When implementing C# Maui XAML popup what can cause error 'InitializeComponent does not exist' or 'Popup could not be found' and what is resolution?

When implementing a C# Maui XAML popup, there are a few types of problems and\or mistakes that can occur which cause it to fail.

The most common error messages appear to be the following:

• The name 'InitializeComponent' does not exist in the current context
• The type or namespace name 'Popup' could not be found (are you
  missing a using directive or an assembly reference?)

What can cause these error messages and what are\is the resolution(s)?

I created this question with the intention of providing this answer also. I want to explain why I think this happens to some people, especially if they are not very experienced in MAUI, XAML, or the MAUI popup.

If other developers know of additional reasons for these error messages and the resolution, please provide that also to help others.

This especially applies if you are following this Microsoft Example at the link below. If you aren't careful, it can get you in trouble.

https://learn.microsoft.com/en-us/dotnet/communitytoolkit/maui/views/popup#building-a-popup

I think Microsoft articles often assume too much that every reader is experienced in the technology, tool, or topic that the article is about (in this case MAUI and ASPX files). They view it from their (experienced) perspective, not from the readers perspective who will have varying levels of experience and confidence. I suppose most people\companies do this. This is unfortunate though because they aren't helping the most important (to their user base) readers: The newbies and the "I'm trying it out to see if I like it" developers.

To give a quick answer, the two error messages could be caused by any one of a combination of, or all three of the following reasons:

1. The namespace in the line that begins with "x:Class=" in the popup’s .xaml file is incorrect or missing.
2. The statement "using CommunityToolkit.Maui.Views;" could be missing from the popup’s .xaml.cs file.
3. The namespace line could be missing from the popup’s .xaml.cs file.

If that answers the problem for you, then feel free to stop here. But, if you are having difficulty resolving these possible issues read on, especially if you are relying on the Microsoft article at the link above.

Most people will probably be looking to add the popup to a project they already are working on. So, their project name (and main project namespace) will already be defined. Note: If you aren't familiar with "namespaces" in C#, then you are new and you should read up on "C# namespaces" before continuing.

In the Microsoft article above provided under: "Defining your Popup" are steps for creating a popup. If you are new, they may be difficult to follow. They also may lead to one of the 3 error messages if you aren't careful when following the steps. The steps below are the same as the section "Defining your Popup" in the Microsoft article with some more detail and pointing out where it might cause one of the 3 reasons (listed above) for the error messages.

1. Right click on your project within Visual Studio
2. Select Add New Item
3. In the menu on the left side select ".NET MAUI"
4. In the "sub-menu" that comes up in the middle, select ".NET MAUI ContentView (XAML)" (Before clicking "Add" button...)
5. For this example, name the file "SimplePopup.xaml" (Before you click "Add" button...) We are using SimplePopup.xaml since that is what is in the example.
6. Now click the "Add" button.
7. The SimplePopup.xaml and the SimplePopup.xaml.cs code behind files will be created.
8. Now this is where you can get in trouble if you aren't experienced enough with Maui and xaml files. Experienced programmers should bite their tongue. We've all been there.
9. The next step they show is to copy and paste their code into your XAML file. So, go ahead and do that. Copy it verbatim like the "Copy" button suggests you should do.
10. The next step they show is to copy and paste their code into your C# (xaml.cs) file. So, go ahead and do that. Copy it verbatim like the "Copy" button suggests you should do.
11. Now build your project (even if you see the errors already)
12. You might notice that in your C# (xaml.cs) file, the IntializeComponent() command is underlined. And if you tried to build your project, you get the following errors:
    (A) The name 'InitializeComponent' does not exist in the current context
    (B) The type or namespace name 'Popup' could not be found (are you missing a using directive or an assembly reference?)
13. You might also notice that Popup in the .cs file is showing underlined and saying that the namespace 'Popup' could not be found.

So, at this point you start trying a bunch of the crazy suggestions given on other sites about this problem.

Before you do all that, stop and check these things first.

First in the article the example code shows the XAML file has x:Class="MyProject.SimplePopup".
The MyProject in that should have a disclaimer like x:Class="[MyProject].SimplePopup" (where [MyProject] is the the namespace used in your project).

If you literally have "x:Class="MyProject.SimplePopup" in your XAML file, you need to change [MyProject] to the namespace of your project. If you don't know the namespace, open another .cs file in the same location as your Popup files. You will see something like "namespace [MyExampleMauiNameSpace];" where [MyExampleMauiNameSpace] is the text you need in your popup .cs file. Most likely the [MyExampleMauiNameSpace] will be similar to the name of your project. Replace the [MyProject] namespace showing in your popup .cs file with the [MyExampleMauiNameSpace] from the other .cs file in your project.

So, once you have that line in your XAML, it will probably look like:

x:Class="[MyExampleMauiNameSpace].SimplePopup">

where [MyExampleMauiNameSpace] should be replaced by the namespace showing in your other .cs file that is in same location as your popup files.

Now, there may be one other change that you need to make if you made the mistake of copying and pasting Microsoft's C# (xaml.cs) code verbatim from their example code (like the "copy" button suggests you should do).

If you did that, you may have overwritten the following lines in your xaml.cs file.

`using CommunityToolkit.Maui.Views;

"namespace [MyExampleMauiNameSpace];" `

...where [MyExampleMauiNameSpace] should be replaced by the namespace showing in your other .cs files in the same location.

If you don't have the using statement shown above or the namespace line above in your xaml.cs file in this example, you probably copied and pasted over them. You need to put them back at the top of your xaml.cs file.

Ultimately your files should look like this:

(SimplePopup.xaml code below)

<?xml version="1.0" encoding="utf-8" ?>
<toolkit:Popup xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
               xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
               xmlns:toolkit="http://schemas.microsoft.com/dotnet/2022/maui/toolkit"
               x:Class="[MyExampleMauiNameSpace].SimplePopup">
    <VerticalStackLayout>
        <Label Text="This is a very important message!" />
    </VerticalStackLayout>
</toolkit:Popup>

(SimplePopup.xaml.cs code below)

using CommunityToolkit.Maui.Views;
namespace [MyExampleMauiNameSpace];

public partial class SimplePopup : Popup
{
    public SimplePopup()
    {
        InitializeComponent();
    }
}

I hope this helps some people having trouble implementing C# Maui XAML popup.