System.Globalization.TextInfo.LastIndexOfStringOrdinalIgnoreCase C# (CSharp) Method

        internal static unsafe int LastIndexOfStringOrdinalIgnoreCase(String source, String value, int startIndex, int count) {

            if (source == null)
                throw new ArgumentNullException("source");

            return nativeLastIndexOfStringOrdinalIgnoreCase(InvariantNativeTextInfo, source, value, startIndex, count);
        }

 public virtual unsafe int LastIndexOf(string source, string value, int startIndex, int count, CompareOptions options)
 {
     if (source == null)
     {
         throw new ArgumentNullException("source");
     }
     if (value == null)
     {
         throw new ArgumentNullException("value");
     }
     if ((((options & ~(CompareOptions.IgnoreWidth | CompareOptions.IgnoreKanaType | CompareOptions.IgnoreSymbols | CompareOptions.IgnoreNonSpace | CompareOptions.IgnoreCase)) != CompareOptions.None) && (options != CompareOptions.Ordinal)) && (options != CompareOptions.OrdinalIgnoreCase))
     {
         throw new ArgumentException(Environment.GetResourceString("Argument_InvalidFlag"), "options");
     }
     if ((source.Length == 0) && ((startIndex == -1) || (startIndex == 0)))
     {
         if (value.Length != 0)
         {
             return(-1);
         }
         return(0);
     }
     if ((startIndex < 0) || (startIndex > source.Length))
     {
         throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index"));
     }
     if (startIndex == source.Length)
     {
         startIndex--;
         if (count > 0)
         {
             count--;
         }
         if (((value.Length == 0) && (count >= 0)) && (((startIndex - count) + 1) >= 0))
         {
             return(startIndex);
         }
     }
     if ((count < 0) || (((startIndex - count) + 1) < 0))
     {
         throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_Count"));
     }
     if (options == CompareOptions.OrdinalIgnoreCase)
     {
         return(TextInfo.LastIndexOfStringOrdinalIgnoreCase(source, value, startIndex, count));
     }
     if (!this.IsSynthetic)
     {
         return(LastIndexOfString(this.m_pSortingTable, this.m_sortingLCID, source, value, startIndex, count, (int)options));
     }
     if (options == CompareOptions.Ordinal)
     {
         return(LastIndexOfString(CultureInfo.InvariantCulture.CompareInfo.m_pSortingTable, this.m_sortingLCID, source, value, startIndex, count, (int)options));
     }
     return(this.SyntheticLastIndexOf(source, value, startIndex, count, GetNativeCompareFlags(options)));
 }