string username;
string password;
string domain;
string location;

void AssignValueFromConfigFile()
{
    username = ConfigurationManager.AppSettings["username"];
    if (username == null)
        throw new ValueNotFound("username");

    password = ConfigurationManager.AppSettings["password"];
}

var values = new Dictionary<string,string>() { {"username",""},{"password",""},{"domain",""},{"location",""}}

void AssignValueFromConfigFile()
{
    foreach(var val in values)
    {
        val.Value = ConfigurationManager.AppSettings[val.Key];
        if (val.Value == null)
            throw new ValueNotFound(val.Value);
    }
}

void AssignValueFromConfigFile(string name, ref string value) {
    value = ConfigurationManager.AppSettings[name];
    if(value == null) {
        throw new ValueNotFound(name);
    }
}

    string username;
    string password;
    string domain;
    string location;
    AssignValueFromConfigFile("username", ref username);
    AssignValueFromConfigFile("password", ref password);
    AssignValueFromConfigFile("domain", ref domain);
    AssignValueFromConfigFile("location", ref location);

string AssignValueFromConfigFile(string name) {
    var value = ConfigurationManager.AppSettings[name];
    if(value == null) {
        throw new ValueNotFound(name);
    }
    return value;
}

    var username = AssignValueFromConfigFile("username");
    var password = AssignValueFromConfigFile("password");
    var domain = AssignValueFromConfigFile("domain");
    var location = AssignValueFromConfigFile("location");