MissingMethodException when opening a form in the designer; runtime works fine

Question

Let's say I have a projoct A with a class A that has this property:

    public bool IsConnected
    {
        get { return m_isConnected; }
        private set { m_isConnected = value; }
    }

In the same solution, I have a project B that references project A and has a user control called Login. This control has this attribute:

    private A m_A = null;

and in the contructor of Login I do this call:

if (m_A != null && m_A.IsConnected) { ... }

In the same project, the main form has on it a user control A that was added with the form designer. The program runs fine and this property is correctly read.

However, when opening the main form in the Designer I get this execption: MissingMethodException: 'Boolean A.get_IsConnected()'

Commenting out m_A.IsConnected let's me use the designer, but this is getting pretty annoying. And sometimes it seems like it randomly just works.

Any ideas?

Answer 1

I've been told in the past that this.DesignMode isn't always perfectly reliable. The other option you can use is preprocessor directives:

#if DESIGN
return;
#else
if (m_A != null && m_A.IsConnected) { /* etc. */ }
#endif

Then add a conditional compilation symbol named DESIGN and you should be golden.

Answer 2

Instead of commenting it out you could use:

if (this.DesignMode)
{
    return;
}

or

if (!this.DesignMode)
{
    if (m_A != null && m_A.IsConnected) { ... }
}

Edit: I remember having problems with a singleton class once. Problem was that the singleton was always initialized although the constructor was never called. If I understand your problem correctly your problem is likewise. Your member m_A is never NULL although it should be.
I managed my problem with the following:
In Visual Studio go to "Tools\Options...\Debugging\General" and disable "Enable property evaluation and other implicit function calls" and enable "Step over properties and operators (Managed only)".
Maybe that helps.

Answer 3

As noted by Ari Roth DesignMode doesn't work right. To compensate for this, I use this extension method:

public static bool IsDesignTime(this Control control)
{
    if (LicenseManager.UsageMode == LicenseUsageMode.Designtime)
    {
        return true;
    }

    if (control.Site != null && control.Site.DesignMode)
    {
        return true;
    }

    var parent = control.Parent;
    while (parent != null)
    {
        if (parent.Site != null && parent.Site.DesignMode)
        {
            return true;
        }
        parent = parent.Parent;
    }
    return false;
}

Use it in the same manner that Simon Linder describes.

Answer 4

It looks like the designer is using an old version of the control DLL, which does not have the IsConnected property.

The MissingMethodException exception is not related to the value of m_A. It is the JIT compiler complaining that the property is missing. If you wrap the call to m_A.IsConnected in a method, it works, because this method is not called (because m_A really is null), thus not jitted, thus the IsConnected property is not needed.

When you put the control in a form in VS, the designer creates a "real" instance of your control in the form, which calls the constructor, which causes the JIT compile problem.