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

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

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

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

source
Type: System.Linq.IQueryable<TSource>

An IQueryable<T> that contains the elements to be counted.

predicate
Type: System.Linq.Expressions.Expression<Func<TSource, Boolean>>

A function to test each element for a condition.

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. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).

ExceptionCondition
ArgumentNullException

source or predicate is null.

OverflowException

The number of elements in source is larger than MaxValue.

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.

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.

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);

    Console.WriteLine(
        "There are {0} unvaccinated animals.",
        numberUnvaccinated);
}

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

Windows 7, Windows Vista, Windows XP SP2, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5

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