How can I qualify symbols within a Roslyn syntax with a CSharpSyntaxRewriter?

Question

I'm writing a Roslyn source generator, where I need to grab the initializer from the VariableDeclaratorSyntax.Initializer of a field, and echo it back into a generated file.

For example, if I wrote this code....

using MyNamespace.SomeStuff;
...
partial class Bar
{
    [Convert]
    Foo _fooField = Foo.Baz;
}

My source generator might produce...

partial class Bar
{
    MyNamespace.SomeStuff.Foo _fooField = MyNamespace.SomeStuff.Foo.Baz;
}

I can get the fully-qualified field Type easily, via IFieldSymbol. However, I don't know how to get a fully-qualified version of the initializer.

This value can be anything. It need not be a static; it could be nested constructors for custom classes; etc. I need to fully-qualify all of the symbols, and print out the result as a string that I can insert into my generated code without bothering with usings.

I think I need to make a custom CSharpSyntaxRewriter that visits the proper nodes and replaces the names with a resolved version from the SemanticModel. But there isn't much information on the internet on how to do this.

Here's what I've got so far:

public override SyntaxNode? VisitMemberAccessExpression(MemberAccessExpressionSyntax node)
{
    var symbol = SemanticModel.GetSymbolInfo(node);
    if (symbol.Symbol is not null)
        return node.WithName(SyntaxFactory.IdentifierName(symbol.Symbol.ToString()));
    else return base.VisitMemberAccessExpression(node);
}

But instead of MyNamespace.SomeStuff.Foo.Baz it produces Foo.MyNamespace.SomeStuff.Foo.Baz which obviously doesn't work. I need to use WithExpression instead of WithName, but the expression could be anything and I don't know how to narrow it down.

Any ideas? Is there a better way of doing this?

EDIT: As requested, here's the full context:

[Generator]
public class StyleSourceGenerator : IIncrementalGenerator
{
    public void Initialize(IncrementalGeneratorInitializationContext context)
    {
        // find any field tagged with [Convert]
        var fields = context.SyntaxProvider.ForAttributeWithMetadataName(typeof(ConvertAttribute).FullName,
            (node, token) =>
            {
                return node is VariableDeclaratorSyntax;
            },
            (ctx, token) =>
            {
                var syntax = (VariableDeclaratorSyntax)ctx.TargetNode;
                syntax.Initializer.Value.ToString(); // this will print Foo.Baz, when I need it to print MyNamespace.SomeStuff.Foo.Baz
            });
    }
}

In var symbol = SemanticModel.GetSymbolInfo(node); where does SemanticModel come from? More generally, can you provide a MRE of your source generator and syntax rewriter? — nalka
– nalka, Commented Oct 13, 2023 at 13:35
I provided some more context in the answer, and added a solution that works for me so far. I'd appreciate another set of eyes though since I don't think it's particularly robust. — Aaa
– Aaa, Commented Oct 14, 2023 at 23:03

Aaa · Accepted Answer · 2023-10-14 23:01:51Z

I've found a solution that works for my case, but I don't know if it's fully bulletproof. There's likely some things (like constructors, maybe?) that wouldn't hold up, but this works for member access.

I'd appreciate advice/edits on how to make it more robust.

From my original example, get the corrected initializer with:

var newInitializer = new (QualifiedWriter(ctx.SemanticModel))
    .Visit(syntax.Initializer) as EqualsValueClauseSyntax;

internal class QualifiedWriter : CSharpSyntaxRewriter
{
    public QualifiedWriter(SemanticModel semanticModel)
    {
        SemanticModel = semanticModel;
    }

    private SemanticModel SemanticModel { get; }

    public override SyntaxNode? VisitMemberAccessExpression(MemberAccessExpressionSyntax node)
    {
        // for example: Microsoft.Xna.Color.Transparent;  identifier name is Transparent, expression is Microsoft.Xna.Color
        // Then we have a russian nesting doll of SimpleMemberAccessExpression until the expression is just an IdentifierNameSyntax

        return ResolveMemberAccessSyntaxTree(node);
    }

    public MemberAccessExpressionSyntax ResolveMemberAccessSyntaxTree(MemberAccessExpressionSyntax node)
    {
        if (node.Expression is MemberAccessExpressionSyntax access) return node.WithExpression(ResolveMemberAccessSyntaxTree(access));

        if (node.Expression is IdentifierNameSyntax name)
        {
            var symbol = SemanticModel.GetSymbolInfo(name);
            if (symbol.Symbol is null) return node; // give up
            return node.WithExpression(name.WithIdentifier(SyntaxFactory.Identifier(symbol.Symbol.ToString())));
        }

        // we don't know how to nest further!
        return node;
    }
}

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