How to pass value from Windows Form to Class file in C#.net Windows Application.

Create a constructor that takes the connection string as a parameter.

public class classfile
{
    private string _connectionString;
    public classfile(string connectionString)
    {
        _connectionString = connectionString;
    }
}

Now everywhere in your class you can use the '_connectionString' member. (if the methods aren't static)

And everywhere you instantiate your classfile class you'll have to pass the connectionstring.