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If you do not provide a default namespace in XAML, the XAML parser will throw a non-descript exception. The message will be:
System.ArgumentNullException: Value cannot be null.
The code can be found here:
It happens here:
if (clrNamespace == null)
throw new ArgumentNullException(nameof(clrNamespace));
The exception that is thrown is here should either be less generic and handled at a higher level, or the message of the exception should be more descriptive and should include a clear reason as to why the exception is being thrown.
Here are a few forum posts on the topic:
If the default xmlns is missing, the partial class won't be generated (XamlG) and the assembly won't compile, so you're not supposed to run the project, and get a runtime issue.
To get this, you're invoking an internal API, and the observed behavior is not a bug.
I got this error again today, so I googled for it, and saw my post here:
Which led my back to this bug. It's still a bug that needs to be fixed. The bug is that the error message needs to more descriptive. I don't know if this is a bug in Xamarin Forms, or a bug in Visual Studio, but regardless, the error message should say something like:
Default xmlns is missing. The partial class won't be generated for XAML file [INSERT FILENAME HERE].
It's not actually the default xmlns thought - it's the default namespace of the project.
I'm now getting this in the latest version of Xamarin Forms. 188.8.131.52203
And, I DO have a default namespace at the project. And, this is now control is defined in XAML:
<?xml version="1.0" encoding="UTF-8"?>
When I remove the XamlCompilation attribute, it compiles fine.
But, when I do remove the attribute, I get nothing on the screen. It's just blank.
I don't understand what's wrong in #c4. Please add a small retro-project.
I'm starting to have this error now too. The clrNamespace param is also null, but it is declared in the XAML file
I can't recreate it. It just happens sometimes and then stops happening.
closing as not reproducible.
if you can provide a reproduction project, please open an issue on https://github.com/xamarin/Xamarin.Forms/issues