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');