Queryable.Average<TSource> Method (IQueryable<TSource>, Expression<Func<TSource, Nullable<Double>>>)


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Computes the average of a sequence of nullable Double values that is obtained by invoking a projection function on each element of the input sequence.

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

public static Nullable<double> Average<TSource>(
	this IQueryable<TSource> source,
	Expression<Func<TSource, Nullable<double>>> selector


Type: System.Linq.IQueryable<TSource>

A sequence of values to calculate the average of.

Type: System.Linq.Expressions.Expression<Func<TSource, Nullable<Double>>>

A projection function to apply to each element.

Return Value

Type: System.Nullable<Double>

The average of the sequence of values, or null if the source sequence is empty or contains only null values.

Type Parameters


The type of the elements of source.

Exception Condition

source or selector is null.

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 Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Nullable<Double>>>) method generates a MethodCallExpression that represents calling Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Nullable<Double>>>) 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 Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Nullable<Double>>>) depends on the implementation of the type of the source parameter. The expected behavior is that it calculates the average of the values in source after invoking selector on each value.

The following code example demonstrates how to use Average<TSource>(IQueryable<TSource>, Expression<Func<TSource, Int32>>) to calculate the average String length in a sequence of values of type String.


This code example uses an overload of this overloaded method that is different from the specific overload that this topic describes. To extend the example to this topic, change the body of the selector function.

string[] fruits = { "apple", "banana", "mango", "orange", "passionfruit", "grape" };

// Determine the average string length in the array.
double average = fruits.AsQueryable().Average(s => s.Length);

Console.WriteLine("The average string length is {0}.", average);

// This code produces the following output:
// The average string length is 6.5. 

Universal Windows Platform
Available since 8
.NET Framework
Available since 3.5
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.1
Windows Phone
Available since 8.1
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