Observable.Where<TSource> Method (IObservable<TSource>, Func<TSource, Boolean>)

Article
2011-06-28

Filters the elements of an observable sequence based on a predicate.

Namespace: System.Reactive.Linq
Assembly: System.Reactive (in System.Reactive.dll)

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function Where(Of TSource) ( _
    source As IObservable(Of TSource), _
    predicate As Func(Of TSource, Boolean) _
) As IObservable(Of TSource)

'Usage
Dim source As IObservable(Of TSource)
Dim predicate As Func(Of TSource, Boolean)
Dim returnValue As IObservable(Of TSource)

returnValue = source.Where(predicate)

public static IObservable<TSource> Where<TSource>(
    this IObservable<TSource> source,
    Func<TSource, bool> predicate
)

[ExtensionAttribute]
public:
generic<typename TSource>
static IObservable<TSource>^ Where(
    IObservable<TSource>^ source, 
    Func<TSource, bool>^ predicate
)

static member Where : 
        source:IObservable<'TSource> * 
        predicate:Func<'TSource, bool> -> IObservable<'TSource>

JScript does not support generic types and methods.

Type Parameters

TSource
The type source.

Parameters

source
Type: System.IObservable<TSource>
An observable sequence whose elements to filter.

predicate
Type: System.Func<TSource, Boolean>
A function to test each source element for a condition.

Return Value

Type: System.IObservable<TSource>
An observable sequence that contains elements from the input sequence that satisfy the condition.

Usage Note

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

Remarks

The Where operator allows you to define a predicate function to test each item in a sequence. Each item from the source sequence which causes the predicate function to return false is filtered from the resulting sequence.

Examples

The following example uses the Where operator with a Language Integerated Query (LINQ) to filter the integer sequence so that only even integers are produced for the sequence.

using System;
using System.Reactive.Linq;

namespace Example
{
  class Program
  {
    static void Main()
    {
      //*********************************************************************************************//
      //*** The mainSequence produces a new long integer from the Interval operator every sec.    ***//
      //*********************************************************************************************//

      var mainSequence = Observable.Interval(TimeSpan.FromSeconds(1));


      //*********************************************************************************************//
      //*** This LINQ statement uses the Where operator to filter the integers in the sequence so ***//
      //*** that only the even integers are returned.                                             ***//
      //***                                                                                       ***//
      //*** you could also call the method explicitly. For example...                             ***//
      //***                                                                                       ***//
      //*** var seqWhereEven = mainSequence.Where(x => x % 2 == 0);                               ***//
      //***                                                                                       ***//
      //*********************************************************************************************//

      var seqWhereEven = from i in mainSequence
                         where i % 2 == 0 
                         select i;


      //*********************************************************************************************//
      //*** Create a subscription and write each of the even integers to the console window.      ***//
      //*********************************************************************************************//

      seqWhereEven.Subscribe(x => Console.WriteLine(x)); 
      Console.ReadLine();
    }
  }
}

The following output was generated by the example code.

Share via