你不应该以这种方式存储和访问你的连接字符串。

按照Microsoft的建议执行： http://msdn.microsoft.com/ en-us / library / ms254494（v = vs.80）.aspx [ ^ ]

im developing a winform and i store my connection in App.config

<add key="CS" value="Server=localhost;User ID=root;Password=123456;Persist Security Info=True;Database=dbMember"/>

and this is how i call the connectionString from C#

string CS = System.Configuration.ConfigurationManager.AppSettings.Get("CS");

yes, it can run smoothly at my machine, how it show error msg: "the connection property has not been set or is null", when i try to run it at another PC.

but it working at another PC if i manually code the connectionstring like this:

string CS= "Server=localhost;User ID=root;Password=123456;Persist Security Info=True;Database=dbMember";

any advice? i already install the same version of sql at both PC

You shouldn''t store and access your connection strings this way.
Do it as suggested by Microsoft: http://msdn.microsoft.com/en-us/library/ms254494(v=vs.80).aspx[^]