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

Solution 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");

Solution 2:

You could encapsulate the string in a struct that implements IFormattable

public struct PaddedString : IFormattable
{
   private string value;
   public PaddedString(string value) { this.value = value; }

   public string ToString(string format, IFormatProvider formatProvider)
   { 
      //... use the format to pad value
   }

   public static explicit operator PaddedString(string value)
   {
     return new PaddedString(value);
   }
}

Then use this like that :

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

result:

"->xxxxxxxxxxxxxxxHello<-"

Solution 3:

Edit: I misunderstood your question, I thought you were asking how to pad with spaces.

What you are asking is not possible using the string.Format alignment component; string.Format always pads with whitespace. See the Alignment Component section of MSDN: Composite Formatting.

According to Reflector, this is the code that runs inside StringBuilder.AppendFormat(IFormatProvider, string, object[]) which is called by string.Format:

int repeatCount = num6 - str2.Length;
if (!flag && (repeatCount > 0))
{
    this.Append(' ', repeatCount);
}
this.Append(str2);
if (flag && (repeatCount > 0))
{
    this.Append(' ', repeatCount);
}

As you can see, blanks are hard coded to be filled with whitespace.