Pad left or right with string.format (not padleft or padright) with arbitrary string

Question

Can I use String.Format() to pad a certain string with arbitrary characters?

Console.WriteLine("->{0,18}<-", "hello");
Console.WriteLine("->{0,-18}<-", "hello");

returns 

->             hello<-
->hello             <-

I now want the spaces to be an arbitrary character. The reason I cannot do it with padLeft or padRight is because I want to be able to construct the format string at a different place/time then the formatting is actually executed.

--EDIT--
Seen that there doesn't seem to be an existing solution to my problem I came up with this (after Think Before Coding's suggestion)
--EDIT2--
I needed some more complex scenarios so I went for Think Before Coding's second suggestion

[TestMethod]
public void PaddedStringShouldPadLeft() {
    string result = string.Format(new PaddedStringFormatInfo(), "->{0:20:x} {1}<-", "Hello", "World");
    string expected = "->xxxxxxxxxxxxxxxHello World<-";
    Assert.AreEqual(result, expected);
}
[TestMethod]
public void PaddedStringShouldPadRight()
{
    string result = string.Format(new PaddedStringFormatInfo(), "->{0} {1:-20:x}<-", "Hello", "World");
    string expected = "->Hello Worldxxxxxxxxxxxxxxx<-";
    Assert.AreEqual(result, expected);
}
[TestMethod]
public void ShouldPadLeftThenRight()
{
    string result = string.Format(new PaddedStringFormatInfo(), "->{0:10:L} {1:-10:R}<-", "Hello", "World");
    string expected = "->LLLLLHello WorldRRRRR<-";
    Assert.AreEqual(result, expected);
}
[TestMethod]
public void ShouldFormatRegular()
{
    string result = string.Format(new PaddedStringFormatInfo(), "->{0} {1:-10}<-", "Hello", "World");
    string expected = string.Format("->{0} {1,-10}<-", "Hello", "World");
    Assert.AreEqual(expected, result);
}

Because the code was a bit too much to put in a post, I moved it to github as a gist:
http://gist.github.com/533905#file_padded_string_format_info

There people can easily branch it and whatever :)

Answer 1

There is another solution.

Implement IFormatProvider to return a ICustomFormatter that will be passed to string.Format :

public class StringPadder : ICustomFormatter
{
  public string Format(string format, object arg,
       IFormatProvider formatProvider)
  {
     // do padding for string arguments
     // use default for others
  }
}

public class StringPadderFormatProvider : IFormatProvider
{
  public object GetFormat(Type formatType)
  { 
     if (formatType == typeof(ICustomFormatter))
        return new StringPadder();

     return null;
  }
  public static readonly IFormatProvider Default =
     new StringPadderFormatProvider();
}

Then you can use it like this :

string.Format(StringPadderFormatProvider.Default, "->{0:x20}<-", "Hello");