March 2020 - Code Improvisation

In a devBetter discussion, a question was raised regarding composing strings: “Why would a developer use String.Format, when string interpolation seems much cleaner and easier to use?”

In case you are not familiar with StringFormat or string interpolation, here’s a code sample that demonstrates using them to compose a string:

var name = "Rick";
var shoeSize = 9.5;

// the manual approach
var x = "My name is " + name + " and my shoe size is " + shoeSize.ToString();

// String.Format
var y = String.Format("My name is {0} and my shoe size is {1}", name, shoeSize);

// string interpolation
var z = $"My name is {name} and my shoe size is {shoeSize}";

var name = "Rick";

var shoeSize = 9.5;

// the manual approach

var x = "My name is " + name + " and my shoe size is " + shoeSize.ToString();

// String.Format

var y = String.Format("My name is {0} and my shoe size is {1}", name, shoeSize);

// string interpolation

var z = $"My name is {name} and my shoe size is {shoeSize}";

Variables x, y, and z will all contain the same string value (“My name is Rick and my shoe size is 9.5”), but you can see how each approach used becomes progressively easier to read and understand. String interpolation is like a shorthand for a call to String.Format.

We’ll soon see that behind the scenes, string interpolation actually compiles down into a call to String.Format, but let’s look at String.Format, first.

String.Format evaluates its format parameter at runtime. Consider the following code that accepts a format string from the console, and displays the result of String.Format():

var name="Rick";
var shoeSize=9;   

while(true)
{
    Console.WriteLine("Enter a format that contains {0} and {1}: ");
    var format = Console.ReadLine();
    Console.WriteLine("You entered: "+format);
    Console.WriteLine("Result: "+String.Format(format,name,shoeSize));
}

var name="Rick";

var shoeSize=9;

while(true)

{

Console.WriteLine("Enter a format that contains {0} and {1}: ");

var format = Console.ReadLine();

Console.WriteLine("You entered: "+format);

Console.WriteLine("Result: "+String.Format(format,name,shoeSize));

}

Here’s a sample of the console output:

Enter a format that contains {0} and {1}:
You entered: Name {0} Shoe {1}
Result: Name Rick Shoe 9
Enter a format that contains {0} and {1}:
You entered: Shoe {1} Name {0}
Result: Shoe 9 Name Rick

In contrast, interpolation builds the format expression at compile time.

Let’s see what happens when we try to use string interpolation like String.Format:

var name = "Rick";
var shoeSize = 9.5;

var format1="Name: {0}, Shoe: {1}";
var format2="Shoe: {1}, Name: {0}";

// Name: Rick, Shoe: 9.5
var result1 = String.Format(format1, name, shoeSize);

// Shoe: 9.5, Name: Rick
var result2 = String.Format(format2, name, shoeSize);

var format3 = "Name: {name}, Shoe: {shoeSize}";

var interpolated = $format3; // this wont compile!

// this displays the string "Name: {name}, Shoe: {shoeSize}" 
// without evaluating the embedded expressions
var interpolated2 =$"{format3}";

var name = "Rick";

var shoeSize = 9.5;

var format1="Name: {0}, Shoe: {1}";

var format2="Shoe: {1}, Name: {0}";

// Name: Rick, Shoe: 9.5

var result1 = String.Format(format1, name, shoeSize);

// Shoe: 9.5, Name: Rick

var result2 = String.Format(format2, name, shoeSize);

var format3 = "Name: {name}, Shoe: {shoeSize}";

var interpolated = $format3; // this wont compile!

// this displays the string "Name: {name}, Shoe: {shoeSize}"

// without evaluating the embedded expressions

var interpolated2 =$"{format3}";

Why don’t we get the same results? The reason is that the compiler “hard codes” the interpolated format.

Let’s look at how the compiler compiles the following string interpolation:

    var name = "Rick";
    var shoeSize = 9;

    var result = $"My name is {name} and my shoe size is {shoeSize}";

var name = "Rick";

var shoeSize = 9;

var result = $"My name is {name} and my shoe size is {shoeSize}";

Here is the IL generated by the compiler.

IL_0000:  nop         
IL_0001:  ldstr       "Rick"
IL_0006:  stloc.0     
IL_0007:  ldc.i4.s    09 
IL_0009:  stloc.1     
IL_000A:  ldstr       "My name is {0} and my shoe size is {1}"
IL_000F:  ldloc.0     
IL_0010:  ldloc.1     
IL_0011:  box         System.Int32
IL_0016:  call        System.String.Format
IL_001B:  stloc.2     
IL_001C:  ret

IL_0000: nop

IL_0001: ldstr "Rick"

IL_0006: stloc.0

IL_0007: ldc.i4.s 09

IL_0009: stloc.1

IL_000A: ldstr "My name is {0} and my shoe size is {1}"

IL_000F: ldloc.0

IL_0010: ldloc.1

IL_0011: box System.Int32

IL_0016: call System.String.Format

IL_001B: stloc.2

IL_001C: ret

Compared to our “changeable” format in the while(true) loop, the format expression (the line starting with IL_000A) is “baked in”: the string “My name is …” will never change.

Both String.Format and string interpolation are valuable tools in our toolbelt. I use string interpolation most of the time (since it was added to C# in version 6), but String.Format can be very useful when you need to have configurable formats.

Monthly Archives: March 2020

To Format Or Interpolate