如何指定由一名前pression树方法返回的对象？ [英] How do I specify the object to return from an expression tree method?

问题描述

我试图创建使用EX pression树，返回一个对象的方法，但我想不出如何实际指定返回对象。我试着读<一href="http://blogs.msdn.com/b/csharpfaq/archive/2009/09/14/generating-dynamic-methods-with-ex$p$pssion-trees-in-visual-studio-2010.aspx">this,但返回值实际上并不似乎没有任何地方规定。

我已经得到了所有的任务和放大器;东西下来，但我怎么指定从方法返回的对象使用前pression树木产生的？

编辑：这些都是V4 EX pression树和方法，我试图创造做这样的事情：

 私有对象ReadStruct（读者BinaryReader在）{
    StructType OBJ =新StructType（）;
    obj.Field1 = reader.ReadSomething（）;
    obj.Field2 = reader.ReadSomething（）;
    //...更多...
    返回OBJ;
}
 

解决方案

显然是返回是<一个href="http://msdn.microsoft.com/en-us/library/system.linq.ex$p$pssions.gotoex$p$pssion.aspx"><$c$c>GotoEx$p$pssion你可以创建<一个href="http://msdn.microsoft.com/en-us/library/system.linq.ex$p$pssions.ex$p$pssion.return.aspx"><$c$c>Ex$p$pssion.Return工厂方法。你需要在年底创建标签跳转到它。事情是这样的：

  //一个前pression重新presenting obj变量
ParameterEx pression objEx pression = ...;

LabelTarget returnTarget =前pression.Label（typeof运算（StructType））;

GotoEx pression returnEx pression =前pression.Return（returnTarget，
    objEx pression的typeof（StructType））;

LabelEx pression returnLabel =前pression.Label（returnTarget，设置defaultValue）;

BlockEx pression块=前pression.Block（
    / * ...变量，其他所有线路和...... * /，
    returnEx pression，
    returnLabel）;
 

标签的目标和GOTO前pression的类型必须匹配。由于标签目标有一个类型，标签前pression必须有一个缺省值。

I'm trying to create a method using an expression tree that returns an object, but I can't figure out how to actually specify the object to return. I've tried reading this, but the return value doesn't actually seem to be specified anywhere.

I've got all the assignments & stuff down, but how do I specify the object to return from a method created using expression trees?

EDIT: these are v4 expression trees, and the method I'm trying to create does something like this:

private object ReadStruct(BinaryReader reader) {
    StructType obj = new StructType();
    obj.Field1 = reader.ReadSomething();
    obj.Field2 = reader.ReadSomething();
    //...more...
    return obj;
}

解决方案

Apparently a return is a GotoExpression that you can create with the Expression.Return factory method. You need to create a label at the end to jump to it. Something like this:

// an expression representing the obj variable
ParameterExpression objExpression = ...;

LabelTarget returnTarget = Expression.Label(typeof(StructType));

GotoExpression returnExpression = Expression.Return(returnTarget, 
    objExpression, typeof(StructType));

LabelExpression returnLabel = Expression.Label(returnTarget, defaultValue);

BlockExpression block = Expression.Block(
    /* ... variables, all the other lines and... */,
    returnExpression,
    returnLabel);

The types of the label target and the goto expression must match. Because the label target has a type, the label expression must have a default value.

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