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

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 extends from the specified index to the last element.

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

public int IndexOf(
	T item,
	int index


Type: T
The object to locate in the List<T>. The value can be null for reference types.
Type: System.Int32
The zero-based starting index of the 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 extends from index to the last element, if found; otherwise, –1.


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

The List<T> is searched forward starting at index and ending at the last element.

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 the number of elements from index to the end of the List<T>.

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.

using System;
using System.Collections.Generic;

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


      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:


IndexOf("Tyrannosaurus"): 0

IndexOf("Tyrannosaurus", 3): 5

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


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.

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