[c#] How do I capitalize first letter of first name and last name in C#?

To get round some of the issues/problems that have ben highlighted I would suggest converting the string to lower case first and then call the ToTitleCase method. You could then use IndexOf(" Mc") or IndexOf(" O\'") to determine special cases that need more specific attention.

inputString = inputString.ToLower();
inputString = CultureInfo.CurrentCulture.TextInfo.ToTitleCase(inputString);
int indexOfMc = inputString.IndexOf(" Mc");
if(indexOfMc  > 0)
{
   inputString.Substring(0, indexOfMc + 3) + inputString[indexOfMc + 3].ToString().ToUpper() + inputString.Substring(indexOfMc + 4);
}