Allow overriding C# method modifiers

[jonpryor]

A C# method modifier are the keywords which can be applied to a method, such as visibility (public, private, …).

Some of these modifiers can be overridden via metadata, e.g. setting //method/@visibility will be inserted as-is into the C# binding.

What is desired is a way to override the default logic for emitting virtual & override.

Sometimes when binding a library, the default logic causes virtual to be emitted when override is needed for the code to compile. (TODO: Insert concrete example here.)

The current workaround is to use metadata to rename the "offending" method and then use a partial class to properly override the method.

What we would like is e.g. a managedOverride attribute in metadata/on //method.

Question: what should the semantics be? Would a "boolean" attribute be desirable, or a string?

<!-- boolean? -->
<attr path="//method[@name='example']" name="managedOverride">true</attr>

<!-- string? -->
<attr path="//method[@name='example']" name="managedOverride">override</attr>

[jonpryor]

Example of what we want to improve

https://github.com/xamarin/FacebookComponents/blob/70e0b91a24a247f4fe0d317d371441a3278a3623/Facebook.Android/source/facebook-core/transforms/Metadata.xml#L32

https://github.com/xamarin/FacebookComponents/blob/70e0b91a24a247f4fe0d317d371441a3278a3623/Facebook.Android/source/facebook-core/additions/additions.cs#L15

A common "source" of the problem tends to be Java generics, in which e.g. FileDownloadTask.doInBackground() has java.lang.String parameter types, which doesn't match the base class. Metadata could be used to "fix" the parameter types to be java.lang.Object, allowing the parameter types to match the base class, but generator wouldn't realize this.

This is the same scenario as: #586

[jpobst]

It feels like this maybe needs to go both ways: changing virtual to override, or override to virtual. Given that, a string would work better:

<attr path="//method[@name='example']" name="managedOverride">override</attr>
<attr path="//method[@name='example']" name="managedOverride">virtual</attr>