定义为lambda表达式的LINQ表达式可以包含其他LINQ表达式吗? [英] Can a LINQ Expression defined as a lambda expression include other LINQ Expressions?
问题描述
使用LINQ表达式时,C#编译器将方便地将C#lambda转换为Expression对象:
When using LINQ Expressions, the C# compiler will conveniently translate C# lambdas into Expression objects:
//using System;
//using System.Linq.Expressions;
Expression<Func<int, bool>> lambda_expression = (int x) => x == 3;
这很方便,与显式构造表达式相比,可以节省很多输入时间:
This is convenient, and can save a lot of typing versus explicitly constructing the expression:
Expression<Func<int, bool>> explicit_expression_object;
{
var x = Expression.Parameter(typeof(int), "x");
explicit_expression =
Expression.Lambda<Func<int, bool>>(Expression.Equal(x, Expression.Constant(3)), x);
}
但是,在某些情况下,例如在运行时动态创建表达式时,有必要使用"longhand"表达式对象语法.因此,我目前发现自己混合使用了表达式lambda"和动态生成的显式"表达式对象.
However there are situations when it is necessary to use the "longhand" Expression object syntax, for example when dynamically creating expressions at run time. As such, I currently find myself using a mix of "expression lambdas" and dynamically generated "explicit" expression objects.
是否可以将Expression对象包含"或嵌入" lambda表达式中?
Is it possible to "include" or "embed" an Expression object into an expression lambda?
例如:
Expression inner_expression_object = Expression.Constant(3);
Expression<Func<int, bool>> wrapper_expression =
(int x) => x == inner_expression_object.Embed();
推荐答案
使用ExpressionVisitor
,可以将Expression
扩展方法的调用替换为Expression
.
Using an ExpressionVisitor
, you can replace calls to an Expression
extension method with the Expression
.
首先,您需要一个ExpressionVisitor
类将Embed
方法调用扩展为它们的值:
First, you need an ExpressionVisitor
class to expand the Embed
method calls to their values:
public class EmbedVisitor : ExpressionVisitor {
public override Expression Visit(Expression node) {
if (node?.NodeType == ExpressionType.Call) {
var callnode = node as MethodCallExpression;
if (callnode.Method.Name == "Embed" && callnode.Method.DeclaringType == typeof(ExpressionExt))
return callnode.Arguments[0].Evaluate<Expression>();
}
return base.Visit(node);
}
}
然后,您需要一个静态类来提供所需的扩展方法:
Then you need a static class for the extension methods needed:
public static class ExpressionExt {
public static T Embed<T>(this Expression e) {
return default(T);
}
public static Expression ExpandEmbed(this Expression orig) => new EmbedVisitor().Visit(orig);
public static T Evaluate<T>(this Expression e) {
//A little optimization for constant expressions
if (e.NodeType == ExpressionType.Constant)
return (T)((ConstantExpression)e).Value;
else
return (T)Expression.Lambda(e).Compile().DynamicInvoke();
}
}
现在,您可以使用它们来扩展嵌入式的Expression
值子表达式:
Now you can use these to expand embedded Expression
valued sub-expressions:
var inner_expression_object = Expression.Constant(3);
Expression<Func<int, bool>> wrapper_expression =
(int x) => x == inner_expression_object.Embed<int>();
var expanded = wrapper_expression.ExpandEmbed();
// Expression<Func<int,bool>> expanded == (int x) => x == 3;
您还可以直接嵌入Expression
表达式并将其展开:
You can also directly embed Expression
expressions and expand them:
Expression<Func<int,bool>> wrap2 = x => x == Expression.Multiply(Expression.Constant(4), Expression.Constant(8)).Embed<int>();
var expanded2 = wrap2.ExpandEmbed();
// Expression<Func<int,bool>> expanded2 = x => x == 4*8;
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