如何：使用表达式树来生成动态查询（C# 和 Visual Basic）
在 LINQ 中，表达式树用于表示针对数据源的结构化查询，这些数据源实现 IQueryable<T>。 例如，LINQ to SQL 提供程序实现 IQueryable<T> 接口，用于查询关系数据存储。 C# 和 Visual Basic 编译器会将针对此类数据源的查询编译为代码，该代码在运行时将生成一个表达式树。 然后，查询提供程序可以遍历表达式树数据结构，并将其转换为适合于数据源的查询语言。
表达式树还可以用在 LINQ 中，用于表示分配给类型为 Expression<TDelegate> 的变量的 Lambda 表达式。
本主题描述如何使用表达式树来创建动态 LINQ 查询。 如果在编译时不知道查询的细节，动态查询将十分有用。 例如，应用程序可能会提供一个用户界面，最终用户可以使用该用户界面指定一个或多个谓词来筛选数据。 为了使用 LINQ 进行查询，这种应用程序必须使用表达式树在运行时创建 LINQ 查询。
示例
下面的示例演示如何使用表达式树依据 IQueryable 数据源构造一个查询，然后执行该查询。 代码将生成一个表达式树来表示以下查询：
C# 查询
companies.Where(company => (company.ToLower() == "coho winery" || company.Length > 16)).OrderBy(company => company)
Visual Basic 查询
companies.Where(Function(company) company.ToLower() = "coho winery" OrElse company.Length > 16).OrderBy(Function(company) company)
System.Linq.Expressions 命名空间中的工厂方法用于创建表达式树，这些表达式树表示构成总体查询的表达式。 表示标准查询运算符方法调用的表达式将引用这些方法的 Queryable 实现。 最终的表达式树将传递到 IQueryable 数据源的提供程序的 CreateQuery<TElement>(Expression) 实现，用于创建类型为 IQueryable 的可执行查询。 结果是通过枚举查询变量获得的。
Dim companies =
{"Consolidated Messenger", "Alpine Ski House", "Southridge Video", "City Power & Light",
"Coho Winery", "Wide World Importers", "Graphic Design Institute", "Adventure Works",
"Humongous Insurance", "Woodgrove Bank", "Margie's Travel", "Northwind Traders",
"Blue Yonder Airlines", "Trey Research", "The Phone Company",
"Wingtip Toys", "Lucerne Publishing", "Fourth Coffee"}
' The IQueryable data to query.
Dim queryableData As IQueryable(Of String) = companies.AsQueryable()
' Compose the expression tree that represents the parameter to the predicate.
Dim pe As ParameterExpression = Expression.Parameter(GetType(String), "company")
' ***** Where(Function(company) company.ToLower() = "coho winery" OrElse company.Length > 16) *****
' Create an expression tree that represents the expression: company.ToLower() = "coho winery".
Dim left As Expression = Expression.Call(pe, GetType(String).GetMethod("ToLower", System.Type.EmptyTypes))
Dim right As Expression = Expression.Constant("coho winery")
Dim e1 As Expression = Expression.Equal(left, right)
' Create an expression tree that represents the expression: company.Length > 16.
left = Expression.Property(pe, GetType(String).GetProperty("Length"))
right = Expression.Constant(16, GetType(Integer))
Dim e2 As Expression = Expression.GreaterThan(left, right)
' Combine the expressions to create an expression tree that represents the
' expression: company.ToLower() = "coho winery" OrElse company.Length > 16).
Dim predicateBody As Expression = Expression.OrElse(e1, e2)
' Create an expression tree that represents the expression:
' queryableData.Where(Function(company) company.ToLower() = "coho winery" OrElse company.Length > 16)
Dim whereCallExpression As MethodCallExpression = Expression.Call(
GetType(Queryable),
"Where",
New Type() {queryableData.ElementType},
queryableData.Expression,
Expression.Lambda(Of Func(Of String, Boolean))(predicateBody, New ParameterExpression() {pe}))
' ***** End Where *****
' ***** OrderBy(Function(company) company) *****
' Create an expression tree that represents the expression:
' whereCallExpression.OrderBy(Function(company) company)
Dim orderByCallExpression As MethodCallExpression = Expression.Call(
GetType(Queryable),
"OrderBy",
New Type() {queryableData.ElementType, queryableData.ElementType},
whereCallExpression,
Expression.Lambda(Of Func(Of String, String))(pe, New ParameterExpression() {pe}))
' ***** End OrderBy *****
' Create an executable query from the expression tree.
