string[20] c = "";

char[20] c = "";

FixedLengthString s = new FixedLengthString(13);
s.Value = "12345678901234567890";

char[] c13 = new char[14];
c13 = "12345678901234567890".ToCharArray();

char[] c13 = new char[14];
c13 = "12345678901234567890".ToCharArray(0,13);

public string HasStringClaimedLength(string value, int claimedLength)
{
    return (value.Length <= claimedLength) ? value : value.Substring(0, claimedLength);
}

Console.WriteLine(HasStringClaimedLength("Hier ein String der nur 10 Zeichen haben darf", 10));

Hier ein S

public static class Extensions
{
    public static string HasStringClaimedLength(this string value, int claimedLength)
    {
        return (value.Length <= claimedLength) ? value : value.Substring(0, claimedLength);
    }
}

Console.WriteLine("Hier ein String der nur 20 Zeichen haben darf".HasStringClaimedLength(20));

public class FixedString
{
 private string _internalString = "";
 private int _maxSize = 65536;

 public FixedString()
 {
 }

 public FixedString(int maxSize)
 {
  _maxSize = maxSize;
 }

 public string FString
 {
  get { return _internalString; }
  set
  {
   if (value.Length <= _maxSize)
   {
    _internalString = value;
   }
   else
   {
    _internalString = value.Substring(0, _maxSize);
   }
  }
 }

 public int FStringMaxSize
 {
  get { return _maxSize; }
  set
  {
   if (value > 0)
   {
    _maxSize = value;
   }
   else
   {
    throw new ArgumentOutOfRangeException("FixedString Length may not be zero!");
   }
  }
 }

 public override string ToString()
 {
  return _internalString;
 }

 public string ToStringPadLeft()
 {
  return _internalString.PadLeft(_maxSize);
 }

 public string ToStringPadLeft(char c)
 {
  return _internalString.PadLeft(_maxSize, c);
 }

 public string ToStringPadRight()
 {
  return _internalString.PadRight(_maxSize);
 }

 public string ToStringPadRight(char c)
 {
  return _internalString.PadRight(_maxSize, c);
 }
}