Get Month Name from Month Number or vice versa

namespace Months
{
  using System;
  using System.Collections.Generic;
  using System.Globalization;

  /// <summary>
  /// Class to help with month name/number conversion
  /// </summary>
  public class MonthsHelper
  {
    /// <summary>
    /// Initializes a new instance of the <see cref="MonthsHelper"/> class.
    /// </summary>
    public MonthsHelper()
      : this(CultureInfo.CurrentCulture)
    {
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="MonthsHelper"/> class.
    /// </summary>
    /// <param name="culture">The culture.</param>
    public MonthsHelper(CultureInfo culture)
      : this((culture ?? CultureInfo.CurrentCulture).DateTimeFormat)
    {
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="MonthsHelper"/> class.
    /// </summary>
    /// <param name="dateFormatInfo">The date format info.</param>
    public MonthsHelper(DateTimeFormatInfo dateFormatInfo)
    {
      DateFormatInfo = dateFormatInfo ?? CultureInfo.CurrentCulture.DateTimeFormat;
      for (int m = 0; m < MonthArrayCount; m++)
      {
        var name = DateFormatInfo.MonthNames[m];
        FullMonthNameToNumberMap[name] = m + 1;
        name = DateFormatInfo.AbbreviatedMonthNames[m];
        ShortMonthNameToNumberMap[name] = m + 1;
        name = DateFormatInfo.MonthGenitiveNames[m];
        FullMonthGenitiveNameToNumberMap[name] = m + 1;
        name = DateFormatInfo.AbbreviatedMonthGenitiveNames[m];
        ShortMonthGenitiveNameToNumberMap[name] = m + 1;
      }
    }

    private const int MonthArrayCount = 13;    // The 4 Month Names arrays are always 13 long. (per MSDN)
    private readonly DateTimeFormatInfo DateFormatInfo;
    private readonly Dictionary<string, int> FullMonthNameToNumberMap = new Dictionary<string, int>(MonthArrayCount, StringComparer.Ordinal);
    private readonly Dictionary<string, int> ShortMonthNameToNumberMap = new Dictionary<string, int>(MonthArrayCount, StringComparer.Ordinal);
    private readonly Dictionary<string, int> FullMonthGenitiveNameToNumberMap = new Dictionary<string, int>(MonthArrayCount, StringComparer.Ordinal);
    private readonly Dictionary<string, int> ShortMonthGenitiveNameToNumberMap = new Dictionary<string, int>(MonthArrayCount, StringComparer.Ordinal);

    /// <summary>
    /// Gets the full name of the month.
    /// </summary>
    /// <param name="monthNumber">The month number.</param>
    /// <returns>Full name of the month, or string.Empty if invalid monthNumber</returns>
    public string FullMonthName(int monthNumber)
    {
      if (monthNumber < 1 || monthNumber > 12)
        return string.Empty;
      return DateFormatInfo.MonthNames[monthNumber - 1];
    }

    /// <summary>
    /// Gets the short name of the month.
    /// </summary>
    /// <param name="monthNumber">The month number.</param>
    /// <returns>Short name of the month, or string.Empty if invalid monthNumber</returns>
    public string ShortMonthName(int monthNumber)
    {
      if (monthNumber < 1 || monthNumber > 12)
        return string.Empty;
      return DateFormatInfo.AbbreviatedMonthNames[monthNumber - 1];
    }

    /// <summary>
    /// Gets the full genitive name of the month.
    /// </summary>
    /// <param name="monthNumber">The month number.</param>
    /// <returns>Full genitive name of the month, or string.Empty if invalid monthNumber</returns>
    public string FullMonthGenitiveName(int monthNumber)
    {
      if (monthNumber < 1 || monthNumber > 12)
        return string.Empty;
      return DateFormatInfo.MonthGenitiveNames[monthNumber - 1];
    }

    /// <summary>
    /// Gets the short genitive name of the month.
    /// </summary>
    /// <param name="monthNumber">The month number.</param>
    /// <returns>Short genitive name of the month, or string.Empty if invalid monthNumber</returns>
    public string ShortMonthGenitiveName(int monthNumber)
    {
      if (monthNumber < 1 || monthNumber > 12)
        return string.Empty;
      return DateFormatInfo.AbbreviatedMonthGenitiveNames[monthNumber - 1];
    }

    /// <summary>
    /// Gets the month number (1-based) for the month name.
    /// </summary>
    /// <param name="monthName">Name of the month.</param>
    /// <remarks>Uses all 4 possible sets of month names to attempt to determine month number.</remarks>
    /// <returns>The month number (1-12) for the month name, or -1 if monthName is invalid.</returns>
    public int MonthNameToNumber(string monthName)
    {
      int monthNumber;
      if (!FullMonthNameToNumberMap.TryGetValue(monthName, out monthNumber) &&
          !ShortMonthNameToNumberMap.TryGetValue(monthName, out monthNumber) &&
          !FullMonthGenitiveNameToNumberMap.TryGetValue(monthName, out monthNumber) &&
          !ShortMonthGenitiveNameToNumberMap.TryGetValue(monthName, out monthNumber))
      {
        monthNumber = -1;
      }
      return monthNumber;
    }

