VisitAllSymbols.cs

public class NamedTypeVisitor : SymbolVisitor
{
    public override void VisitNamespace(INamespaceSymbol symbol)
    {
        Console.WriteLine(symbol);
        
        foreach(var childSymbol in symbol.GetMembers())
        {
            //We must implement the visitor pattern ourselves and 
            //accept the child symbols in order to visit their children
            childSymbol.Accept(this);
        }
    }

    public override void VisitNamedType(INamedTypeSymbol symbol)
    {
        Console.WriteLine(symbol);
        
        foreach (var childSymbol in symbol.GetTypeMembers())
        {
            //Once againt we must accept the children to visit 
            //all of their children
            childSymbol.Accept(this);
        }
    }
}

//Now we need to use our visitor
var tree = CSharpSyntaxTree.ParseText(@"
class MyClass
{
    class Nested
    {
    }
    void M()
    {
    }
}");

var mscorlib = MetadataReference.CreateFromFile(typeof(object).Assembly.Location);
var compilation = CSharpCompilation.Create("MyCompilation",
    syntaxTrees: new[] { tree }, references: new[] { mscorlib });

var visitor = new NamedTypeVisitor();
visitor.Visit(compilation.GlobalNamespace);