How can I set application settings at install time (via installer class)

What I do for my installers is to use the "file" attribute in App.Config. The appSettings block takes a "file" attribute, like so:

<appSettings file="user.config">
    <add key="foo" value="some value unchanged by setup"/>
</appSettings>

The "file" attribute is sort of like CSS, in that the most specific setting wins. If you have "foo" defined in user.config as well as App.config, the value in user.config is used.

Then, I have a config generator that writes out a second appSettings block to user.config (or whatever you want to call it), using values in a dictionary.

using System.Collections.Generic;
using System.Text;
using System.Xml;

namespace Utils
{
    public class ConfigGenerator
    {
        public static void WriteExternalAppConfig(string configFilePath, IDictionary<string, string> userConfiguration)
        {
            using (XmlTextWriter xw = new XmlTextWriter(configFilePath, Encoding.UTF8))
            {
                xw.Formatting = Formatting.Indented;
                xw.Indentation = 4;
                xw.WriteStartDocument();
                xw.WriteStartElement("appSettings");

                foreach (KeyValuePair<string, string> pair in userConfiguration)
                {
                    xw.WriteStartElement("add");
                    xw.WriteAttributeString("key", pair.Key);
                    xw.WriteAttributeString("value", pair.Value);
                    xw.WriteEndElement();
                }

                xw.WriteEndElement();
                xw.WriteEndDocument();
            }
        }
    }
}

In your installer, just add something like the following in your Install method:

string configFilePath = string.Format("{0}{1}User.config", targetDir, Path.DirectorySeparatorChar);

IDictionary<string, string> userConfiguration = new Dictionary<string, string>();

userConfiguration["Server"] = Context.Parameters["Server"];
userConfiguration["Port"] = Context.Parameters["Port"];

ConfigGenerator.WriteExternalAppConfig(configFilePath, userConfiguration);

We use it for our test, training, and production servers, so all we have to do is specify the machine name and password during the install, and everything's taken care of for us. It used to be a 3-hour process, including going through multiple config files to set passwords. Now it's almost entirely automated.

Hope this helps.