Queryable.First<TSource> Method (IQueryable<TSource>, Expression<Func<TSource, Boolean>>)
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Returns the first element of a sequence that satisfies a specified condition.
Namespace: System.Linq
Assembly: System.Core (in System.Core.dll)
Syntax
'Declaration
<ExtensionAttribute> _
Public Shared Function First(Of TSource) ( _
source As IQueryable(Of TSource), _
predicate As Expression(Of Func(Of TSource, Boolean)) _
) As TSource
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.
Exceptions
| Exception | Condition |
|---|---|
| ArgumentNullException | source or predicate is nulla null reference (Nothing in Visual Basic). |
| InvalidOperationException | No element satisfies the condition in predicate. -or- The source sequence is empty. |
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.
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.
Dim numbers() As Integer = {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.
Dim first As Integer = numbers.AsQueryable().First(Function(number) number > 80)
outputBlock.Text &= first & vbCrLf
' This code produces the following output:
'
' 92
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);
outputBlock.Text += first + "\n";
/*
This code produces the following output:
92
*/
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.