mysql - PHP Fatal error: Call to a member function fetch_assoc() on string

Can anyone help me solve this? I having this line code with error of

PHP Fatal error: Call to a member function fetch_assoc() on string in
C:\inetpub\wwwroot\dbcontroller.php on line 26

Line 26 is this line of code: $result = $query->fetch_assoc();

class DBController {
private $host = "localhost";
private $user = "root";
private $password = " ";
private $database = "test";
function __construct() {
    $conn = $this->connectDB();
    if(!empty($conn)) {
        $this->selectDB($conn);
    }
}
function connectDB() {
    $conn = mysqli_connect($this->host,$this->user,$this->password);
    return $conn;
}
function selectDB($conn) {
    mysqli_select_db($conn,$this->database);
}
function runQuery($query) {
    $result = $query->fetch_assoc();
    // $result = mysqli_query($query);
    while($row=mysqli_fetch_assoc($result)) {
        $resultset[] = $row;
    }
    if(!empty($resultset))
        return $resultset;
}
function numRows($query) {
    $result  = mysqli_query($query);
    $rowcount = mysqli_num_rows($result);
    return $rowcount;
}
}
?>