Article
11/18/2011

List<T>.IndexOf Method (T, Int32, Int32)

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Searches for the specified object and returns the zero-based index of the first occurrence within the range of elements in the List<T> that starts at the specified index and contains the specified number of elements.

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

Syntax

'Declaration
Public Function IndexOf ( _
    item As T, _
    index As Integer, _
    count As Integer _
) As Integer

public int IndexOf(
    T item,
    int index,
    int count
)

Parameters

item
Type: T
The object to locate in the List<T>. The value can be nulla null reference (Nothing in Visual Basic) for reference types.

index
Type: System.Int32
The zero-based starting index of the search.

count
Type: System.Int32
The number of elements in the section to search.

Return Value

Type: System.Int32
The zero-based index of the first occurrence of item within the range of elements in the List<T> that starts at index and contains count number of elements, if found; otherwise, –1.

Exceptions

Exception	Condition
ArgumentOutOfRangeException	index is outside the range of valid indexes for the List<T>. -or- count is less than 0. -or- index and count do not specify a valid section in the List<T>.

ArgumentOutOfRangeException

index is outside the range of valid indexes for the List<T>.

-or-

count is less than 0.

-or-

index and count do not specify a valid section in the List<T>.

Remarks

The List<T> is searched forward starting at index and ending at index plus count minus 1, if count is greater than 0.

This method determines equality using the default equality comparer EqualityComparer<T>.Default for T, the type of values in the list.

This method performs a linear search; therefore, this method is an O(n) operation, where n is count.

Examples

The following code example demonstrates all three overloads of the IndexOf method. A List<T> of strings is created, with one entry that appears twice, at index location 0 and index location 5. The IndexOf(T) method overload searches the list from the beginning, and finds the first occurrence of the string. The IndexOf(T, Int32) method overload is used to search the list beginning with index location 3 and continuing to the end of the list, and finds the second occurrence of the string. Finally, the IndexOf(T, Int32, Int32) method overload is used to search a range of two entries, beginning at index location two; it returns –1 because there are no instances of the search string in that range.

Imports System.Collections.Generic

Public Class Example

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

      Dim dinosaurs As New List(Of String)

      dinosaurs.Add("Tyrannosaurus")
      dinosaurs.Add("Amargasaurus")
      dinosaurs.Add("Mamenchisaurus")
      dinosaurs.Add("Brachiosaurus")
      dinosaurs.Add("Deinonychus")
      dinosaurs.Add("Tyrannosaurus")
      dinosaurs.Add("Compsognathus")

      outputBlock.Text &= vbCrLf
      For Each dinosaur As String In dinosaurs
         outputBlock.Text &= dinosaur & vbCrLf
      Next

      outputBlock.Text &= String.Format(vbLf & _
          "IndexOf(""Tyrannosaurus""): {0}", _
          dinosaurs.IndexOf("Tyrannosaurus")) & vbCrLf

      outputBlock.Text &= String.Format(vbLf & _
          "IndexOf(""Tyrannosaurus"", 3): {0}", _
          dinosaurs.IndexOf("Tyrannosaurus", 3)) & vbCrLf

      outputBlock.Text &= String.Format(vbLf & _
          "IndexOf(""Tyrannosaurus"", 2, 2): {0}", _
          dinosaurs.IndexOf("Tyrannosaurus", 2, 2)) & vbCrLf

   End Sub
End Class

' This code example produces the following output:
'
'Tyrannosaurus
'Amargasaurus
'Mamenchisaurus
'Brachiosaurus
'Deinonychus
'Tyrannosaurus
'Compsognathus
'
'IndexOf("Tyrannosaurus"): 0
'
'IndexOf("Tyrannosaurus", 3): 5
'
'IndexOf("Tyrannosaurus", 2, 2): -1

using System;
using System.Collections.Generic;

public class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      List<string> dinosaurs = new List<string>();

      dinosaurs.Add("Tyrannosaurus");
      dinosaurs.Add("Amargasaurus");
      dinosaurs.Add("Mamenchisaurus");
      dinosaurs.Add("Brachiosaurus");
      dinosaurs.Add("Deinonychus");
      dinosaurs.Add("Tyrannosaurus");
      dinosaurs.Add("Compsognathus");

      outputBlock.Text += "\n";
      foreach (string dinosaur in dinosaurs)
      {
         outputBlock.Text += dinosaur + "\n";
      }

      outputBlock.Text += String.Format("\nIndexOf(\"Tyrannosaurus\"): {0}",
          dinosaurs.IndexOf("Tyrannosaurus")) + "\n";

      outputBlock.Text += String.Format("\nIndexOf(\"Tyrannosaurus\", 3): {0}",
          dinosaurs.IndexOf("Tyrannosaurus", 3)) + "\n";

      outputBlock.Text += String.Format("\nIndexOf(\"Tyrannosaurus\", 2, 2): {0}",
          dinosaurs.IndexOf("Tyrannosaurus", 2, 2)) + "\n";
   }
}

/* This code example produces the following output:

Tyrannosaurus
Amargasaurus
Mamenchisaurus
Brachiosaurus
Deinonychus
Tyrannosaurus
Compsognathus

IndexOf("Tyrannosaurus"): 0

IndexOf("Tyrannosaurus", 3): 5

IndexOf("Tyrannosaurus", 2, 2): -1
 */

Version Information

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

Platforms

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

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