String interpolation by user input

Question

is possible to make interpolation based on user input?

For example something like this:

public string Foo(string input)
{
    string user = "random_user_name";
    string address = "random_address";
    string city = "random_city";

    return $"{input}";
}

• Example input1: "{user}, {city}"
• Expected output1: "random_user_name, random_city"

• Example input2: "{address}"
• Expected output2: "random_address"

• Example input3: "{city}, {address}"
• Expected output3: "random_city, random_address"

I need that user can define which variables want and their order. Btw user will know variable names.

Answer 1

Taking @TheGeneral's suggestion, I did the following:

The first thing I did was to change your three strings into a Dictionary<string, string>

var tokens = new Dictionary<string, string> {
    { "user", "random_user_name" },
    { "address", "random_address" },
    { "city",  "random_city" }
};

Then I created a worker function that does the replacement in the way @TheGeneral described

public static string DoReplacement(string key, Dictionary<string, string> tokens)
{
    var result = new StringBuilder(key);
    foreach (var kvp in tokens)
    {
        result.Replace("{" + kvp.Key + "}", kvp.Value);
    }
    return result.ToString();
}

And finally, I wrapped it all together:

public static string Test(string input)
{
    var tokens = new Dictionary<string, string> {
        { "user", "random_user_name" },
        { "address", "random_address" },
        { "city",  "random_city" }
    };

    return $"{DoReplacement(input, tokens)}";
}

To exercise this, I called it with the OP's examples:

var tests = new List<string> {
    "{user}, {city}",
    "{address}",
    "{city}, {address}",
};
foreach (var s in tests)
{
    var replaced = StringReplacement.Test(s);
    Debug.WriteLine(replaced);
}

and got this output:

random_user_name, random_city
random_address
random_city, random_address