Статья
11/17/2011

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)

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function Average(Of TSource) ( _
    source As IQueryable(Of TSource), _
    selector As Expression(Of Func(Of TSource, Nullable(Of Double))) _
) As Nullable(Of Double)

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

Type Parameters

TSource
The type of the elements of source.

Parameters

source
Type: System.Linq.IQueryable<TSource>
A sequence of values to calculate the average of.

selector
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 nulla null reference (Nothing in Visual Basic) if the source sequence is empty or contains only nulla null reference (Nothing in Visual Basic) values.

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 selector is nulla null reference (Nothing in Visual Basic).

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 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.

Examples

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.

Note:
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.

      Dim fruits() As String = {"apple", "banana", "mango", "orange", "passionfruit", "grape"}

      ' Determine the average string length in the array.
      Dim average As Double = fruits.AsQueryable().Average(Function(s) s.Length)

      outputBlock.Text &= String.Format("The average string length is {0}.", _
                                        average) & vbCrLf

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

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

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

         outputBlock.Text += String.Format("The average string length is {0}.", average) + "\n";

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

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.

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