C# 4 "dynamic" in expression trees

Question

I'm trying to figure out how to put all the pieces together, and would appreciate a concrete source code sample for a simple case to start with.

Consider the following C# code:

Func<int, int, int> f = (x, y) => x + y;

I can produce an equivalent function at runtime using expression trees as follows:

var x = Expression.Parameter(typeof(int), "x");
var y = Expression.Parameter(typeof(int), "y");
Func<int, int, int> f =
    Expression.Lambda<Func<int, int, int>>(
        Expression.Add(x, y),
        new[] { x, y }
    ).Compile();

Now given the following lambda:

Func<dynamic, dynamic, dynamic> f = (x, y) => x + y;

how would I generate the equivalent using expression trees (and, presumably, Expression.Dynamic)?

Answer 1

You can create an expression tree that represents a dynamic C# addition expression by passing the CallSiteBinder for a dynamic C# addition expression into Expression.Dynamic. You can discover the code to create the Binder by running Reflector on the original dynamic expression. Your example would go something like this:

var x = Expression.Parameter(typeof(object), "x");
var y = Expression.Parameter(typeof(object), "y");
var binder = Binder.BinaryOperation(
    CSharpBinderFlags.None, ExpressionType.Add, typeof(Program),
    new CSharpArgumentInfo[] { 
        CSharpArgumentInfo.Create(CSharpArgumentInfoFlags.None, null), 
        CSharpArgumentInfo.Create(CSharpArgumentInfoFlags.None, null)});
Func<dynamic, dynamic, dynamic> f =
    Expression.Lambda<Func<object, object, object>>(
        Expression.Dynamic(binder, typeof(object), x, y),
        new[] { x, y }
    ).Compile();