LINQ的Expression.Quote方法的目的是什么？ [英] What is the purpose of LINQ's Expression.Quote method?

问题描述

MSDN文档指出：

Expression.Quote

方法创建
UnaryExpression表示
表达式，其值为
，类型为Expression。

Method Creates a UnaryExpression that represents an expression that has a constant value of type Expression.

我已经能够通过使用Expression类手动构建LINQ查询来构建谓词表达式，但从未遇到过Expression.Quote的需求。

I've been able to build predicate expressions for use in LINQ queries by manually constructing them using the Expression class, but have never come across the need for Expression.Quote.

你什么时候和为什么要使用这个？从LINQ表达式我已经看到有这些，他们似乎包装现有的表达式，而不添加任何值。

When and why would you use this? From the LINQ expressions I've seen that have them, they just seem to wrap existing expressions without adding any value.

Quote方法/节点类型的目的是什么？

What is the purpose of the Quote method/node type?

推荐答案

Expression.Quote 指定要对待的lambda作为一个表达式树而不是一个函数。它在其操作数上引发关闭语义。

Expression.Quote specifies that a lambda is to be treated as an expression tree and not as a function. It induces closure semantics on its operand.

当您使用表达式构造一个 MethodCallExpression .Call ，任何lambda表达式的参数（ LambdaExpression / Expression< TDelegate> ）必须使用 Expression.Quote 来传递参数。

When you are constructing a MethodCallExpression using Expression.Call, any parameters that are lambda expressions (LambdaExpression/Expression<TDelegate>) must use Expression.Quote to wrap the parameter before passing in.

因此，对于<$ c类型的参数$ c> Expression< Func< bool>> ，当您创建一个实例，如：（）=> true ，表达式的类型属性将是 Func< bool> 而表达式的类型调用 GetType ）将是表达式< Func< bool>

So for a parameter of type Expression<Func<bool>>, when you create an instance such as: () => true, the expression's Type property would be Func<bool> whereas the expression's type (calling GetType) would be Expression<Func<bool>>

所以要获得一个表达式，它具有 Type 属性的正确值，可以将lambda表达式传递给 Expression.Quote 并将其作为参数传递给 Expression.Call 。

So to get an Expression that has the correct value for the Type property you pass the lambda expression into Expression.Quote and pass that as the parameter to Expression.Call.

我通过反光镜看过 Expression.Quote ，而唯一的参数是表达式 ，它必须从 LambdaExpression 派生，这在方法中被检查。感兴趣的是，任何人都知道为什么MS不会使参数类型为 LambdaExpression ？

I had a look at Expression.Quote through reflector and while the sole parameter is of type Expression, it must derive from LambdaExpression and this is checked inside the method. Out of interest, anyone know why MS didn't just make the parameter type be LambdaExpression?

由于StevenH指出out， Expression.Quote 用于实现LINQ查询提供程序。 可查询中的所有方法都采用lambda表达式，例如 Where ， OrderBy ， GroupBy 等内部构造一个 MethodCallExpression 使用 Expression.Call 并用 Expression.Quote 调用包装lambda表达式参数。

As StevenH pointed out, Expression.Quote is used in implementing LINQ Query Providers. All the methods on Queryable that take a lambda expression such as Where, OrderBy, GroupBy, etc internally construct a MethodCallExpression using Expression.Call and wrap the lambda expression parameters with Expression.Quote calls.

有关更详细的说明 Expression.Quote 阅读此答案。

For a more detailed explanation of Expression.Quote read this answer.

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