How to use lambda expression parameter in “sub” expression?

Question

I\'d like to be able to build expressions like the following delegate:

Func createSomeType1 = (args) =>
{
    return new SomeType1         


        

Answer 1

I'm on iPod, so can't give a full example at the moment: but:

• declare a param of type object[] (Expression.Param(typeof(object[]))) and store that in a variable
• for each term from the array, use the array indexer to obtain an expression to the indexer, and "Convert" or "cast" (iPod again!) to cast it
• use Expression.Invoke, passing the inner expression plus the indexer+cast you generated above

I'll be happy to do a complete example later if you need (when I'm at a PC)

---

Full example:

Type[] types = new Type[] { typeof(int), typeof(float), typeof(string) };

var constructorInfo = typeof(SomeType).GetConstructor(types);
var parameters = types.Select((t,i) => Expression.Parameter(t, "p" + i)).ToArray();
var someType1Exp = Expression.New(constructorInfo, parameters);
var inner = Expression.Lambda(someType1Exp, parameters);

var args = Expression.Parameter(typeof(object[]), "args");          
var body = Expression.Invoke(inner,
    parameters.Select((p,i) => Expression.Convert(Expression.ArrayIndex(args, Expression.Constant(i)), p.Type)).ToArray());
var outer = Expression.Lambda<Func<object[], object>>(body, args);
var func = outer.Compile();

object[] values = {1, 123.45F, "abc"};
object obj = func(values);
Console.WriteLine(obj);

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Or as a single expression:

Type[] types = new Type[] { typeof(int), typeof(float), typeof(string) };   
var constructorInfo = typeof(SomeType).GetConstructor(types);

var args = Expression.Parameter(typeof(object[]), "args");          
var body = Expression.New(constructorInfo,
    types.Select((t,i) => Expression.Convert(Expression.ArrayIndex(args, Expression.Constant(i)), t)).ToArray());
var outer = Expression.Lambda<Func<object[], object>>(body, args);
var func = outer.Compile();

object[] values = {1, 123.45F, "abc"};
object obj = func(values);
Console.WriteLine(obj);