如何:对查询结果分页(实体框架)

本主题介绍如何对查询结果分页。 本示例先跳过查询结果(按 Product.ListPrice 排序)中的前三个 Product 对象,然后获取五个对象。 将使用以下每种实体框架查询技术说明同一示例:

  • LINQ to Entities

  • Entity SQL 以及 ObjectQuery<T>

  • ObjectQuery<T> 的查询生成器方法

本主题中的示例基于 Adventure Works 销售模型。若要运行本主题中的代码,则必须已经将 Adventure Works 销售模型添加到了您的项目中,并且已经将项目配置为使用实体框架。有关更多信息,请参见如何:使用实体数据模型向导(实体框架)如何:手动配置实体框架项目如何:手动定义实体数据模型(实体框架)

示例

这是 LINQ to Entities 示例。

Using context As New AdventureWorksEntities
    'LINQ to Entities only supports Skip on ordered collections.
    Dim products As IOrderedQueryable(Of Product) = _
            context.Products.OrderBy(Function(p) p.ListPrice)

    Dim allButFirst3Products As IQueryable(Of Product) = products.Skip(3)

    Console.WriteLine("All but first 3 products:")
    For Each product As Product In allButFirst3Products
        Console.WriteLine("Name: {0} \t ID: {1}", _
                product.Name, _
                product.ProductID)
    Next
End Using
using (AdventureWorksEntities context = new AdventureWorksEntities())
{
    // LINQ to Entities only supports Skip on ordered collections.
    IOrderedQueryable<Product> products = context.Products
            .OrderBy(p => p.ListPrice);

    IQueryable<Product> allButFirst3Products = products.Skip(3);

    Console.WriteLine("All but first 3 products:");
    foreach (Product product in allButFirst3Products)
    {
        Console.WriteLine("Name: {0} \t ID: {1}",
            product.Name,
            product.ProductID);
    }
}

这是 Entity SQL 示例。

Using context As New AdventureWorksEntities()
    ' Create a query that takes two parameters. 
    Dim queryString As String = "SELECT VALUE product FROM AdventureWorksEntities.Products AS product " & _
        " order by product.ListPrice SKIP @skip LIMIT @limit"

    Dim productQuery As New ObjectQuery(Of Product)(queryString, context)

    ' Add parameters to the collection. 
    productQuery.Parameters.Add(New ObjectParameter("skip", 3))
    productQuery.Parameters.Add(New ObjectParameter("limit", 5))

    ' Iterate through the collection of Contact items. 
    For Each result As Product In productQuery
        Console.WriteLine("ID: {0}; Name: {1}", result.ProductID, result.Name)
    Next
End Using
using (AdventureWorksEntities context =
    new AdventureWorksEntities())
{
    // Create a query that takes two parameters.
    string queryString =
        @"SELECT VALUE product FROM 
          AdventureWorksEntities.Products AS product 
          order by product.ListPrice SKIP @skip LIMIT @limit";

    ObjectQuery<Product> productQuery =
        new ObjectQuery<Product>(queryString, context);

    // Add parameters to the collection.
    productQuery.Parameters.Add(new ObjectParameter("skip", 3));
    productQuery.Parameters.Add(new ObjectParameter("limit", 5));

    // Iterate through the collection of Contact items.
    foreach (Product result in productQuery)
        Console.WriteLine("ID: {0}; Name: {1}",
        result.ProductID, result.Name);
}

这是查询生成器方法示例。

Using context As New AdventureWorksEntities()
    ' Define the parameters used to define the "page" of returned data. 
    Dim skipValue As Integer = 3
    Dim limitValue As Integer = 5

    ' Define a query that returns a "page" or the full 
    ' Product data using the Skip and Top methods. 
    ' When Top() follows Skip(), it acts like the LIMIT statement. 
    Dim query As ObjectQuery(Of Product) = _
        context.Products.Skip("it.ListPrice", "@skip", _
            New ObjectParameter("skip", skipValue)).Top("@limit", New ObjectParameter("limit", limitValue))

    ' Iterate through the page of Product items. 
    For Each result As Product In query
        Console.WriteLine("ID: {0}; Name: {1}", result.ProductID, result.Name)
    Next
End Using
using (AdventureWorksEntities context =
    new AdventureWorksEntities())
{
    // Define the parameters used to define the "page" of returned data.
    int skipValue = 3;
    int limitValue = 5;

    // Define a query that returns a "page" or the full 
    // Product data using the Skip and Top methods. 
    // When Top() follows Skip(), it acts like the LIMIT statement.
    ObjectQuery<Product> query = context.Products
        .Skip("it.ListPrice", "@skip",
                new ObjectParameter("skip", skipValue))
        .Top("@limit", new ObjectParameter("limit", limitValue));

    // Iterate through the page of Product items.
    foreach (Product result in query)
        Console.WriteLine("ID: {0}; Name: {1}",
        result.ProductID, result.Name);
}

另请参见

参考

SKIP (Entity SQL)
LIMIT (Entity SQL)
Skip
Top

概念

分页 (Entity SQL)
Entity SQL 语言