如何将表达式树编译成可调用的方法，C＃？ [英] How do I compile an Expression Tree into a callable method, C#?

问题描述

我有一个表达式树，我通过使用C＃中的表达式类解析Xml创建的。 看到这个问题。

我只在表达式树中添加，减除，除数，乘法，参数和和。
有没有办法将此ExpressionTree转换成可调用的方法？
...或者我必须手动发出IL？

请问，

解决方案

这是两种方法的一个例子。如果我错过了一些东西，或者想要更多的信息，只需让我知道。

  static void Main（）
 {
 //尝试做x +（3 * x）
 
 var single = BuildSingle< decimal>（）; 
 var composite = BuildComposite< decimal>（）; 
 
 Console.WriteLine（{0} vs {1}，单（13.2M），复合（13.2M））; 
} 
 //实用程序方法来获取3作为正确的类型，因为并不总是有一个int x T
 static Expression ConvertConstant< TSource，TDestination>（TSource value）
 {
 return Expression.Convert（Expression.Constant（value，typeof（TSource）），typeof（TDestination））; 
} 
 //选项1：单个表达式树;这是最有效的
 static Func< T，T> BuildSingle< T>（）
 {
 var param = Expression.Parameter（typeof（T），x）; 
表达式body = Expression.Add（param，Expression.Multiply（
 ConvertConstant< int，T>（3），param））; 
 var lambda = Expression.Lambda< Func< T，T>（body，param）; 
 return lambda.Compile（）; 
} 
 //选项2：嵌套表达式树：
 static Func< T，T> BuildComposite< T>（）
 {
 
 //步骤1：执行乘法：
 var paramInner = Expression.Parameter（typeof（T），inner）; 
表达式bodyInner = Expression.Multiply（
 ConvertConstant< int，T>（3），paramInner）; 
 var lambdaInner = Expression.Lambda（bodyInner，paramInner）; 
 
 //步骤2：添加，调用现有树
 var paramOuter = Expression.Parameter（typeof（T），outer）; 
表达式bodyOuter = Expression.Add（paramOuter，Expression.Invoke（lambdaInner，paramOuter））; 
 var lambdaOuter = Expression.Lambda< Func< T，T>（bodyOuter，paramOuter）; 
 
 return lambdaOuter.Compile（）; 
} 
  

个人而言，我将瞄准第一种方法;它更简单和更有效率。这可能涉及将原始参数传递到堆栈的嵌套代码，但是如此。我有一些代码在某个地方采用Invoke方法（复合），并重新编写树作为第一种方法（单） - 但它是相当复杂和漫长的。但对于实体框架（不支持Expression.Invoke）非常有用。

I have an expression tree I have created by parsing an Xml using the expression class in C#. See this question.

I only have Add, Subtract, Divide, Multiply, Parameters, And and Or in my Expression Tree. Is there a way to convert this ExpressionTree into a callable method? ...or do I have to emit the IL manually?

Kind regards,

解决方案

Here's an example of both approaches. If I have missed something, or you want more information, just let me know.

static void Main()
{
    // try to do "x + (3 * x)"

    var single = BuildSingle<decimal>();
    var composite = BuildComposite<decimal>();

    Console.WriteLine("{0} vs {1}", single(13.2M), composite(13.2M));
}
// utility method to get the 3 as the correct type, since there is not always a "int x T"
static Expression ConvertConstant<TSource, TDestination>(TSource value)
{
    return Expression.Convert(Expression.Constant(value, typeof(TSource)), typeof(TDestination));
}
// option 1: a single expression tree; this is the most efficient
static Func<T,T> BuildSingle<T>()
{        
    var param = Expression.Parameter(typeof(T), "x");
    Expression body = Expression.Add(param, Expression.Multiply(
        ConvertConstant<int, T>(3), param));
    var lambda = Expression.Lambda<Func<T, T>>(body, param);
    return lambda.Compile();
}
// option 2: nested expression trees:
static Func<T, T> BuildComposite<T>()
{

    // step 1: do the multiply:
    var paramInner = Expression.Parameter(typeof(T), "inner");
    Expression bodyInner = Expression.Multiply(
        ConvertConstant<int, T>(3), paramInner);
    var lambdaInner = Expression.Lambda(bodyInner, paramInner);

    // step 2: do the add, invoking the existing tree
    var paramOuter = Expression.Parameter(typeof(T), "outer");
    Expression bodyOuter = Expression.Add(paramOuter, Expression.Invoke(lambdaInner, paramOuter));
    var lambdaOuter = Expression.Lambda<Func<T, T>>(bodyOuter, paramOuter);

    return lambdaOuter.Compile();
}

Personally, I would aim towards the first method; it it both simpler and more efficient. This might involve passing the original parameter throughout a stack of nested code, but so be it. I have got some code somewhere that takes the "Invoke" approach (composite), and re-writes the tree as the first approach (single) - but it is quite complex and long. But very useful for Entity Framework (which doesn't support Expression.Invoke).

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