There is no option to get the list of the PaaS and SaaS Service alone using the API.
However, you can get all the resources under a particular Subscriptionid
and filter
using it's type
using Azure List Resouce API
1st You should find your Paas/Saas Resouce type, for an instance, say it's is Microsoft.DataFactory/factories
var credentials = SdkContext.AzureCredentialsFactory.FromServicePrincipal("<Clientid>", "<ClientSecret?", "<TenantId>",AzureEnvironment.AzureGlobalCloud);
var azure = Microsoft.Azure.Management.Fluent.Azure.Configure().Authenticate(credentials).WithDefaultSubscription();
var resouceManagementClient = new ResourceManagementClient(credentials) { SubscriptionId = "<SubID>" };
var resource = resouceManagementClient.Resources.ListAsync(new ODataQuery<GenericResourceFilterInner>(x => x.ResourceType == "Microsoft.DataFactory/factories")).Result;
Like that you can add many filters to get that particular resource types.