Searching Active Directory B2C by custom property on User

I am not using that library, but we are doing a very similar search using the Graph API. I have constructed a filter that will look for users that match two extension attribute values I am looking for. The filter looks like this:

var filter = $"$filter={idpExtensionAttribute} eq '{userType.ToString()}' and {emailExtensionAttribute} eq '{emailAddress}'";

We have also used REST calls via PowerShell to the Graph API that will return the desired users. The URI with the associated filter looks like this:

https://graph.windows.net/$AzureADDomain/users?`$filter=extension_d2fbadd878984184ad5eab619d33d016_idp eq '$idp' and extension_d2fbadd878984184ad5eab619d33d016_email eq '$email'&api-version=1.6

Both of these options will return any users that match the filter criteria.

I would use normal DirectorySearcher Class from System.DirectoryServices

private void Search()
{
    // GetDefaultDomain as start point is optional, you can also pass a specific 
    // root object like new DirectoryEntry ("LDAP://OU=myOrganisation,DC=myCompany,DC=com");
    // not sure if GetDefaultDomain() works in B2C though :(
    var results = FindUser("extPropName", "ValueYouAreLookingFor", GetDefaultDomain());

    foreach (SearchResult sr in results)
    {
        // query the other properties you want for example Accountname
        Console.WriteLine(sr.Properties["sAMAccountName"][0].ToString());
    }
    Console.ReadKey();
}

private DirectoryEntry GetDefaultDomain()
{   // Find the default domain
    using (var dom = new DirectoryEntry("LDAP://rootDSE"))
    {
        return new DirectoryEntry("LDAP://" + dom.Properties["defaultNamingContext"][0].ToString());
    }
}

private SearchResultCollection FindUser(string extPropName, string searchValue, DirectoryEntry startNode)
{
    using (DirectorySearcher dsSearcher = new DirectorySearcher(startNode))
    {
        dsSearcher.Filter = $"(&(objectClass=user)({extPropName}={searchValue}))";
        return dsSearcher.FindAll();
    }
}