Например:
string maskedString = MaskString("ABCDEFGHIJLK", 2, 3, 'X');
Выход: ABXXXXXXXJLK
string maskedString = MaskString("123456", 2, 3, '*');
Выход: 12*456
string MaskString(string inputString, int leftUnMaskLength, int rightUnMaskLength, char maskCharacter)
{
//What is the simplest way to code this efficiently in C# .Net 3.5
}