Parameter Problem while running sql query in C#

Error :

The SqlParameterCollection only accepts non-null SqlParameter type objects, not String objects

This is the error I'm getting when I run this code

MenuItem masterItem = new MenuItem((string)masterRow["Parentitem"]);
string mp =(string) masterItem.Value;

SqlP开发者_StackOverflow中文版arameter parameter = new SqlParameter();
parameter.ParameterName = "@mp";

parameter.SqlDbType = SqlDbType.NVarChar;
parameter.Direction = ParameterDirection.Input;
parameter.Value = mp;

string q = "select aspnet_PersonalizationPerUser.hasRights from Menu, aspnet_Users, 
            aspnet_Paths, aspnet_PersonalizationPerUser 
            where Menu.Parentitem = @mp 
            and Menu.Url = aspnet_Paths.Path 
            and aspnet_Paths.PathId = aspnet_PersonalizationPerUser.PathId 
            and aspnet_Users.UserName ='admin' 
            and aspnet_PersonalizationPerUser.UserId = aspnet_Users.userId ";

SqlCommand cm = new SqlCommand(q, conn);
cm.Parameters.Add(mp);
string b = (string)cm.ExecuteScalar();

I'm getting the exception when I'm adding parameter to the command.. Can u let me know the mistake..

Change this:

cm.Parameters.Add(mp);

to this:

cm.Parameters.Add(parameter);