Fix namespace generation issue

[rockfordlhotka]

I've tested and the PrivatAssets works. But omitting the namespace declaration dosn't work, because then the generated partial class is in the global namespace and this is not the same as the original namespace.

But I think I've found the solution 😀

During my tests I noticed that the problem only occurs, if I used file scoped namespaces. If I changed my code to use block-scoped namespaces all went fine. So I looked in TypeDefinitionExtractor.GetNamespace() and found the following line:

namespaceDeclaration = containingTypeDeclaration.Parent as NamespaceDeclarationSyntax;

NamespaceDeclarationSyntax only matches block-scoped namespaces. File scoped namespaces need FileScopedNamespaceDeclarationSyntax.

The final solution I got from Andrew Locks blog

// determine the namespace the class/enum/struct is declared in, if any
static string GetNamespace(BaseTypeDeclarationSyntax syntax)
{
    // If we don't have a namespace at all we'll return an empty string
    // This accounts for the "default namespace" case
    string nameSpace = string.Empty;

    // Get the containing syntax node for the type declaration
    // (could be a nested type, for example)
    SyntaxNode? potentialNamespaceParent = syntax.Parent;
    
    // Keep moving "out" of nested classes etc until we get to a namespace
    // or until we run out of parents
    while (potentialNamespaceParent != null &&
            potentialNamespaceParent is not NamespaceDeclarationSyntax
            && potentialNamespaceParent is not FileScopedNamespaceDeclarationSyntax)
    {
        potentialNamespaceParent = potentialNamespaceParent.Parent;
    }

    // Build up the final namespace by looping until we no longer have a namespace declaration
    if (potentialNamespaceParent is BaseNamespaceDeclarationSyntax namespaceParent)
    {
        // We have a namespace. Use that as the type
        nameSpace = namespaceParent.Name.ToString();
        
        // Keep moving "out" of the namespace declarations until we 
        // run out of nested namespace declarations
        while (true)
        {
            if (namespaceParent.Parent is not NamespaceDeclarationSyntax parent)
            {
                break;
            }

            // Add the outer namespace as a prefix to the final namespace
            nameSpace = $"{namespaceParent.Name}.{nameSpace}";
            namespaceParent = parent;
        }
    }

    // return the final namespace
    return nameSpace;
}

This will fix it. Same method must be changed in TypeDefinitionExtractor in the project AutoSerialization.

Originally posted by @jensbrand in #4446