NamespaceDeclarationSyntax namespaceDeclarationSyntax = null;
if (!SyntaxNodeHelper.TryGetParentSyntax(classDeclarationSyntax,out namespaceDeclarationSyntax))
{
    return; // or whatever you want to do in this scenario
}

var namespaceName = namespaceDeclarationSyntax.Name.ToString();
var fullClassName = namespace.Name + "." + classDeclarationSyntax.Identifier.ToString();

和助手：

static class SyntaxNodeHelper
{
    public static bool TryGetParentSyntax<T>(SyntaxNode SyntaxNode,out T result) 
        where T : SyntaxNode
    {
        // set defaults
        result = null;

        if (SyntaxNode == null)
        {
            return false;
        }

        try
        {
            SyntaxNode = SyntaxNode.Parent;

            if (SyntaxNode == null)
            {
                return false;
            }

            if (SyntaxNode.GetType() == typeof (T))
            {
                result = SyntaxNode as T;
                return true;
            }

            return TryGetParentSyntax<T>(SyntaxNode,out result);
        }
        catch
        {
            return false;
        }
    }
}

没有什么太复杂的事情在这里…这是有道理的命名空间将“上”的语法树(因为该类包含在命名空间中),所以你只需要“上”语法树直到找到命名空间并追加到ClassDeclarationSyntax的标识符.