Add Forms Based user to SharePoint site programatically - sharepoint

Question

I have a web app in SharePoint 2010 whose Authentication mode is Windows. I have extended the web app (Zone:Intranet) where Forms Based Authentication is used.
I have created a sample C# Console application to add the forms based user to the site and it adds the users successfully.
The method:
private void AddFormsBasedUserToSite(){
try
{
string url = "https://appserver:36732";
SPSecurity.RunWithElevatedPrivileges(delegate
{
using (SPSite site = new SPSite(url))
{
using (SPWeb web = site.OpenWeb())
{
string userName = "i:0#.f|aspnetsqlmembershipprovider|forms_user";
web.SiteUsers.Add(userName, null, null, null);
web.Update();
}
}
});
}
catch (Exception e)
{
// Log the exception
}
}
But when the same method is exposed in WCF service (hosted as windows service), and when the client calls this method, it could not add the user. It throws following exception:
The user does not exist or is not unique.
The SharePoint logs is:
Error resolving http://schemas.microsoft.com/sharepoint/2009/08/claims/userlogonname: forms_user from membership provider AspNetSqlMembershipProvider: System.Web.HttpException: Unable to connect to SQL Server database. ---> System.Data.SqlClient.SqlException: A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)
at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection)
at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj)
at System.Data.SqlClient.TdsParser.Connect(ServerInfo serverInfo, SqlInternalConnectionTds connHandler, Boolean ignoreSniOpenTimeout, Int64 timerExpire, Boolean encrypt, Boolean trustServerCert, Boolean integratedSecurity, SqlConnection owningObject)
at System.Data.SqlClient.SqlInternalConnectionTds.AttemptOneLogin(ServerInfo serverInfo, String newPassword, Boolean ignoreSniOpenTimeout, Int64 timerExpire, SqlConnection owningObject)
at System.Data.SqlClient.SqlInternalConnectionTds.LoginNoFailover(String host, String newPassword, Boolean redirectedUserInstance, SqlConnection owningObject, SqlConnectionString connectionOptions, Int64 timerStart)
at System.Data.SqlClient.SqlInternalConnectionTds.OpenLoginEnlist(SqlConnection owningObject, SqlConnectionString connectionOptions, String newPassword, Boolean redirectedUserInstance)
at System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, Object providerInfo, String newPassword, SqlConnection owningObject, Boolean redirectedUserInstance)
at System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection)
at System.Data.ProviderBase.DbConnectionFactory.CreateNonPooledConnection(DbConnection owningConnection, DbConnectionPoolGroup poolGroup)
at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection)
at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory)
at System.Data.SqlClient.SqlConnection.Open()
at System.Web.Management.SqlServices.GetSqlConnection(String server, String user, String password, Boolean trusted, String connectionString) -
-- End of inner exception stack trace ---
at System.Web.Management.SqlServices.GetSqlConnection(String server, String user, String password, Boolean trusted, String connectionString)
at System.Web.Management.SqlServices.SetupApplicationServices(String server, String user, String password, Boolean trusted, String connectionString, String database, String dbFileName, SqlFeatures features, Boolean install)
at System.Web.DataAccess.SqlConnectionHelper.CreateMdfFile(String fullFileName, String dataDir, String connectionString)
at System.Web.DataAccess.SqlConnectionHelper.EnsureSqlExpressDBFile(String connectionString)
at System.Web.DataAccess.SqlConnectionHelper.GetConnection(String connectionString, Boolean revertImpersonation)
at System.Web.Security.SqlMembershipProvider.GetUser(String username, Boolean userIsOnline)
at Microsoft.SharePoint.Utilities.SPMembershipProviderPrincipalResolver.ResolvePrincipal(String input, Boolean inputIsEmailOnly, SPPrincipalType scopes, SPPrincipalSource sources, SPUserCollection usersContainer)
at Microsoft.SharePoint.Administration.Claims.SPFormsClaimProvider.Resolve(SPPrincipalResolver resolver, Boolean inputIsEmailOnly, SPPrincipalSource pricipalSource, SPPrincipalType pricipalType, String resolveInput, List1 resolved)
at Microsoft.SharePoint.Administration.Claims.SPFormsClaimProvider.FillResolve(Uri context, Boolean allZones, String[] entityTypes, SPClaim resolveInput, List1 resolved)
Please help me to figure it out?
Thanks
Prakash