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Back to Basics: String Interpolation in C#

One of my favorite features of C# 6.0 - which has been out for a while now - is String Interpolation. String Interpolation is a fancy name for compiler based string templates that allow you to embed expression values into literal strings with the compiler expanding the embedded values into the string using the $ operator:

var msg = $".NET Version: {Environment.Version}" 
// .NET Version: 4.0.30319.42000

You can think of String Interpolation of compiler sugar around the string.Format() function for string literals, although there are some differences between how you use String Interpolation and string.Format() that I describe later in this post.

String Interpolation - Inline Template Literals

String Interpolation is like an inlined, much more readable version of string.Format(). Rather than using string.Format() with {0}, {1} etc. for value embedding, you can use {varNameOrExpression} directly inside of the string content.

To demonstrate let's look at creating an embedded string using string.Format() first. I'm using LinqPad here and you can find these examples in a Gist on GitHub:


string name = "Rick";
int accesses = 10;
string output = string.Format("{0}, you've been here {1:n0} times.",
                              name, accesses);

The same code written using C# 6.0 string interpolation looks like this:

String Interpolation

string name = "Rick";
int accesses = 10;
string output = $"{name}, you've been here {accesses:n0} times.";

Both produce:

Rick, you've been here 10 times.

Notice that like String.Format() the expressions you use can provide a format expression, which is the same value you would use in a .ToString("n0") function.

The second example is much more readable, especially if you have a few expressions that you're pushing into a string. You don't have to worry about order as you can simply embed any valid and in-scope C# expression into the string template.

{Expressions} can be any valid C# expression.

The following is a simple math expression:

int x = 20;
int y = 15
string = $"Adding {x} + {y} equals {x + y}"

You can also use object properties and call methods on objects. The following uses the DateTime and Exception objects:

catch(Exception ex) 
    mmApp.Log($"{DateTime.UtcNow.ToString("dd-MM-yyyy HH:mm:ss")} - DOM Doccument update failed: {ex.Message}",ex);

Note I'm using .ToString() explicitly in the code above to demonstrate that you can use a method, but really you can use the formatting expression syntax:

$"Time: {DateTime.UtcNow:dd-MM-yyyy HH:mm:ss}"

You can use object properties and methods just as easily as local variables declared inside of the current method. Any valid C# expression that's in scope is usable for an expression.


String templates also work across multiple lines so you can expand text into a verbatim string literal prefixed with the @:

// parameterized values
DateTime releaseDate = DateTime.Now.Date;
decimal version = 1.01M;
string newStuff = @"
* Fixed blah
* Updated foo
* Refactored stuff

// actual string literal
string message = $@"Version {version} of Markdown Monster is now available.

Released on: {releaseDate:MMM dd, yyyy}	


Which produces:

Version 1.01 of Markdown Monster is now available.
  Released on: Dec 26, 2016  
  * Fixed blah
  * Updated foo
  * Refactored stuff

The combination of multi line literals and embedded string expressions make for a much more readable experience when dealing with long strings. This is useful for message dialogs, log entries and any other situations where you need to write out larger blocks of text with embedded values.

Interpolated Strings are not a Template Solution

At first blush interpolated strings look like an easy way to create string templates that evaluate expressions. But it's important to understand that String Interpolation in C# is merely compiler generated syntactic sugar that dynamically generates string.Format() code with compile time expressions that are parameterized. The format string has to be a static string literal.

The String literals part is important: You can't load an Interpolated Format String string like "Hello {name}" from a file and 'evaluate' that. Would be nice if that worked, but no cigar...

This means that unlike string.Format() which does let you explicitly specify a format string at runtime, string interpolation requires that the format string is a static string literal in your source code.

Interpolated Strings must be Static String Literals

Interpolated strings have to exist in their entirety at compile time as string literals, and all the expressions embedded in the string must be properly in scope in order for the compiler to embed them into the generated code. Otherwise a compiler error is generated.

What this means is that you can't parameterize the format string with String Interpolation. This concept does not work:

var format = "Time is {DateTime.Now}";

But you can parameterize the format string when using string.Format(). This does work:

var format = "Time is {0}";
Console.WriteLine(String.Format(format, DateTime.Now));

Looking at the IL Code

To understand how this works you can look at the generated IL code on an Interpolated string.

Let's look at the first example again:

string name = "Rick";
int accesses = 10;
string output = $"{name}, you've been here {accesses:n0} times.";

which turns into this IL code (as decompiled by LinqPad):

IL_0000:  nop         
IL_0001:  ldstr       "Rick"
IL_0006:  stloc.0     // name
IL_0007:  ldc.i4.s    0A 
IL_0009:  stloc.1     // accesses
IL_000A:  ldstr       "{0}, you've been here {1:n0} times."
IL_000F:  ldloc.0     // name
IL_0010:  ldloc.1     // accesses
IL_0011:  box         System.Int32
IL_0016:  call        System.String.Format
IL_001B:  stloc.2     // output
IL_001C:  ldloc.2     // output
IL_001D:  call        LINQPad.Extensions.Dump<String>
IL_0022:  pop         
IL_0023:  ret

You can see how the compiler is turning our Interpolated String literal into a string.Format() method call. The compiler creates creates local variables and embeds those into the string.Format() call as parameters.

