下面是一个使用AAD B2C按PrincipalName搜索用户的示例代码：

using Microsoft.Graph;
using Microsoft.Identity.Client;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace AADB2CUserSearch
{
    class Program
    {
        // AAD B2C tenant details
        private static string tenantId = "your-tenant-id";
        private static string clientId = "your-client-id";
        private static string clientSecret = "your-client-secret";
        private static string policy = "your-sign-in-policy";

        static async Task Main(string[] args)
        {
            // Initialize the GraphServiceClient
            GraphServiceClient graphClient = await GetGraphServiceClient();

            // Search for users by PrincipalName
            string principalName = "user@example.com";
            var users = await SearchUsersByPrincipalName(graphClient, principalName);

            // Display the search results
            Console.WriteLine($"Found {users.Count} user(s) with PrincipalName '{principalName}':");
            foreach (var user in users)
            {
                Console.WriteLine($"- {user.DisplayName} ({user.Id})");
            }
        }

        static async Task GetGraphServiceClient()
        {
            // Create a confidential client application
            IConfidentialClientApplication confidentialClientApp = ConfidentialClientApplicationBuilder
                .Create(clientId)
                .WithClientSecret(clientSecret)
                .WithAuthority($"https://login.microsoftonline.com/{tenantId}/v2.0")
                .Build();

            // Acquire an access token
            string[] scopes = new string[] { "https://graph.microsoft.com/.default" };
            AuthenticationResult authResult = await confidentialClientApp.AcquireTokenForClient(scopes).ExecuteAsync();

            // Initialize the GraphServiceClient
            GraphServiceClient graphClient = new GraphServiceClient(new DelegateAuthenticationProvider((requestMessage) =>
            {
                requestMessage.Headers.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Bearer", authResult.AccessToken);
                return Task.CompletedTask;
            }));

            return graphClient;
        }

        static async Task> SearchUsersByPrincipalName(GraphServiceClient graphClient, string principalName)
        {
            var queryOptions = new List
            {
                new QueryOption("$filter", $"signInNames/any(x: x/value eq '{principalName}' and x/type eq 'emailAddress')")
            };

            var request = graphClient.Users.Request(queryOptions);
            var response = await request.GetAsync();

            return response.CurrentPage.ToList();
        }
    }
}

注意替换以下变量值：

• tenantId：AAD B2C租户的ID
• clientId：应用程序的客户端ID
• clientSecret：应用程序的客户端密钥
• policy：登录策略的名称

此示例使用Microsoft Graph .NET SDK和Microsoft Identity Client库来与AAD B2C进行交互。首先，我们将创建一个机密客户端应用程序，并使用其凭据获取访问令牌。然后，我们使用GraphServiceClient进行搜索并获取用户信息。

在Main方法中，我们首先初始化GraphServiceClient，然后调用SearchUsersByPrincipalName方法来搜索具有指定PrincipalName的用户。最后，我们遍历搜索结果并在控制台上显示每个用户的显示名称和ID。