Method IndexOf

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IndexOf<TSource>(IEnumerable<TSource>, TSource)

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

Declaration
public static int IndexOf<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 first 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.

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 an element in a sequence using IndexOf.

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

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

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

// This code produces the following output:
// Index: 2
Exceptions
Type Condition
ArgumentNullException

source is null.
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IndexOf<TSource>(IEnumerable<TSource>, TSource, Index)

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

Declaration
public static int IndexOf<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 starting element within the sequence.
Returns
Type Description
int

The zero-based index of the first occurrence of item within the the range of elements in the IEnumerable<T> that extends from index to the last 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 index and ending at the last element.

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 an element in a sequence using IndexOf.

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

// Find the element `3` in the sequence in the range [5..]
var result = sequence
    .IndexOf(3, 5);

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

// This code produces the following output:
// Index: 7
Exceptions
Type Condition
ArgumentNullException

source is null.
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IndexOf<TSource>(IEnumerable<TSource>, TSource, Index, int)

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

Declaration
public static int IndexOf<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 starting 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 first occurrence of item within the the range of elements in the IEnumerable<T> that that starts 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 index and ending at index plus count minus 1, if count is greater than 0.

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 an element in a sequence using IndexOf.

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

// Find the element `3` in the sequence in the range [5..7]
var result = sequence
    .IndexOf(3, 5, 2);

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

// This code produces the following output:
// Index: -1
Exceptions
Type Condition
ArgumentNullException

source is null.
ArgumentOutOfRangeException

count is less than 0.