The following code uses an exception object:

public void Log(Exception ex)
 	string val = $"{DateTime.UtcNow.ToString("dd-MM-yyyy HH:mm:ss")} - DOM Doccument update failed: {ex.Message}";

which turns into:

IL_0000:  nop         
IL_0001:  ldstr       "{0} - DOM Doccument update failed: {1}"
IL_0006:  call        System.DateTime.get_UtcNow
IL_000B:  stloc.1     
IL_000C:  ldloca.s    01 
IL_000E:  ldstr       "dd-MM-yyyy HH:mm:ss"
IL_0013:  call        System.DateTime.ToString
IL_0018:  ldarg.1     
IL_0019:  callvirt    System.Exception.get_Message
IL_001E:  call        System.String.Format
IL_0023:  stloc.0     // val
IL_0024:  ldloc.0     // val
IL_0025:  call        LINQPad.Extensions.Dump<String>
IL_002A:  pop         
IL_002B:  ret   

It's a neat trick, but it clearly demonstrates that you can't dynamically load a string with expressions and expect to evaluate the string. The code is generated at compile time and hard codes the expressions in the string into the compiled code which means the expressions are fixed at runtime.

Effectively this means you can use this feature only for inlining expressions into literal strings.

Posted in C#  .NET  

The Voices of Reason


Grzegorz Danowski
December 27, 2016

# re: Back to Basics: String Interpolation in C#

Please correct String.Format example: string output = string.Format("{0}, you've been here {1:n0} times.") to string output = string.Format("{0}, you've been here {1:n0} times.", name, accesses);

And why do you use ToString in string interpolation? For example, why: var msg = $".NET Version: {Environment.Version.ToString()}" but not simply: var msg = $".NET Version: "

December 27, 2016

# re: Back to Basics: String Interpolation in C#

Hi Rick, I use string interpolation everyday and it is a great replacement for string.format. Can you give a small example when you say that, "you can't dynamically load a string with expressions and expect to evaluate the string". I am able to run your Log function in linqpad just fine. Thanks.

Rick Strahl
December 27, 2016

# re: Back to Basics: String Interpolation in C#

@Grzegorz - thanks for doing my proof reading 😃 Guess that will teach me to post at 1am in the morning... I had initially used .ToString() in one of the examples to demonstrate use of methods, but failed to clarify. I've added that in and removed it in the first example. Thanks for the feedback - appreciated!

December 27, 2016

# re: Back to Basics: String Interpolation in C#

Definitely one of my favorite features as well.

For example, it simplifies my go-to byte[] to hex string converter:

 //Sample data var data = Encoding.ASCII.GetBytes("Hello Rick");
//Original version data.Select(b => String.Format("0x{0:X2}", b)).Aggregate((l,r) => String.Concat(l, ", ", r))

//Version with string interpolation data.Select (b => $"0x{b:X2}").Aggregate((l,r) => $"{l}, {r}");").Aggregate((l,r) => $", ");

December 28, 2016

# re: Back to Basics: String Interpolation in C#

@Mitul I think what he means is that you can't do something like this

var a = "hello it's currently {DateTime.Now}";

but you could do

var a = "hello it's currently {0}";
Console.WriteLine(String.Format(a, DateTime.Now));

Thomas Levesque
December 28, 2016

# re: Back to Basics: String Interpolation in C#

You can use String Interpolation with C# 6.0, and any post 4.5 version of .NET assuming you are using the C# 6 or later Roslyn compiler. Although Roslyn can compile down to .NET 4, String Interpolation relies on newer features of the framework to work properly.

There is no such restriction; I used string interpolation in a project targeting .NET 2.0. FormattableString is not necessary in most cases, but if you need it, you can add it yourself, as explained here, or use this NuGet package

Rick Strahl
December 28, 2016

# re: Back to Basics: String Interpolation in C#

Thanks @Thomas. I was looking for more info on this and came up blank. Thanks for the link - I've referenced it in the post and added to the post's resources.

David Scheppers
January 04, 2017

# re: Back to Basics: String Interpolation in C#

There's so much syntactic sugar in c# 6, I may have contracted syntactic diabetes 😃

January 04, 2017

# re: Back to Basics: String Interpolation in C#

Things I wish you'd have said:

  • Interpolation uses the current culture. This is important to know if you're creating a string for machines rather than humans (e.g. building up markup or something).
  • Talk about the built-in Invariant() helper method.
  • Talk about getting a FormattableString from the interpolation rather than a string.
  • Mention that everything is boxed, an array is allocated and everything (including plain strings) go through a formatting step. For trivial usages like $"Hello {name}"this makes interpolation a lot more expensive than "Hello " + name with little benefits.

January 04, 2017

# re: Back to Basics: String Interpolation in C#

This is one of my favorite features in C# 6 as well. I've been using it heavily. I have an app that generates Entity Framework classes, a web app, and other source files, from reverse engineering a database. I've been using T4 templates to do a lot of it. But, I'm starting to wonder if it wouldn't be easier to just ditch T4 altogether and use multi-line strings and string interpolation for everything in a standard C# file. I did this for some of what I'm working on. The source files that I'm generating have so much conditional code in them that there is more conditional logic than static text. Plus, I'm annoyed by the fact that Microsoft doesn't even have syntax coloring for T4 template files. I've been using the extensions to Visual Studio for support for that, but, they don't recognize interpolated strings and in general are kind of flaky. I also use LINQ a lot. string.Join() is another one of my favorites, although I think that was always in .NET, though I didn't find out about it until later.

    private DbCommand <#= string.Join(", ", Context.Entities.Where(e => !e.IsView && e.Properties.Any(p => !p.NotMapped && !p.IsKey && !p.IsDatabaseGenerated && !p.IsNavigationProperty)).Select(e => $"update{e.Name}DbCommand")) #>;

June 01, 2017

# re: Back to Basics: String Interpolation in C#

Hey, thanks so much for the article. Though it seems like a simple enough concept, I've just come up blank when trying to use string interpolation in one of our projects with a file as my source. Your information helped me save a lot of time trying to figure out my problem here and I'm quite happy to read about this in more detail. Thanks!


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