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Queryable.First<TSource> Method (IQueryable<TSource>, Expression<Func<TSource, Boolean>>)

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

Namespace:  System.Linq
Assemblies:   System.Linq.Queryable (in System.Linq.Queryable.dll)
  System.Core (in System.Core.dll)

public static TSource First<TSource>(
	this IQueryable<TSource> source,
	Expression<Func<TSource, bool>> predicate
)

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Linq.IQueryable<TSource>

An IQueryable<T> to return an element from.

predicate
Type: System.Linq.Expressions.Expression<Func<TSource, Boolean>>

A function to test each element for a condition.

Return Value

Type: TSource
The first element in source that passes the test in predicate.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable<TSource>. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

ExceptionCondition
ArgumentNullException

source or predicate is null.

InvalidOperationException

No element satisfies the condition in predicate.

-or-

The source sequence is empty.

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.

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.

            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
            */

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library
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