项目
11/18/2011

Queryable.Cast<TResult> Method

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Converts the elements of an IQueryable to the specified type.

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

Syntax

'Declaration
<ExtensionAttribute> _
Public Shared Function Cast(Of TResult) ( _
    source As IQueryable _
) As IQueryable(Of TResult)

public static IQueryable<TResult> Cast<TResult>(
    this IQueryable source
)

Type Parameters

TResult
The type to convert the elements of source to.

Parameters

source
Type: System.Linq.IQueryable
The IQueryable that contains the elements to be converted.

Return Value

Type: System.Linq.IQueryable<TResult>
An IQueryable<T> that contains each element of the source sequence converted to the specified type.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IQueryable. When you use instance method syntax to call this method, omit the first parameter.

Exceptions

Exception	Condition
ArgumentNullException	source is nulla null reference (Nothing in Visual Basic).
InvalidCastException	An element in the sequence cannot be cast to type TResult.

Remarks

The Cast<TResult>(IQueryable) method generates a MethodCallExpression that represents calling Cast<TResult>(IQueryable) itself as a constructed generic method. It then passes the MethodCallExpression to the CreateQuery(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 Cast<TResult>(IQueryable) depends on the implementation of the type of the source parameter. The expected behavior is that it converts the values in source to type TResult.

Examples

The following code example demonstrates how to use Cast<TResult>(IQueryable) to convert objects in a sequence to type String.


      ' Create a list of objects.
      Dim words As New List(Of Object)(New Object() {"green", "blue", "violet"})

      ' Cast the objects in the list to type 'string'
      ' and project the first letter of each string.
      Dim query As IEnumerable(Of String) = _
          words.AsQueryable() _
                  .Cast(Of String)() _
                  .Select(Function(str) str.Substring(0, 1))

      For Each s As String In query
         outputBlock.Text &= s & vbCrLf
      Next

      ' This code produces the following output:
      '
      ' g
      ' b
      ' v


         // Create a list of objects.
         List<object> words =
             new List<object> { "green", "blue", "violet" };

         // Cast the objects in the list to type 'string'
         // and project the first letter of each string.
         IEnumerable<string> query =
             words.AsQueryable()
             .Cast<string>()
             .Select(str => str.Substring(0, 1));

         foreach (string s in query)
            outputBlock.Text += s + "\n";

         /*  This code produces the following output:

             g
             b
             v
         */

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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