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);
                        }
                    }