sql语句内的filename标量未声明

 public List<GraduateFileLook> SearchGraduateFile(string FileName, string UploadClassification)

 {

   

    SqlParameter[] parameter = {

     new SqlParameter("@FileName", SqlDbType.NVarChar,50),

     new SqlParameter("@UploadClassification", SqlDbType.NVarChar,50)

     };

    parameter[0].Value = FileName;

    parameter[1].Value = UploadClassification;

    string sql = @"SELECT GraduateFileUpload.FileName,GraduateFileUpload.UploadClassification,GraduateFileUpload.Remark 

    FROM GraduateFileUpload WHERE GraduateFileUpload.FileName = @FileName AND GraduateFileUpload.UploadClassification = @UploadClassification";

    List<GraduateFileLook> GraduateFileLookList = new List<GraduateFileLook>();

    using (SqlDataReader gfl = SqlHelper.ExecuteReader(SqlHelper.ConnectionString, CommandType.Text, sql, null))

    {

	while (gfl.Read())

	{

	 GraduateFileLook GraduateFileLook = new GraduateFileLook

	 {

	FileName = gfl["FileName"].ToString(),

	UploadClassification = gfl["UploadClassification"].ToString(),

	Remark = gfl["Remark"].ToString()

  };

    GraduateFileLookList.Add(GraduateFileLook);

}

gfl.Close();

    }

    return GraduateFileLookList;

    

}

运行后提示“为生命变量@FileName”，就是这个错误。求指导。
------解决方案--------------------
using (SqlDataReader gfl = SqlHelper.ExecuteReader(SqlHelper.ConnectionString, CommandType.Text, sql, null)) //这里的null是否应该为parameter？