Assigning local variables in a bulk way

Question

I've been trying to mull over a question I am not sure how to solve and it probably has a simple solution however I am trying to work out a simple solution.

The following scenario is as such. Let's say we have the following variables.

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

and I want to run the same function across them

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

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

..... etc for each one

This is obviously really tedious and the wrong way to do this. We can make this work in a dictionary if we had:

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);
    }
}

But then we lose the ability to call the local variables and have to know what they are in the dictionary. How is this achieved?

Answer 1

There's no need for a dictionary (unless you want one, of course). You could assign each variable by reference, e.g.

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);

However, a much neater solution would be to apply the command/query separation pattern, e.g.

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

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

You have one function that reads the data and guarantees that it is not null. The calling code then decides what to do with the data.

Answer 2

You might be able to get cheeky with reflection if you want to do this to all properties of a class, for example:

public class LoginViewModel 
{
    string username;
    string password;
    string domain;
    string location;

    public void init()
    {
        var props = this.GetType().GetProperties();
        foreach(var prop in props) 
        {
            prop.SetValue(this, ConfigurationManager.AppSettings[prop.Name]);
            if (prop.GetValue(this) == null)
                throw new ValueNotFound(prop.Name);
        }
    }
}