• Article

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

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Returns the number of elements in the specified sequence that satisfies a condition.

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

Syntax

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

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

Type Parameters

  • TSource
    The type of the elements of source.

Parameters

Return Value

Type: System.Int32
The number of elements in the sequence that satisfies the condition in the predicate function.

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).
OverflowException

The number of elements in source is larger than MaxValue.

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 Count<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) method generates a MethodCallExpression that represents calling Count<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 Count<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) depends on the implementation of the type of the source parameter. The expected behavior is that it counts the number of items in source that satisfy the condition specified by predicate.

Examples

The following code example demonstrates how to use Count<TSource>(IQueryable<TSource>, Expression<Func<TSource, Boolean>>) to count the elements in a sequence that satisfy a condition.

   Structure Pet
      Public Name As String
      Public Vaccinated As Boolean
   End Structure

   Shared Sub CountEx2()
      ' Create an array of Pet objects.
      Dim pets() As Pet = {New Pet With {.Name = "Barley", .Vaccinated = True}, _
                     New Pet With {.Name = "Boots", .Vaccinated = False}, _
                     New Pet With {.Name = "Whiskers", .Vaccinated = False}}

      ' Count the number of unvaccinated pets in the array.
      Dim numberUnvaccinated As Integer = pets.AsQueryable().Count(Function(p) p.Vaccinated = False)

      outputBlock.Text &= String.Format("There are {0} unvaccinated animals.", _
                                        numberUnvaccinated) & vbCrLf
   End Sub

   ' This code produces the following output:
   '
   ' There are 2 unvaccinated animals.


         class Pet
         {
            public string Name { get; set; }
            public bool Vaccinated { get; set; }
         }

         public static void CountEx2()
         {
            // Create an array of Pet objects.
            Pet[] pets = { new Pet { Name="Barley", Vaccinated=true },
                               new Pet { Name="Boots", Vaccinated=false },
                               new Pet { Name="Whiskers", Vaccinated=false } };

            // Count the number of unvaccinated pets in the array.
            int numberUnvaccinated =
                pets.AsQueryable().Count(p => p.Vaccinated == false);

            outputBlock.Text += String.Format(
                "There are {0} unvaccinated animals.",
                numberUnvaccinated) + "\n";
         }

         // This code produces the following output:
         //
         // There are 2 unvaccinated animals.

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.