只需将声明移到循环之外：

public string getkampioen(string selecteditem)
{
    string kampioen; //declare here
    using (MySqlConnection connection = new MySqlConnection(connectionString))
    {
        connection.Open();
        string query = ("select * FROM clubs where naam = '" + selecteditem + "'");
        MySqlCommand cmd = new MySqlCommand(query, connection);
        MySqlDataReader Reader = cmd.ExecuteReader();
        while (Reader.Read())
        {
           kampioen = (string)Reader["aantalkampioenschappen"].ToString();
        }
        return kampioen; 
    }
}

正如其他人所说，行动起来 kampioen 在圈外。但是，如果您希望获得一个特定值而不是随机值（如果返回多个记录），则可能需要连接字符串或将结果限制在第一行，如下所示：

public string getkampioen(string selecteditem)
{
    string kampioen = string.empty; // declared outside of the while 
    using (MySqlConnection connection = new MySqlConnection(connectionString))
    {
        connection.Open();
        string query = ("select * FROM clubs where naam = '" + selecteditem + "' limit one");
        MySqlCommand cmd = new MySqlCommand(query, connection);
        MySqlDataReader Reader = cmd.ExecuteReader();
        while (Reader.Read())
        {
           kampioen = (string)Reader["aantalkampioenschappen"].ToString();
        }
        return kampioen; 
    }
}

可以在while循环之前声明字符串。

public string getkampioen(string selecteditem)
{
    using (MySqlConnection connection = new MySqlConnection(connectionString))
    {
        connection.Open();
        string query = ("select * FROM clubs where naam = '" + selecteditem + "'");
        MySqlCommand cmd = new MySqlCommand(query, connection);
        MySqlDataReader Reader = cmd.ExecuteReader();
        string kampioen = "";
        while (Reader.Read())
        {
           kampioen = (string)Reader["aantalkampioenschappen"].ToString();
        }
        return kampioen; 
    }
}