LDAP Authentication for asp.net Web API

I'm working on WEB API project using ASP.NET technology. This Web API need to check the user from AD Active Directory, as domain authentication using LDEP://

    [HttpGet]
    public IHttpActionResult ListProperties(string domainName, string userName, string password)
    {
        try
        {
            using (DirectoryEntry dEntry = new DirectoryEntry("LDAP://" + domainName, userName, password))
            {
                DirectorySearcher dSearcher = new DirectorySearcher(dEntry)
                {
                    Filter = "(&(objectClass=user)(mail=" + userName + "))"
                };
                SearchResult sResult = dSearcher.FindOne();
                Dictionary<string, string> resultDictionary = new Dictionary<string, string>
                {
                    {"Name", GetProperty(sResult,"cn")},
                    {"Email", GetProperty(sResult,"mail")}
                };

                return Ok(resultDictionary.ToList());
            }
        }
        catch (Exception ex)
        {
            return BadRequest(ex.Message);
        }
    }


    private string GetProperty(SearchResult searchResult, string propertyName)
    {
        if (searchResult.Properties.Contains(propertyName))
        {
            return searchResult.Properties[propertyName][0].ToString();
        }
        return string.Empty;
    }

so I call this method with ajax for test only

$(document).ready(function () { 


    $.ajax({
        type: "GET",
        url: "../api/xxxxxxx/ListProperties",
        data: { domainName: "mydomain.xxx.xx", userName: "MyUsername", password: "MyPassword" },
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function (data) { console.log(JSON.stringify(data)); },
        failure: function (data) { console.log(0); },
        error: function (data)   { console.log(1); }
    });
});

Unfortunately, I always receiving bad request or this below error

System.Runtime.InteropServices.COMException HResult=0x8007203A Message=The server is not operational.

Can you please provide me with a guide how to resolve this issue, as I have never work with security programming before.

The error is down to your application failing to bind to the LDAP server. Firstly, I would recommend adding a target LDAP server to your query string and then formatting the query string correctly to represent the domain DN= as well as any specific Organisation Units OU= etc...

The query string would look something like this:

LDAP://contoso.local/DC=contoso,DC=local

I've created an example below which performs the request using a GET (Not recommended for obvious reasons) along with a helper method to convert the domain into a friendly LDAP string. The response outputs the result properties into JSON so you can manipulate how you see fit.

   public JsonResult CheckAdCreds(string server, string domain, string username, string password)
    {
        try
        {
            var ldapDomainString = LdapStringFromDomain(domain, server);

            using (var entry = new DirectoryEntry(ldapDomainString, username, password))
            {
                using (var search = new DirectorySearcher(entry))
                {
                    search.Filter = $"(&(objectClass=user)(objectCategory=user) (sAMAccountName={username}))";
                    var result = search.FindOne();
                    return Json(result.Properties, JsonRequestBehavior.AllowGet);
                }
            }
        }
        catch (Exception ex)
        {
            return Json(new { Error = ex.Message }, JsonRequestBehavior.AllowGet);
        }
    }

Helper method which converts the domain string into an LDAP friendly string:

    private string LdapStringFromDomain(string domain, string server)
    {
        var ldapString = $"LDAP://{server}/";
        var domainArr = domain.Split('.');

        for (int i = 0; i < domainArr.Length; i++)
        {
            ldapString += $"DC={domainArr[i]}";

            if (i != domainArr.Length - 1)
                ldapString += ",";
        }
        return ldapString;
    }

Hope it helps.