IndexOf Method (T, Int32, Int32)
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List<'T>.IndexOf Method ('T, Int32, 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 starts at the specified index and contains the specified number of elements.

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

member IndexOf : 
        item:'T *
        index:int *
        count:int -> int


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. 0 (zero) is valid in an empty list.

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.

Exception Condition

index is outside the range of valid indexes for the List<'T>.


count is less than 0.


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

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.

The following 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.

No code example is currently available or this language may not be supported.

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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