Queryable.First Method

Definition

Returns the first element of a sequence.

Overloads

First<TSource>(IQueryable<TSource>)

Returns the first element of a sequence.

First<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)

Returns the first element of a sequence that satisfies a specified condition.

First<TSource>(IQueryable<TSource>)

Source:
Queryable.cs
Source:
Queryable.cs
Source:
Queryable.cs

Returns the first element of a sequence.

C#
public static TSource First<TSource>(this System.Linq.IQueryable<TSource> source);

Type Parameters

TSource

The type of the elements of source.

Parameters

source
IQueryable<TSource>

The IQueryable<T> to return the first element of.

Returns

TSource

The first element in source.

Exceptions

source is null.

The source sequence is empty.

Examples

The following code example demonstrates how to use First<TSource>(IQueryable<TSource>) to return the first element in a sequence.

C#
int[] numbers = { 9, 34, 65, 92, 87, 435, 3, 54,
                    83, 23, 87, 435, 67, 12, 19 };

int first = numbers.AsQueryable().First();

Console.WriteLine(first);

/*
    This code produces the following output:

    9
*/

Remarks

The First<TSource>(IQueryable<TSource>) method generates a MethodCallExpression that represents calling First<TSource>(IQueryable<TSource>) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling First<TSource>(IQueryable<TSource>) depends on the implementation of the type of the source parameter. The expected behavior is that it returns the first element in source.

Applies to

.NET 9 and other versions
Product Versions
.NET Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard 2.0, 2.1
UWP 10.0

First<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>)

Source:
Queryable.cs
Source:
Queryable.cs
Source:
Queryable.cs

Returns the first element of a sequence that satisfies a specified condition.

C#
public static TSource First<TSource>(this System.Linq.IQueryable<TSource> source, System.Linq.Expressions.Expression<Func<TSource,bool>> predicate);

Type Parameters

TSource

The type of the elements of source.

Parameters

source
IQueryable<TSource>

An IQueryable<T> to return an element from.

predicate
Expression<Func<TSource,Boolean>>

A function to test each element for a condition.

Returns

TSource

The first element in source that passes the test in predicate.

Exceptions

source or predicate is null.

No element satisfies the condition in predicate.

-or-

The source sequence is empty.

Examples

The following code example demonstrates how to use First<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) to return the first element of a sequence that satisfies a condition.

C#
int[] numbers = { 9, 34, 65, 92, 87, 435, 3, 54,
                  83, 23, 87, 435, 67, 12, 19 };

// Get the first number in the array that is greater than 80.
int first = numbers.AsQueryable().First(number => number > 80);

Console.WriteLine(first);

/*
    This code produces the following output:

    92
*/

Remarks

This method has at least one parameter of type Expression<TDelegate> whose type argument is one of the Func<T,TResult> types. For these parameters, you can pass in a lambda expression and it will be compiled to an Expression<TDelegate>.

The First<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) method generates a MethodCallExpression that represents calling First<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) itself as a constructed generic method. It then passes the MethodCallExpression to the Execute<TResult>(Expression) method of the IQueryProvider represented by the Provider property of the source parameter.

The query behavior that occurs as a result of executing an expression tree that represents calling First<TSource>(IQueryable<TSource>, Expression<Func<TSource,Boolean>>) depends on the implementation of the type of the source parameter. The expected behavior is that it returns the first element in source that satisfies the condition specified by predicate.

Applies to

.NET 9 and other versions
Product Versions
.NET Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard 2.0, 2.1
UWP 10.0