某クライアント

ChatWorkClient.cs

public class ChatWorkAuthenticationHandler : DelegatingHandler
{
    readonly string apiToken;

    public ChatWorkAuthenticationHandler(string apiToken)
        : this(apiToken, new System.Net.Http.HttpClientHandler())
    {

    }
    public ChatWorkAuthenticationHandler(string apiToken, HttpMessageHandler innerHandler)
        : base(innerHandler)
    {
        this.apiToken = apiToken;
    }


    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)
    {
        request.Headers.Add("X-ChatWorkToken", apiToken);
        return base.SendAsync(request, cancellationToken);
    }
}

public class ChatWorkClient
{
    readonly HttpClient httpClient;

    public long MaxResponseContentBufferSize
    {
        get
        {
            return httpClient.MaxResponseContentBufferSize;
        }
        set
        {
            httpClient.MaxResponseContentBufferSize = value;
        }
    }

    public TimeSpan Timeout
    {
        get
        {
            return httpClient.Timeout;
        }
        set
        {
            httpClient.Timeout = value;
        }
    }

    public ChatWorkClient(string apiToken)
    {
        this.httpClient = new HttpClient(new ChatWorkAuthenticationHandler(apiToken));
    }

    public ChatWorkClient(string apiToken, HttpMessageHandler innerHandler)
    {
        this.httpClient = new HttpClient(new ChatWorkAuthenticationHandler(apiToken, innerHandler));
    }

    // 以下略
}