c # database connection error SqlHelper

This is a problem plagued me for several days, first look at the error message

Code:

private static string connectionString = ConfigurationManager.ConnectionStrings["connstr"].ConnectionString;

Profiles

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <connectionStrings>
      <add name="connstr" connectionString="server=localhost;port=3306;user=root;password=123456;database=test"/>
    </connectionStrings>
</configuration>

At first I thought it was the connection string, and then use the connection string directly public static string connectionString = "server=localhost;port=3306;user=root;password=123456;database=test";
so there is no problem

See a bit connectionString returned as null
is not a configuration file format or method of reading problems
then put into a configuration file

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="connstr" value="server=localhost;port=3306;user=root;password=123456;database=test"/>
  </appSettings>
</configuration>

Reading method private static string connectionString = ConfigurationSettings.AppSettings[
so although it is not being given to the connection string read or null

So experienced a number of Baidu Google, see a problem, there are two main

1. Problems connected string
   server=localhost;port=3306;uid=root;pwd=123456;database=test
   username changed into uid password pwd
2. File location
   down on the ui layer project

​