C# string interpolation with variable format

Question

I need to format a variable with string interpolation, and the format string is another variable:

here is my sample code:

static void Main(string[] args)
{
    int i = 12345;

    Console.WriteLine($"Test 1: {i:N5}");

    var formatString = "N5";

    Console.WriteLine($"Test 2: {i:formatString}");
}

Test 1 works, Test 2 don't work.

What's the exact syntax for Test 2 ?

Answer 1

Your code is equivalent to:

Console.WriteLine(String.Format("Test 2: {0:formatString}", i));

As the formatString is in the format string, you would nest String.Format calls to put the value in the format string:

Console.WriteLine(String.Format(String.Format("Test 2: {{0:{0}}}", formatstring), i));

This isn't supported with string interpolation.

Answer 2

The shortest way you can do this 'syntactically' without String.Format, is using ToString:

$"Test 2: {i.ToString(formatString)}"