Code used by the Salesforce Toolkits for .NET to perform a token refresh

CallTokenRefreshAsync.cs

var auth = new AuthenticationClient();
await auth.TokenRefreshAsync(ConsumerKey, _token.RefreshToken);

FormatRefreshTokenUrl.cs

public static string FormatRefreshTokenUrl(
    string tokenRefreshUrl,
    string clientId,
    string refreshToken,
    string clientSecret = "")
{
    if (tokenRefreshUrl == null) throw new ArgumentNullException("tokenRefreshUrl");
    if (clientId == null) throw new ArgumentNullException("clientId");
    if (refreshToken == null) throw new ArgumentNullException("refreshToken");

    var clientSecretQuerystring = "";
    if (!string.IsNullOrEmpty(clientSecret))
    {
        clientSecretQuerystring = string.Format("&client_secret={0}", clientSecret);
    }

    var url =
    string.Format(
        "{0}?grant_type=refresh_token&client_id={1}{2}&refresh_token={3}",
        tokenRefreshUrl,
        clientId,
        clientSecretQuerystring,
        refreshToken);

    return url;
}

TokenRefreshAsync.cs

public async Task TokenRefreshAsync(string clientId, string refreshToken, string clientSecret, string userAgent, string tokenRequestEndpointUrl)
{
    _httpClient.DefaultRequestHeaders.UserAgent.ParseAdd(string.Concat(userAgent, "/", ApiVersion));

    var url = Common.FormatRefreshTokenUrl(
        tokenRequestEndpointUrl,
        clientId,
        refreshToken,
        clientSecret);

    var request = new HttpRequestMessage()
    {
        Method = HttpMethod.Post,
        RequestUri = new Uri(url)
    };

    var responseMessage = await _httpClient.SendAsync(request);
    var response = await responseMessage.Content.ReadAsStringAsync();

    if (responseMessage.IsSuccessStatusCode)
    {
        var authToken = JsonConvert.DeserializeObject<AuthToken>(response);

        AccessToken = authToken.access_token;
        InstanceUrl = authToken.instance_url;
    }
    else
    {
        var errorResponse = JsonConvert.DeserializeObject<AuthErrorResponse>(response);
        throw new ForceException(errorResponse.error, errorResponse.error_description);
    }
}