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

int p = 3; // fixed length padding
int n = 55; // number to test

string t = n.ToString("D" + p); // magic     

Console.WriteLine("Hello, world! >> {0}", t);

// outputs: 
// Hello, world! >> 055