances during design. Similar to this exception, there will also be database/ The file system is read, the variable does not have an initial value attached, and Page-like properties such as Session, Page.Request.QueryString, etc. are called. When designing the page, because some of the above solutions, especially the related methods called to the Page, will always exist, we will not often see exceptions from them. We should pay more attention when designing the controls.

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