Jan 022016
 January 2, 2016  Posted by at 8:51 am Not So Stupid Questions, Uncategorized  Add comments

I had been stuck on some legacy projects and forgotten the world of sexy new language features until I started doing some work for a new client here in Gothenburg. While coding at a café my coding buddy for the day noticed some work I was doing and asked about the new string interpolation in C#.

So let’s talk string interpolation,- here is my best attempt at explaining and please do join in 🙂 :

Interpolate comes from latin, interpolare and means ‘to give new appearance’ (‘to polish’). Interpolation is thus used to describe when one puts something between other things or parts, often more specifically inserting words into a text. This phrase becomes even more descriptive when we take a look at two alternatives also used, variable substitution and variable expansion.

FIY on the Stupid Question series:

[To celebrate my first year of programming I will ask a ‘stupid’ questions daily on my blog for a year, to make sure I learn at least 365 new things during my second year as a developer]

In C# you might be familiar with composite formatting, functions that takes a list of objects and a string as input, and we can insert our objects into the string.

String.Format(“Hello {0}”, person.Name) is one example I’m sure you are more than familiar with.
C# 6 introduced string interpolation, a feature that has been around for quite some time in many languages- and it looks very much like templating albeit for smaller phrases 🙂 JavaScript (for example) does have template strings ( string literals that allows you to embed expressions), and you can use string interpolation with them. But templating is not quite the same as interpolation.


var spinach = { bioavailability : 0.05, calcium : 300};
console.log(`From a cup of spinach only ${ spinach.calcium * spinach.bioavailability} mg of calcium is absorbed in calicium deficient subjects`);


var spinach = new {Bioavailability = 0.05, Calcium = 300};

// ‘Manual’ string concat
Console.WriteLine("From a cup of spinach only " + (spinach.Calcium * spinach.Bioavailability) + " mg of calcium is absorbed in deficient subjects");

// Composite formatting
Console.WriteLine("From a cup of spinach only {0} mg of calcium is absorbed in deficient subjects",(spinach.Calcium * spinach.Bioavailability));

// String interpolation
Console.WriteLine($"From a cup of spinach only {spinach.Calcium * spinach.Bioavailability} mg of calcium is absorbed in deficient subjects");

So how does it really work? What is it doing in the background? Well, it’s more or less syntactic sugar (and very sweet sugar that is). Using TryRoslyn we can see that it does a String.Format(), and String.Format() uses a stringbuilder internally which uses the .AppendFormat() method to do exactly that.

public static string Format(IFormatProvider provider, string format, params object[] args)
    if ((format == null) || (args == null))
        throw new ArgumentNullException((format == null) ? "format" : "args");

    StringBuilder builder = new StringBuilder(format.Length + (args.Length * 8));
    builder.AppendFormat(provider, format, args);

    return builder.ToString();

  5 Responses to “(Not so) Stupid Question 283: What is string interpolation?”

  1. I’ve only recently started using interpolation (mainly due to Resharper) and am in two minds. Embedding variables and logic in strings just feels wrong and leads to overly long strings that are a lot harder to read. It introduces additional brain work to separate out text and code.

    String.Format, on the other hand, clearly lays out the variables and logic being inserted in to the string, but requires a little additional brain power to tie up the variable to the place holder.

    On balance, I feel that string.Format leads to more readable code. I think it is something I will eventually use all the time, but I find myself still writing string.Format and letting Resharper do the conversion for me.

    • I think I agree with you, Steve, at least partially. At first look, this seems nice – just write the whole string you want in the order you want it – but I suspect that it may make the code more difficult to read. Might depend on how complex what you’re inserting is; if you had something like:

      Console.WriteLine(“His name was {Name} and he had been there for {Tenure} years”)

      that reads pretty well. It’s just the inline calculations that look messy.

  2. So, to be clear then, this doesn’t do anything other than giving you a “shortcut” for string.Format? I agree that the syntax is really nice, though!

  3. Python:

    name = ‘EvilKiru’
    print ‘My name is %s.’ %(name)

  4. @Steve

    For 1 or 2 parameter String.Formats, it’s probably not too bad either way. But with more complex ones, I imagine that it’s a lot easier to understand what the result will be if you can read inline with meaningful variable names, rather than needing to do the mental gymnastics to translate {0} {1} {2} …

    Plus it’s semantically identical to PowerShell string handling.

    A caveat is that you’d end up needing to go back the other way for doing internationalization/localization work.

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