Question

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,

Answer 1

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).