使用Roslyn替换方法

Question

I am trying to merge two C# file contents using Roslyn. I need to replace certain methods (marked for update) in the older version with newer ones. I have traversed all the members in both the documents and marked the methods that require changes into a list.

class Token
{
    public string Name;  // unique identifier
    public SyntaxNode Node;  // actual node in the document
}

Now I have two Token lists viz, OldTokens and NewTokens. I need to overwrite the old methods with corresponding new ones.

foreach (Token oldt in OldTokens)
{
    Token newt = NewTokens.Find(m => m.Name == oldt.Name);
    if (newt != null)
    {
        DocumentRoot = DocumentRoot.ReplaceNode(oldt.Node, newt.Node);
    }
}

It wasn't working. I thought it could be because the DocumentRoot doesn't hold the old node in it immediate member list, as it would appear deep inside the hierarchy of namespace->class->member. So I tried something like:

var nd = existing.Node.Parent.ReplaceNode(existing.Node, miss.Node);
var d2 = existing.Node.Parent.Parent.ReplaceNode(existing.Node.Parent, nd);
DocumentRoot = existing.Node.Parent.Parent.Parent.ReplaceNode(existing.Node.Parent.Parent, d2) as CompilationUnitSyntax;

Which is working but too difficult to handle. It would become a mess when there exists nested classes. Is there a better way?

Secondly, in the result, I am missing the code comments. I tried with the overload for ReplaceNode method, which is quite confusing as it won't take the new method node.

Thanks

Answer 1

Due to the fact that Roslyn SyntaxNode objects are immutable, every time you call "ReplaceNode", you get a whole new tree back. The normal way to solve this problem is to add a SyntaxAnnotation to each node you want to replace. This is a stable marker that will be preserved through tree transformations as long as the node itself isn't replaced.

Alternatively, when making many changes to a file like this, you could look at implementing a SyntaxRewriter , instead of just calling ReplaceNode a bunch of times. SyntaxRewriter will visit the tree bottom-up, so the node passed to the method will always be from the original SyntaxTree . However, make sure you first call base.Visit() in any method you override and use the result so that you have the updated version of any nodes that are descendants of the current node.

Finally, regarding comments, you'll need to copy the trivia ( GetLeadingTrivia() / GetTrailingTrivia() ) from the nodes you are replacing in order to retain it.