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  • Artykuł

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

Microsoft Silverlight will reach end of support after October 2021. Learn more.

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
Assembly:  System.Core (in System.Core.dll)

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function FirstOrDefault(Of TSource) ( _
    source As IQueryable(Of TSource), _
    predicate As Expression(Of Func(Of TSource, Boolean)) _
) As TSource

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

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.

Exceptions

Exception Condition
ArgumentNullException

source or predicate is nulla null reference (Nothing in Visual Basic).

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 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.

Examples

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.

      Dim names() As String = {"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.
      Dim firstLongName As String = _
                  names.AsQueryable().FirstOrDefault(Function(name) name.Length > 20)

      outputBlock.Text &= String.Format("The first long name is '{0}'.", _
                                        firstLongName) & vbCrLf

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

      outputBlock.Text &= String.Format( _
          "There is {0} name that is longer than 30 characters.", _
          IIf(String.IsNullOrEmpty(firstVeryLongName), "NOT a", "a")) & vbCrLf

      ' 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.


         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);

         outputBlock.Text += String.Format("The first long name is '{0}'.", firstLongName) + "\n";

         // 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);

         outputBlock.Text += String.Format(
             "There is {0} name that is longer than 30 characters.",
             string.IsNullOrEmpty(firstVeryLongName) ? "NOT a" : "a") + "\n";

         /*
             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.
         */

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.