Method LastIndexOf

LastIndexOf<TSource>(IEnumerable<TSource>, TSource)

Searches for the specified object and returns the zero-based index of the last occurrence within the entire IEnumerable<T>.

Declaration

public static int LastIndexOf<TSource>(this IEnumerable<TSource> source, TSource item)

Parameters

Type	Name	Description
IEnumerable<TSource>	source	The source sequence.
TSource	item	The object to locate in the IEnumerable<T>. The value can be null for reference types.

Returns

Type	Description
int	The zero-based index of the last occurrence of item within the entire IEnumerable<T>, if found; otherwise, -1.

Type Parameters

Name	Description
TSource	The type of elements of source

Remarks

The IEnumerable<T> is searched forward starting at the first element and ending at the last element, and the index of the last instance of item is returned.

This method determines equality using the default equality comparer Default for TSource, the type of values in the list.

This operator executes immediately.

Examples

The following code example demonstrates how to find the index of the last instance of an element in a sequence using LastIndexOf.

var sequence = new[]
{
    1, 2, 3, 4, 5,
    1, 2, 3, 4, 5,
}.AsEnumerable();

// Find the element `3` in the sequence
var result = sequence
    .LastIndexOf(3);

Console.WriteLine($"Index: {result}");

// This code produces the following output:
// Index: 7

Exceptions

Type	Condition
ArgumentNullException	source is null.

LastIndexOf<TSource>(IEnumerable<TSource>, TSource, Index)

Searches for the specified object and returns the zero-based index of the last occurrence within the range of elements in the IEnumerable<T> that extends backwards from the specified index to the first element.

Declaration

public static int LastIndexOf<TSource>(this IEnumerable<TSource> source, TSource item, Index index)

Parameters

Type	Name	Description
IEnumerable<TSource>	source	The source sequence.
TSource	item	The object to locate in the IEnumerable<T>. The value can be null for reference types.
Index	index	The Index of the ending element within the sequence.

Returns

Type	Description
int	The zero-based index of the last occurrence of item within the the range of elements in the IEnumerable<T> that extends backwards from index to the first element, if found; otherwise, -1.

Type Parameters

Name	Description
TSource	The type of elements of source

Remarks

The IEnumerable<T> is searched forward starting at the first element and ending at index, and the index of the last instance of item is returned.

This method determines equality using the default equality comparer Default for TSource, the type of values in the list.

This operator executes immediately.

Exceptions

Type	Condition
ArgumentNullException	source is null.

LastIndexOf<TSource>(IEnumerable<TSource>, TSource, Index, int)

Searches for the specified object and returns the zero-based index of the last occurrence within the range of elements in the IEnumerable<T> that ends at the specified index to the last element and contains the specified number of elements.

Declaration

public static int LastIndexOf<TSource>(this IEnumerable<TSource> source, TSource item, Index index, int count)

Parameters

Type	Name	Description
IEnumerable<TSource>	source	The source sequence.
TSource	item	The object to locate in the IEnumerable<T>. The value can be null for reference types.
Index	index	The Index of the ending element within the sequence.
int	count	The number of elements in the section to search.

Returns

Type	Description
int	The zero-based index of the last occurrence of item within the the range of elements in the IEnumerable<T> that that ends at index and contains count number of elements, if found; otherwise, -1.

Type Parameters

Name	Description
TSource	The type of elements of source

Remarks

The IEnumerable<T> is searched forward starting at the first element and ending at index, and the index of the last instance of item no earlier in the sequence than count items before index is returned.

This method determines equality using the default equality comparer Default for TSource, the type of values in the list.

This operator executes immediately.

Exceptions

Type	Condition
ArgumentNullException	source is null.
ArgumentOutOfRangeException	count is less than 0.