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

Question

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,

Answer 1

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.

Answer 2

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).