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

Returns the first element of a sequence that satisfies a specified condition or a default value if no such element is found.

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

public static TSource FirstOrDefault<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
default(TSource) if source is empty or if no element passes the test specified by predicate; otherwise, the first element in source that passes the test specified by 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.

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 FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) method generates a MethodCallExpression that represents calling FirstOrDefault<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 FirstOrDefault<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 in predicate, or a default value if no element satisfies the condition.

The following code example demonstrates how to use FirstOrDefault<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) by passing in a predicate. In the second query, there is no element in the sequence that satisfies the condition.

            string[] names = { "Hartono, Tommy", "Adams, Terry", 
                                 "Andersen, Henriette Thaulow", 
                                 "Hedlund, Magnus", "Ito, Shu" };

            // Get the first string in the array that is longer 
            // than 20 characters, or the default value for type 
            // string (null) if none exists. 
            string firstLongName =
                names.AsQueryable().FirstOrDefault(name => name.Length > 20);

            Console.WriteLine("The first long name is '{0}'.", firstLongName);

            // Get the first string in the array that is longer 
            // than 30 characters, or the default value for type 
            // string (null) if none exists. 
            string firstVeryLongName =
                names.AsQueryable().FirstOrDefault(name => name.Length > 30);

            Console.WriteLine(
                "There is {0} name that is longer than 30 characters.",
                string.IsNullOrEmpty(firstVeryLongName) ? "NOT a" : "a");

            /*
                This code produces the following output:

                The first long name is 'Andersen, Henriette Thaulow'.
                There is NOT a name that is longer than 30 characters.
            */

.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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