If you already have access to your Document class from your workspace (Using Workspaces) it is easy to access the root of your Syntax tree.

 Document document = ... // Get document from workspace or other source 

 var syntaxRoot = await document.GetSyntaxRootAsync();

You can easily navigate the a Syntax Tree using LINQ. For example it is easy to get all the ClassDeclarationSyntax nodes (declared classes), that have a name starting with the letter A:

var allClassesWithNameStartingWithA = syntaxRoot.DescendantNodes()
    .OfType<ClassDeclarationSyntax>()
    .Where(x => x.Identifier.ToString().StartsWith("A"));

Or getting all the classes that have attributes:

var allClassesWithAttriutes = syntaxRoot.DescendantNodes()
    .OfType<ClassDeclarationSyntax>()
    .Where(x => x.AttributeLists.Any(y => y.Attributes.Any()));

The CSharpSyntaxWalker class is out of the box implementation of the Visitor pattern, that we can use to traverse our Syntax Tree. Here is a simple example of a Syntax Walker that collects all the struct-s that have a name, starting with the letter A:

public class StructCollector : CSharpSyntaxWalker
{
    public StructCollector()
    {
        this.Structs = new List<StructDeclarationSyntax>();
    }

    public IList<StructDeclarationSyntax> Structs { get; }

    public override void VisitStructDeclaration(StructDeclarationSyntax node)
    {
        if (node.Identifier.ToString().StartsWith("A"))
        {
            this.Structs.Add(node);
        }
    }
}

We can use our SyntaxWalker in the following way:

var structCollector = new StructCollector();
structCollector.Visit(syntaxRoot); // Or any other syntax node
Console.WriteLine($"The number of structs that have a name starting with the letter 'A' is {structCollector.Structs.Count}");