var gatewayFactory = new GatewayFactory();
var masterClient = gatewayFactory.CreateMasterClient("someBaseAddress");

public IMasterService CreateMasterClient(string baseAddress)
{
    return (MasterServiceClient)CreateHttpClient<IMasterService>($"{baseAddress}api/master/");
}

private DalServiceBaseClient CreateHttpClient<T>(string baseAddress)
{
    var httpClient = new HttpClient
    {
        BaseAddress = new Uri(baseAddress)
    };

    if (typeof(T) == typeof(IMasterService))
    {
        return new MasterServiceClient(httpClient);
    }
    if (typeof(T) == typeof(ISlaveService))
    {
        return new SlaveServiceClient(httpClient);
    }
    if (typeof(T) == typeof(IRoleService))
    {
        return new RoleServiceClient(httpClient);
    }

    // A lot more ifs..

    throw new ArgumentException("No type exists for the provided interface");
}

IDictionary<Type, Type> mappings; //Populated with all interface to implementation types

private DalServiceBaseClient CreateHttpClient<T>(string baseAddress) {
    var httpClient = new HttpClient {
        BaseAddress = new Uri(baseAddress)
    };
    var implementationType = mappings[typeof(T)];
    if (implementationType == null)
        throw new ArgumentException("No type exists for the provided interface");

    return (DalServiceBaseClient)Activator.CreateInstance(implementationType, httpClient);
}

mappings = new Dictionary<Type, Type>();
mappings[typeof(IMasterService)] = typeof(MasterServiceClient);
mappings[typeof(ISlaveService)] = typeof(SlaveServiceClient);

//...other types;

IDictionary<Type, Func<HttpClient, DalServiceBaseClient>> mappings; //Populated with all interface to implementation types

private DalServiceBaseClient CreateHttpClient<T>(string baseAddress) {
    var httpClient = new HttpClient {
        BaseAddress = new Uri(baseAddress)
    };
    var implementationFactory = mappings[typeof(T)];
    if (implementationFactory == null)
        throw new ArgumentException("No type exists for the provided interface");

    return implementationFactory(httpClient);
}

mappings = new Dictionary<Type, Func<HttpClient, DalServiceBaseClient>>();
mappings[typeof(IMasterService)] = (httpClient) => new MasterServiceClient(httpClient);
mappings[typeof(ISlaveService)] = (httpClient) => new SlaveServiceClient(httpClient);

//...other types;

private HttpClient CreateHttpClient(string baseAddress)
{
    return new HttpClient()
    {
        BaseAddress = new Uri(baseAddress)
    };
}

public IMasterService CreateMasterClient(string baseAddress)
{
    return new MasterServiceClient(
        CreateHttpClient($"{baseAddress}api/master/")
    );
}