.net – host a Windows Form inside a control

There is actually a (subtle) difference between the two. Imagine you have the following code in File1.cs:

// File1.cs
using System;
namespace Outer.Inner
{
    class Foo
    {
        static void Bar()
        {
            double d = Math.PI;
        }
    }
}

Now imagine that someone adds another file (File2.cs) to the project that looks like this:

// File2.cs
namespace Outer
{
    class Math
    {
    }
}

The compiler searches Outer before looking at those using directives outside the namespace, so it finds Outer.Math instead of System.Math. Unfortunately (or perhaps fortunately?), Outer.Math has no PI member, so File1 is now broken.

This changes if you put the using inside your namespace declaration, as follows:

// File1b.cs
namespace Outer.Inner
{
    using System;
    class Foo
    {
        static void Bar()
        {
            double d = Math.PI;
        }
    }
}

Now the compiler searches System before searching Outer, finds System.Math, and all is well.

Some would argue that Math might be a bad name for a user-defined class, since there's already one in System; the point here is just that there is a difference, and it affects the maintainability of your code.

It's also interesting to note what happens if Foo is in namespace Outer, rather than Outer.Inner. In that case, adding Outer.Math in File2 breaks File1 regardless of where the using goes. This implies that the compiler searches the innermost enclosing namespace before it looks at any using directive.

The conversion you have done of the original C# code is not perfect (you should understand what each part does, instead of copying bit by bit). In the SetVisible/SetInvisible part this is the problem:

Public Shared Function SetInvisible(page As TabPage, frm As Form) 'As Boolean
    page = frm.Controls(page.Name)

    If IsVisible(page) Then
        Dim tabCtrl As TabControl = DirectCast(page.Parent, TabControl)
        Dim tpinfo As TabPageData

        tpinfo = New TabPageData(tabCtrl.TabPages.IndexOf(page), tabCtrl, page)

        tabCtrl.TabPages.Remove(page)
        hiddenPages.Add(TabPageData.GetKey(tabCtrl, page), tpinfo)
    End If
End Function

(this should be a Sub rather than a Function) You are adding a bit not present in the original code: page = frm.Controls(page.Name); I guess that it is an adaptation to make the code work under your specific conditions (you have added to the form a TabPage alone, instead one inside a TabControl, what is the normal behaviour). This would be fine, but you are not adapting the SetVisible function to this reality:

Public Shared Sub SetVisible(page As TabPage, parent As TabControl)
    If parent IsNot Nothing AndAlso Not parent.IsDisposed Then
        Dim tpinfo As TabPageData
        Dim key As String = TabPageData.GetKey(parent, page)

        If hiddenPages.ContainsKey(key) Then
            tpinfo = hiddenPages(key)

            If tpinfo.Index < parent.TabPages.Count Then
                parent.TabPages.Insert(tpinfo.Index, tpinfo.Page)
            Else
                ' add the page in the same position it had
                parent.TabPages.Add(tpinfo.Page)
            End If

            hiddenPages.Remove(key)
        Else
            PrintAllKeys()
        End If
    End If
End Sub

Understand what both functions do: the first one (modified by you) expects a TabPage added to the form directly (thus with no parent TabControl); the second one (as in the original C# code) expects a TabPage with a parent TabControl but your input does not have that. How I know that? If your TabPage would have a TabControl as a parent, the page = frm.Controls(page.Name) would be Nothing.

If you want to use this code you have to provide the expected inputs, that is, TabPages inside a TabControl. Otherwise, you should modify it accordingly (not just one part, all the parts). Easy test for you to understand what is required:

1- Open a new project and add a new TabControl via "Design View".
2- Copy your class but let SetInvisible as in the original version (delete page = frm.Controls(page.Name)).
3- Test your class with the main form and see that it works fine. Sample code (these are the default names when you add a new TabControl):

Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load   
    Dim test As clsTabManager = New clsTabManager()

    test.SetInvisible(TabPage1, Me)

    test.SetVisible(TabPage1, TabControl1)
End Sub