String interpolation into a variable

Question

I'm attempting to insert a person's name into the body of an email as dynamic values using string interpolation in c# 6.0.

Proof of Concept

public class Program
{
    public static void Main()
    {
        var p = new Person { FirstName = "John", LastName = "Smith" };
        var msg = $"{p.FirstName} says 'Hello World!'";
        
        Console.WriteLine($"Message: {msg}");
    }
}
public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

This proof of concept outputs the string:

Message: John says 'Hello World!'

Implementation

So now when I'm trying to use the concept to build my MailMessage, I'm getting {Contact.FirstName} output in my email. Here's the code:

private string _Body;
public string Body
{
    get { return _Body; }
    set
    {
        _Body = value;
        RaisePropertyChangedEvent("Body");
    }
}

var Contacts = Context.Contacts
  .Where(x => x.ContactGroupId == ContactGroupId && x.Deleted == false);

// By this point, Body will already have a value containing the interpolation.
// For example: Body = "Message: {Contact.Name} says \"Hello World!\"";
foreach (var Contact in Contacts)
{
    MailMessage Msg = new MailMessage();
    
    MailMessage.Body = ${Body}
}

Problem

This should render the body to read as:

Message: John says "Hello World!"

Instead, what renders is:

Message: {Contact.FirstName} says "Hello World!"

How can I interpolate my variables into the string if I'm not actually hard coding the string in my code?

String interpolation is a compile time feature. It cannot be used to replace tokens at runtime. — Tim Schmelter
– Tim Schmelter, Commented Jul 21, 2017 at 11:07

Zein Makki · Accepted Answer · 2017-07-21 11:16:54Z

4

${Body} doesn't compile.

You can't generate the interpolation at run-time, it is a syntactic sugar that gets transformed by the compiler into String.Format.

You can solve your problem by using Replace from StringBuilder or String:

sb.Replace("{p.FirstName}", p.FirstName);

edited Jul 21, 2017 at 11:16

answered Jul 21, 2017 at 11:08

Zein Makki

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6 Comments

Ortund Over a year ago

String.Format is fine but won't it have the same problem? Anyway I can't have the user typing in {0} {1} etc tags in the mail body when he's writing the email... Suppose I could always leave it as {Contact.FirstName} and then .Replace() the hell out of it...

DavidG Over a year ago

@Ortund Yes, that's what you need to do, though I suggest using StringBuilder for that.

Zein Makki Over a year ago

@Ortund You can solve your problem multiple ways, String.Replace is one of the options. String.Replace("{p.FirstName}", p.FirstName); StringBuilder is a better option though.

Ortund Over a year ago

I guess either way I go, the issue of interpolation is now moot so accepting an answer becomes a strange issue

DavidG Over a year ago

@Ortund Actually it's a valid question, I'm sure other people have had the same issue.

|

Kevin Li · Accepted Answer · 2017-07-21 11:22:37Z

0

String interpolation is a compile-time feature. That’s why you can do some neat things like:

Console.WriteLine($"When {condition} is true, {(condition ? "it's true!" : "It's False")}");

You are essentially putting C# code inside the string!

However, if it were implemented as a dynamic feature where you could load strings and have them evaluate at runtime, then you would have a security risk much like the use of eval in JavaScript. It makes sense, then, that it wouldn’t be possible to do what you want to do.

answered Jul 21, 2017 at 11:22

Kevin Li

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