    /// <summary>
    /// Gets the month number (1-based) for the month name.
    /// </summary>
    /// <param name="monthName">Full name of the month.</param>
    /// <returns>The month number (1-12) for the month name, or -1 if monthName is invalid.</returns>
    public int FullMonthNameToNumber(string monthName)
    {
      int monthNumber;
      if (!FullMonthNameToNumberMap.TryGetValue(monthName, out monthNumber))
      {
        monthNumber = -1;
      }
      return monthNumber;
    }

    /// <summary>
    /// Gets the month number (1-based) for the month name.
    /// </summary>
    /// <param name="monthName">Short name of the month.</param>
    /// <returns>The month number (1-12) for the month name, or -1 if monthName is invalid.</returns>
    public int ShortMonthNameToNumber(string monthName)
    {
      int monthNumber;
      if (!ShortMonthNameToNumberMap.TryGetValue(monthName, out monthNumber))
      {
        monthNumber = -1;
      }
      return monthNumber;
    }

    /// <summary>
    /// Gets the month number (1-based) for the month name.
    /// </summary>
    /// <param name="monthName">Full genitive name of the month.</param>
    /// <returns>The month number (1-12) for the month name, or -1 if monthName is invalid.</returns>
    public int FullMonthGenitiveNameToNumber(string monthName)
    {
      int monthNumber;
      if (!FullMonthGenitiveNameToNumberMap.TryGetValue(monthName, out monthNumber))
      {
        monthNumber = -1;
      }
      return monthNumber;
    }

    /// <summary>
    /// Gets the month number (1-based) for the month name.
    /// </summary>
    /// <param name="monthName">Short genitive name of the month.</param>
    /// <returns>The month number (1-12) for the month name, or -1 if monthName is invalid.</returns>
    public int ShortMonthGenitiveNameToNumber(string monthName)
    {
      int monthNumber;
      if (!ShortMonthGenitiveNameToNumberMap.TryGetValue(monthName, out monthNumber))
      {
        monthNumber = -1;
      }
      return monthNumber;
    }
  }
}

using System;
using System.Diagnostics;

namespace Months
{
  class Program
  {
    static readonly string[] FullMonths = { "null", "January", "February", "March", "April", "May", "June", 
                                            "July", "August", "September", "October", "November", "December" };
    //static readonly string[] ShortMonths = { "null", "Jan", "Feb", "Mar", "Apr", "May", "Jun", 
    //                                         "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" };
    static void Main(string[] args)
    {
      Stopwatch sw = new Stopwatch();
      sw.Start();
      const int repeat = 100000;
      for (int r = 0; r < repeat; r++)
      {
        for (int m = 1; m <= 12; m++)
        {
          DateTime dtDate = new DateTime(2000, m, 1);
          //var sMonthName = dtDate.ToString("MMM");
          var sMonthFullName = dtDate.ToString("MMMM");
        }
      }
      sw.Stop();
      Console.WriteLine("Original: DateTime.ToString = {0}", TimeSpan.FromTicks(sw.ElapsedTicks));
      sw.Restart();
      for (int r = 0; r < repeat; r++)
      {
        for (int m = 1; m <= 12; m++)
        {
          DateTime dtDate = new DateTime(2000, m, 1);
          //var sMonthName = ShortMonths[m];
          //var iMonthNo = Convert.ToDateTime("01-" + sMonthName + "-2011").Month;
          var sMonthName = FullMonths[m];
          var iMonthNo = Convert.ToDateTime("01-" + sMonthName + "-2011").Month;
        }
      }
      sw.Stop();
      Console.WriteLine("Original: Convert.ToDateTime = {0}", TimeSpan.FromTicks(sw.ElapsedTicks));
      MonthsHelper helper = new MonthsHelper();
      sw.Restart();
      for (int r = 0; r < repeat; r++)
      {
        helper = new MonthsHelper();
      }
      sw.Stop();
      Console.WriteLine("New: initializing new MonthsHelper() = {0}", TimeSpan.FromTicks(sw.ElapsedTicks));
      sw.Restart();
      for (int r = 0; r < repeat; r++)
      {
        for (int m = 1; m <= 12; m++)
        {
          //var sMonthName = helper.ShortMonthName(m);
          var sMonthFullName = helper.FullMonthName(m);
        }
      }
      sw.Stop();
      Console.WriteLine("New: MonthsHelper.FullMonthName = {0}", TimeSpan.FromTicks(sw.ElapsedTicks));
      sw.Restart();
      for (int r = 0; r < repeat; r++)
      {
        for (int m = 1; m <= 12; m++)
        {
          //var sMonthName = ShortMonths[m];
          //var iMonthNo = helper.ShortMonthNameToNumber(sMonthName);
          var sMonthName = FullMonths[m];
          var iMonthNo = helper.FullMonthNameToNumber(sMonthName);
        }
      }
      sw.Stop();
      Console.WriteLine("New: MonthsHelper.FullMonthNameToNumber = {0}", TimeSpan.FromTicks(sw.ElapsedTicks));
      Console.ReadLine();
    }
  }
}

public static class DateTimeExt
{
    public static string GetShortMonthName(this DateTime dateTime)
    {
        return dateTime.ToString("MMM");
    }

    public static string GetFullMonthName(this DateTime dateTime)
    {
        return dateTime.ToString("MMMM");
    }
}