See if user is part of Active Directory group in C# + Asp.net

With 3.5 and System.DirectoryServices.AccountManagement this is a bit cleaner:

public List<string> GetGroupNames(string userName)
{
  var pc = new PrincipalContext(ContextType.Domain);
  var src = UserPrincipal.FindByIdentity(pc, userName).GetGroups(pc);
  var result = new List<string>();
  src.ToList().ForEach(sr => result.Add(sr.SamAccountName));
  return result;
}

How about this

How to write LDAP query to test if user is member of a group?

You could try the following code:

public bool Check_If_Member_Of_AD_Group(string username, string grouptoCheck, string domain, string ADlogin, string ADpassword)
{
    
     try {
        
        string EntryString = null;
        EntryString = "LDAP://" + domain;
        
        DirectoryEntry myDE = default(DirectoryEntry);
        
        grouptoCheck = grouptoCheck.ToLower();
        
        
        myDE = new DirectoryEntry(EntryString, ADlogin, ADpassword);
        
        DirectorySearcher myDirectorySearcher = new DirectorySearcher(myDE);
        
        myDirectorySearcher.Filter = "sAMAccountName=" + username;
        
        myDirectorySearcher.PropertiesToLoad.Add("MemberOf");
        
        SearchResult myresult = myDirectorySearcher.FindOne();
        
        int NumberOfGroups = 0;
        
        NumberOfGroups = myresult.Properties["memberOf"].Count - 1;
        
        string tempString = null;
        
        while ((NumberOfGroups >= 0)) {
            
            tempString = myresult.Properties["MemberOf"].Item[NumberOfGroups];
            tempString = tempString.Substring(0, tempString.IndexOf(",", 0));
            
            tempString = tempString.Replace("CN=", "");
            
            tempString = tempString.ToLower();
            tempString = tempString.Trim();
            
            if ((grouptoCheck == tempString)) {
                
                    
                return true;
            }
            
                
            NumberOfGroups = NumberOfGroups - 1;
        }
        
            
        return false;
    }
    catch (Exception ex) {
        
        System.Diagnostics.Debugger.Break();
    }
    //HttpContext.Current.Response.Write("Error: <br><br>" & ex.ToString)
}

How to check user is in AD member and specific AD group member

//This Reference and DLL must be attach in your project         
//using System.DirectoryServices.AccountManagement;        


         public bool IsAuthenticated(string username, string pwd)
        {

            using (PrincipalContext pc = new PrincipalContext(ContextType.Domain, "xxx.com"))   // Your Domain Name
            {
                if (pc.ValidateCredentials(username, password))  //User and Password is OK for Active Directory 
                {
                    UserPrincipal user = UserPrincipal.FindByIdentity(pc, username);  //Get User Active Directory Information Details
                    if (user != null)
                    {

                        var groups = user.GetAuthorizationGroups();   // Get User Authorized Active Directory Groups
                        foreach (GroupPrincipal group in groups)
                        {
                            if (group.Name.Equals("SpecificActiveDirectoryGroupName"))  //Check if user specific group members
                            { 
                                return true;
                            }

                        }
                    }
                }
            }
            return false;
        }

This should work in .NET 3.5+

// using System.DirectoryServices.AccountManagement;
public static bool IsUserMemberOfGroup(string username, string group)
{
    using (var ctx = new PrincipalContext(ContextType.Domain))
    using (var usr = UserPrincipal.FindByIdentity(ctx, username))
        return usr.IsMemberOf(ctx, IdentityType.Name, group);
}

This is similar to a lot of answers here, but this:

• only finds the user, then checks if the user is a member of a group using just its name (doesn't need to find the Group or iterate over the users/groups)
• Disposes the objects (using usings)