Επεξεργασία

Κοινή χρήση μέσω

ObjectQuery<T>.SelectValue<TResultType> Method

  • Reference

Definition

Namespace:
System.Data.Objects
Assembly:
System.Data.Entity.dll

Limits the query results to only the property specified in the projection.

public:
generic <typename TResultType>
 System::Data::Objects::ObjectQuery<TResultType> ^ SelectValue(System::String ^ projection, ... cli::array <System::Data::Objects::ObjectParameter ^> ^ parameters);
public System.Data.Objects.ObjectQuery<TResultType> SelectValue<TResultType> (string projection, params System.Data.Objects.ObjectParameter[] parameters);
member this.SelectValue : string * System.Data.Objects.ObjectParameter[] -> System.Data.Objects.ObjectQuery<'ResultType>
Public Function SelectValue(Of TResultType) (projection As String, ParamArray parameters As ObjectParameter()) As ObjectQuery(Of TResultType)

Type Parameters

TResultType

The type of the ObjectQuery<T> returned by the SelectValue<TResultType>(String, ObjectParameter[]) method.

Parameters

projection
String

The projection list.

parameters
ObjectParameter[]

An optional set of query parameters that should be in scope when parsing.

Returns

ObjectQuery<TResultType>

A new ObjectQuery<T> instance of a type compatible with the specific projection. The returned ObjectQuery<T> is equivalent to the original instance with SELECT VALUE applied.

Exceptions

ArgumentNullException

projection is null.

-or-

parameters is null.

ArgumentException

The projection is an empty string.

Examples

This example creates a new ObjectQuery<T> that contains results that are a sequence of ProductID values projected from the results of the existing query.

using (AdventureWorksEntities context =
    new AdventureWorksEntities())
{
    string queryString =
        @"SELECT VALUE product FROM
            AdventureWorksEntities.Products AS product";

    ObjectQuery<Product> productQuery1 =
        new ObjectQuery<Product>(queryString,
            context, MergeOption.NoTracking);

    ObjectQuery<Int32> productQuery2 =
        productQuery1.SelectValue<Int32>("it.ProductID");

    foreach (Int32 result in productQuery2)
    {
        Console.WriteLine("{0}", result);
    }
}

Remarks

SelectValue is used to return values that are simple types, entity types, or complex types. Use the Select method for projections that require a row type instead of a value type. For more information, see Object Queries.

The SelectValue method applies the projection specified by the projection parameter. The ObjectQuery<T> returned by the SelectValue method must be of a value type that is compatible with the projection and must be the same type as the type of SelectValue.

Applies to

See also