HasFullName.cs

using System;
using System.Linq;
using Microsoft.CodeAnalysis;

internal static class INamespaceOrTypeSymbolExtensions
{
    /// <summary>
    /// Checks the full name of a type or namespace without incurring the expense of building a full name string.
    /// </summary>
    public static bool HasFullName(this INamespaceOrTypeSymbol symbol, params string[] segments)
    {
        if (segments.Length < 1)
            throw new ArgumentException("At least one name segment must be specified.", nameof(segments));

        if (segments.Any(string.IsNullOrEmpty))
            throw new ArgumentException("Name segments must not be null or empty.", nameof(segments));

        if (symbol.ContainingType is object) return false; // Nested types can't be specified with this method

        if (symbol.Name != segments.Last()) return false;

        var currentNamespace = symbol.ContainingNamespace;

        for (var i = segments.Length - 2; i >= 0; i--)
        {
            if (currentNamespace.IsGlobalNamespace) return false;
            if (currentNamespace.Name != segments[i]) return false;
            currentNamespace = currentNamespace.ContainingNamespace;
        }

        return currentNamespace.IsGlobalNamespace;
    }
}