C# convert int to string with padding zeros?

Question

In C# I have an integer value which need to be convereted to string but it needs to add zeros before:

For Example:

int i = 1;

When

Answer 1

C# 6.0 style string interpolation

int i = 1;
var str1 = $"{i:D4}";
var str2 = $"{i:0000}";