What is the optional argument in C# interpolated string for?

Question

Interpolated strings is one of the new features of C# 6.0.

According to MSDN, the syntax of the embedded C# expressions can contain an optional, comma-separated val

Answer 1

The number is the alignment, documented in the Alignment Component here.

The formatted data in the field is right-aligned if alignment is positive and left-aligned if alignment is negative.

In your example, alignment will pad the p.ProcessName with spaces if it is less than 5 characters long. Where string length is less than the absolute value of alignment (like in your example), alignment has no effect.

Example

var text = "MyText";
Console.WriteLine($"x{text}x");
Console.WriteLine($"x{text, 3}x");
Console.WriteLine($"x{text, 10}x");
Console.WriteLine($"x{text, -10}x");

Result

xMyTextx
xMyTextx
x    MyTextx
xMyText    x