C# 最初の文字を大文字にする

using System;
class Program {
  static void Main() {
    string str = "delftstack";

    if (str.Length == 0) {
      Console.WriteLine("Empty String");
    } else if (str.Length == 1) {
      Console.WriteLine(char.ToUpper(str[0]));
    } else {
      Console.WriteLine(char.ToUpper(str[0]) + str.Substring(1));
    }
  }
}

using System;
using System.Text.RegularExpressions;

class Program {
  static public string UpperCaseFirstChar(string text) {
    return Regex.Replace(text, "^[a-z]", m => m.Value.ToUpper());
  }
  static void Main() {
    string str = "delftstack";
    Console.WriteLine(UpperCaseFirstChar(str));
  }
}

Delftstack

using System;

class Program {
  public static string UpperCaseFirstChar(string s) {
    if (string.IsNullOrEmpty(s)) {
      return string.Empty;
    }
    char[] a = s.ToCharArray();
    a[0] = char.ToUpper(a[0]);
    return new string(a);
  }
  public static void Main() {
    Console.WriteLine(UpperCaseFirstChar("delftstack"));
  }
}

Delftstack