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

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

Usage Note

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.

.NET Framework

.NET Framework Client Profile

Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

Reference