从其他两个（链接表达式）创建动态表达式lambda [英] Create dynamic Expression lambda from two others (chaining the Expressions)

问题描述

 表达式< Func<标识，对象>> ; fx = _ => _。ID; 
  

还有一个将对象转换为识别实例的转换lambda：

  ParameterExpression p = Expression.Parameter（typeof（object），o）; 
 Expression @new = Expression.Lambda（Expression.Convert（p，typeof（Identification）），p）; 
  

如何构建一个执行 @new （取出识别实例），并将结果传递到 fx 。我需要 @new 的结果绑定到 fx 的第一个参数，我找不到一个例子。

我需要将结果作为表达式，它应该是类型 Expression< Func< object，object>> ，它应将入站参数转换为标识，然后获取 Id property。

解决方案

首先，请注意，如果键入 @新的适当地，即：

  LambdaExpression @new = ... 
  

，因为它可以轻松访问 @ new.Body 和 @ new.Parameters ;这样做，
Expression.Invoke 可以在这里有用：

  var combinedParam = Expression.Parameter（typeof（object），o）; 
 var combined = Expression.Lambda&FunC< object，object>（
 Expression.Invoke（fx，
 Expression.Invoke（@new，combinedParam）），combinedParam）; 
  

虽然对于更干净的表达式，您也可以使用 ExpressionVisitor 完全替换内部表达式：

  var inject = new SwapVisitor（fx.Parameters [0]，@new。体）.Visit（fx.Body）; 
 var combined = Expression.Lambda   

with：

 code> class SwapVisitor：ExpressionVisitor {
 private readonly Expression from，to; 
 public SwapVisitor（Expression from，Expression to）{
 this.from = from; 
 this.to = to; 
} 
 public override表达式访问（表达式节点）{
 return node == from？ to：base.Visit（node）; 
} 
} 
  

这是什么：

• 检查 fx.Body 树，替换所有 _ （参数）与 @ new.Body （注意，这将包含对 o的引用参数（aka p ）


• 然后我们从替换的表达式中构建一个新的lambda，重新使用来自 @new ，确保我们注入的值正确绑定

Given a lambda that takes an Identification object, and returns a property:

Expression<Func<Identification, object>> fx = _ => _.Id;

And a conversion lambda that converts an object into an Identification instance:

ParameterExpression p = Expression.Parameter(typeof(object), "o");
Expression @new = Expression.Lambda(Expression.Convert(p, typeof(Identification)), p);

How do I build a new lambda that executes @new (getting out the Identification Instance) and passes the result into fx. I need @new's result to bind to the first parameter of fx somehow, and I cannot find an example.

I need the result to be an Expression, it should be of type Expression<Func<object, object>> and it should convert the inbound parameter to an Identification and then get the Id property.

解决方案

Firstly, note that this is easier if you type @new appropriately, i.e.:

LambdaExpression @new = ...

since that provides easy access to @new.Body and @new.Parameters; that done, Expression.Invoke can be useful here:

var combinedParam = Expression.Parameter(typeof(object), "o");
var combined = Expression.Lambda<Func<object, object>>(
    Expression.Invoke(fx,
        Expression.Invoke(@new, combinedParam)), combinedParam);

although for a cleaner expression, you can also use ExpressionVisitor to completely replace the inner expressions:

var injected = new SwapVisitor(fx.Parameters[0], @new.Body).Visit(fx.Body);
var combined = Expression.Lambda<Func<object, object>>(injected,@new.Parameters);

with:

class SwapVisitor : ExpressionVisitor {
    private readonly Expression from, to;
    public SwapVisitor(Expression from, Expression to) {
        this.from = from;
        this.to = to;
    }
    public override Expression Visit(Expression node) {
        return node == from ? to : base.Visit(node);
    }
}

what this does is:

• inspect the fx.Body tree, replacing all instances of _ (the parameter) with the @new.Body (note that this will contain references to the o parameter (aka p)
• we then build a new lambda from the replaced expression, re-using the same parameters from @new, which ensures that the values we injected will be bound correctly

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