Creating a Partial Class for a Form

I开发者_JAVA百科 would like to create a partial class for my form. I have a lot of events, and it gets messy, so I would like to break up sections into their own files.

The problem: When I create a partial class of my form, say:

Partial Public Class Form1

End Class

Visual Studio decides I need another form for this partial class.

Questions:

1. How do I create a partial class for a form?

2. If I cant do that, how can I break up all the events in my form into different files?

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Yeah, it does. As soon as you drop a control on this phantom form, you'll get the design-time code (InitializeComponent) generated into that source code file. This is compatibility behavior for .NET 1.x, it didn't support the Partial keyword. Which will break the build, there are now two of them. It is somewhat avoidable with careful clicking, but you know it's going to happen sooner or later.

Other things go wrong too btw, the designer can no longer track an event handler when you move it from one file to another. And will readily let you add another, a much trickier source of bugs.

This just doesn't work very well, abandon hope of relying on it to solve your problem.

The generic diagnostic is that a convoluted user interface begets convoluted code. But that ship has sailed, no doubt. A more structural solution is pursuing the MVC model, separating the data from the view of the data. You'll still have a lot of event handlers but they won't do anything more than calling a method of a class that does the real work. Whose source code can of course live in another source code file. The typical hangup is that Windows Forms has no support whatsoever built in for this, you have to craft it by hand. Nothing similar to the MVVM model in WPF.

Something that can work well is isolating control + code into a separate UserControl. You have to do so carefully though, you don't want to have to add a bunch of properties and events that expose internal controls.

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Sometimes I create partial classes for better readibility, especially when I have very large classes. But when I click on the partial class, then the VS IDE will open the form editor showing me an empty form. If I do not care, than I could damage the main form (it seems to be a bug of VS 2008/2010)

A possibility could be using DesignerCategoryAttribute Class

Mark the partial class with the attribute "code".

 <System.ComponentModel.DesignerCategory("code")>
 Partial Class Form1

 End Class

In this way when you click on the file, you open the class in the code editor. Of course this will apply to all files, also to the main form file. If you want to edit again your form in the form editor, you have to quote the attribute:

 '<System.ComponentModel.DesignerCategory("code")>

Some more details here.

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While it does not answer the original question, I found using regions made my code a little more manageable/readable.

#Region "RegionA"

#End Region

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I orginally called this method a "hack", thus the comment below.

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Not sure what you mean be "Visual Studio decides you need another form", however, are you sure the new Form1 partial class is declared in the corresponding original namespace?

All partial classes for a given .NET type must be declared in the same namespace of course (whatever files they're stored on).

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I appreciate the answers given by Hans and I'm not disputing these at all. It is curious though that in Visual Studio 2010, when you create a form called say Main you get a Main.designer.vb which is a partial class. It says 'Partial Class Main' at the top. This class doesn't open a form when clicked. It also includes reference to Event Handlers. So I was wondering how do they get around this? Is there a way to create one of these 'special' partial classes that work as we would expect.

I noticed that when I created a Form Partial class, that the icon went from a class icon to a form icon. The icon associated with the Main.designer.vb file looks like a class icon with a arrow.

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what worked for me (VS 2010) was naming Form1 class, already saved in Form1.vb with its own designer (Form1.Designer.vb) as:

Public Class Main 'saved in Form1.vb

VS updated the name in the designer as:

Partial Class Main 'saved in Form1.Designer.vb

then I created another "partial class" with the same name:

Partial Class Main 'saved in Main.vb

Whether I am editing Form1.vb or Main.vb VS shows me on the top navigation pan all the routines, functions, subs, even background workers and timers. For event handlers, to avoid the loophole mentioned earlier (you click on a control in the layout designer and a brand new event handler will be created in the original Form1.vb) I go:

Partial Public Class Main 'in Form1.vb file
    Private Sub SomeControl_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles SomeControl.Click
        Call SomeControlClick(sender, e)
    End Sub
End Class

Partial Public Class Main 'then in Main.vb file
    Private Sub SomeControlClick(ByVal sender As Object, ByVal e  As System.EventArgs)
        'blah blah
    End Sub
End Class