Finally i got it to work :
SPWebApplication app = SPContext.Current.Site.WebApplication;
SPAlternateUrl u = app.AlternateUrls[Request.Url];
SPUrlZone zone = u.UrlZone;
SPIisSettings settings = app.IisSettings[zone];
string components = Request.Url.GetComponents(UriComponents.Query, UriFormat.SafeUnescaped);
string AuthProviderString;
if (flag == 1)
AuthProviderString = "SAML_STS";
else
AuthProviderString = "Staging External Users";
foreach (SPAuthenticationProvider provider in settings.ClaimsAuthenticationProviders)
{
if (string.Compare(provider.DisplayName, AuthProviderString, true, System.Globalization.CultureInfo.CurrentUICulture) == 0
|| string.Compare(provider.ClaimProviderName, AuthProviderString, true, System.Globalization.CultureInfo.CurrentUICulture) == 0)
{
string url = provider.AuthenticationRedirectionUrl.ToString();
SPUtility.Redirect(url, SPRedirectFlags.Default, this.Context, components);
}
}