Dim results As IQueryable(Of String) = queryableData.Provider.CreateQuery(Of String)(orderByCallExpression)
' Enumerate the results.
For Each company As String In results
Console.WriteLine(company)
Next
' This code produces the following output:
'
' Blue Yonder Airlines
' City Power & Light
' Coho Winery
' Consolidated Messenger
' Graphic Design Institute
' Humongous Insurance
' Lucerne Publishing
' Northwind Traders
' The Phone Company
' Wide World Importers
string[] companies = { "Consolidated Messenger", "Alpine Ski House", "Southridge Video", "City Power & Light",
"Coho Winery", "Wide World Importers", "Graphic Design Institute", "Adventure Works",
"Humongous Insurance", "Woodgrove Bank", "Margie's Travel", "Northwind Traders",
"Blue Yonder Airlines", "Trey Research", "The Phone Company",
"Wingtip Toys", "Lucerne Publishing", "Fourth Coffee" };
// The IQueryable data to query.
IQueryable<String> queryableData = companies.AsQueryable<string>();
// Compose the expression tree that represents the parameter to the predicate.
ParameterExpression pe = Expression.Parameter(typeof(string), "company");
// ***** Where(company => (company.ToLower() == "coho winery" || company.Length > 16)) *****
// Create an expression tree that represents the expression 'company.ToLower() == "coho winery"'.
Expression left = Expression.Call(pe, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
Expression right = Expression.Constant("coho winery");
Expression e1 = Expression.Equal(left, right);
// Create an expression tree that represents the expression 'company.Length > 16'.
left = Expression.Property(pe, typeof(string).GetProperty("Length"));
right = Expression.Constant(16, typeof(int));
Expression e2 = Expression.GreaterThan(left, right);
// Combine the expression trees to create an expression tree that represents the
// expression '(company.ToLower() == "coho winery" || company.Length > 16)'.
Expression predicateBody = Expression.OrElse(e1, e2);
// Create an expression tree that represents the expression
// 'queryableData.Where(company => (company.ToLower() == "coho winery" || company.Length > 16))'
MethodCallExpression whereCallExpression = Expression.Call(
typeof(Queryable),
"Where",
new Type[] { queryableData.ElementType },
queryableData.Expression,
Expression.Lambda<Func<string, bool>>(predicateBody, new ParameterExpression[] { pe }));
// ***** End Where *****
// ***** OrderBy(company => company) *****
// Create an expression tree that represents the expression
// 'whereCallExpression.OrderBy(company => company)'
MethodCallExpression orderByCallExpression = Expression.Call(
typeof(Queryable),
"OrderBy",
new Type[] { queryableData.ElementType, queryableData.ElementType },
whereCallExpression,
Expression.Lambda<Func<string, string>>(pe, new ParameterExpression[] { pe }));
// ***** End OrderBy *****
// Create an executable query from the expression tree.
IQueryable<string> results = queryableData.Provider.CreateQuery<string>(orderByCallExpression);
// Enumerate the results.
foreach (string company in results)
Console.WriteLine(company);
/* This code produces the following output:
Blue Yonder Airlines
City Power & Light
Coho Winery
Consolidated Messenger
Graphic Design Institute
Humongous Insurance
Lucerne Publishing
Northwind Traders
The Phone Company
Wide World Importers
*/
此代码在传递到 Queryable.Where 方法的谓词中使用固定数量的表达式。 但是，您可以编写一个视用户输入而定来合并可变数量谓词表达式的应用程序。 视用户输入而定，您也可以更改在查询中调用的标准查询运算符。
编译代码
在 Visual Studio 中创建一个新的**“控制台应用程序”**项目。
添加对 System.Core.dll 的引用（如果尚未引用它的话）。
包括 System.Linq.Expressions 命名空间。
从示例中复制代码，并将其粘贴到 Main 方法 (C#) 或 Main Sub 过程 (Visual Basic) 中。
请参见
任务
概念
其他资源
LINQ Farm Seed: Using the Expression Tree Visualizer（LINQ Farm Seed：使用表达式树可视化工具）