How do I compile an Expression Tree into a callable method, C# ?

Hi, 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,

From stackoverflow

• You need to create a lambda - i.e.

  var lambda = Expression.Lambda<Func<float,int>>(body, param);
  Func<float,int> method = lambda.Compile();
  int v = method(1.0); // test
  

  where "body" is your expression tree (taking a float, returning an int) involving the ParameterExpression param.

  You might also find this and this helpful.

  SharePoint Newbie : The problem is ((x + 2) + y)/z When there are more than one parameters part of different sub expressions in the tree, what do I do?

  Marc Gravell : Multiple parameters are possible (it is a params array of ParameterExpression); for sub-expressions, you need to invoke the inner expression (Expression.Invoke?)

  Marc Gravell : You can also build the entire tree into a single expression; this is slightly more efficient, but is more complex to do.

  SharePoint Newbie : I was initially trying to build the entire tree into a single expression. Then could not figure out a way to get to the parameters in the sub-expressions. Any info on that will help.

  Marc Gravell : If you have a single expression, you can use as many ParameterExpression instances as you need. For sub-expressions, you simply need to map things in the outermost expression to parameters in the *inner* expression. For example, you might need to create a new ParameterExpression for the outer...

  Marc Gravell : ... and simply pass that into the parameters set when using Expression.Invoke - this then maps as the value into the inner expression (which must also be a LambdaExpression via Expression.Lambda)

  Marc Gravell : If you have a concrete example, I can fill in some blanks - but I have to nip off for a few hours... back in a bit ;-p

  SharePoint Newbie : Wouldn't wanna give up without trying your suggestions...thx

• 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).

  SharePoint Newbie : That did the trick...Thanks a lot.