AddHttpClient和AddHttpClient都是在ASP.NET Core的依赖注入容器中注册一个HttpClient实例。区别在于，AddHttpClient用于注册一个可以定制的HttpClient实现，而AddHttpClient只是注册了HttpClient的默认实现。

具体代码示例如下：

//AddHttpClient示例 services.AddHttpClient();

//AddHttpClient示例 services.AddHttpClient() .SetHandlerLifetime(TimeSpan.FromMinutes(5)) //将Handler的生命周期设置为5分钟 .AddPolicyHandler(GetRetryPolicy()); //添加一个重试策略

public static IAsyncPolicy GetRetryPolicy()
{
    return HttpPolicyExtensions
        .HandleTransientHttpError()
        .OrResult(msg => msg.StatusCode == HttpStatusCode.NotFound)
        .WaitAndRetryAsync(6, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));
} 

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