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String.LastIndexOf Method (Char, Int32, Int32)

Updated: October 2010

Reports the zero-based index position of the last occurrence of the specified Unicode character in a substring within this instance. The search starts at a specified character position and examines a specified number of character positions.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)

'Declaration
<SecuritySafeCriticalAttribute> _
Public Function LastIndexOf ( _
	value As Char, _
	startIndex As Integer, _
	count As Integer _
) As Integer

Parameters

value
Type: System.Char
A Unicode character to seek.
startIndex
Type: System.Int32
The starting position of a substring within this instance.
count
Type: System.Int32
The number of character positions to examine.

Return Value

Type: System.Int32
The zero-based index position of value if that character is found, or -1 if it is not found or if the current instance equals String.Empty.

ExceptionCondition
ArgumentOutOfRangeException

The current instance does not equal String.Empty, and startIndex is less than zero or greater than or equal to the length of this instance.

-or-

The current instance does not equal String.Empty, and startIndex - count + 1 is less than zero.

Index numbering starts from zero. That is, the first character in the string is at index 0 and the last is at Length - 1.

This method begins searching at the startIndex character position of this instance and proceeds backward toward the beginning until either value is found or count character positions have been examined. For example, if startIndex is Length - 1, the method searches backward count characters from the last character in the string. The search is case-sensitive.

This method performs an ordinal (culture-insensitive) search, where a character is considered equivalent to another character only if their Unicode scalar value are the same. To perform a culture-sensitive search, use the CompareInfo.LastIndexOf method, where a Unicode scalar value representing a precomposed character, such as the ligature 'Æ' (U+00C6), might be considered equivalent to any occurrence of the character's components in the correct sequence, such as "AE" (U+0041, U+0045), depending on the culture.

The following code example finds the index of all occurrences of a character in a substring, working from the end of the substring to the start of the substring.


' Sample for String.LastIndexOf(Char, Int32, Int32)
 _

Class Example

   Public Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)

      Dim br1 As String = "0----+----1----+----2----+----3----+----4----+----5----+----6----+-"
      Dim br2 As String = "0123456789012345678901234567890123456789012345678901234567890123456"
      Dim str As String = "Now is the time for all good men to come to the aid of their party."
      Dim start As Integer
      Dim at As Integer
      Dim count As Integer
      Dim [end] As Integer

      start = str.Length - 1
      [end] = start / 2 - 1
      outputBlock.Text &= String.Format("All occurrences of 't' from position {0} to {1}.", start, [end]) & vbCrLf
      outputBlock.Text &= String.Format("{1}{0}{2}{0}{3}{0}", vbCrLf, br1, br2, str) & vbCrLf
      outputBlock.Text &= "The letter 't' occurs at position(s): "

      count = 0
      at = 0
      While start > -1 And at > -1
         count = start - [end] 'Count must be within the substring.
         at = str.LastIndexOf("t"c, start, count)
         If at > -1 Then
            outputBlock.Text &= String.Format("{0} ", at)
            start = at - 1
         End If
      End While
      outputBlock.Text &= String.Format("{0}{0}{0}", vbCrLf)
   End Sub 'Main 
End Class 'Sample
'
'This example produces the following results:
'All occurrences of 't' from position 66 to 32.
'0----+----1----+----2----+----3----+----4----+----5----+----6----+-
'0123456789012345678901234567890123456789012345678901234567890123456
'Now is the time for all good men to come to the aid of their party.
'
'The letter 't' occurs at position(s): 64 55 44 41 33
'
'


Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

Date

History

Reason

October 2010

Noted that an exception is not thrown for an empty string if the index or length parameter is invalid.

Content bug fix.

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