How to use Expression to build an Anonymous Type?

Question

In C# 3.0 you can use Expression to create a class with the following syntax:

var exp = Expression.New(typeof(MyClass));
var lambda = LambdaExpression.Lambda(exp);
object myObj = lambda.Compile().DynamicInvoke();

But how do you use Expression to create an Anonymous class?

//anonymousType = typeof(new{ Name="abc", Num=123});
Type anonymousType = Expression.NewAnonymousType???  <--How to do ?
var exp = Expression.New(anonymousType);
var lambda = LambdaExpression.Lambda(exp);
object myObj = lambda.Compile().DynamicInvoke();

Answer 1

Since an anonymous type doesn't have a default empty constructor, you cannot use the Expression.New(Type) overload ... you have to provide the ConstructorInfo and parameters to the Expression.New method. To do that, you have to be able to get the Type ... so you need to make a "stub" instance of the anonymous type, and use that to get the Type, and the ConstructorInfo, and then pass the parameters to the Expression.New method.

Like this:

        var exp = Expression.New(new { Name = "", Num = 0 }.GetType().GetConstructors()[0], Expression.Constant("abc", typeof(string)), Expression.Constant(123, typeof(int)));
        var lambda = LambdaExpression.Lambda(exp);
        object myObj = lambda.Compile().DynamicInvoke();

Answer 2

You're close, but you have to be aware that anonymous types don't have default constructors. The following code prints { Name = def, Num = 456 }:

Type anonType = new { Name = "abc", Num = 123 }.GetType();
var exp = Expression.New(
            anonType.GetConstructor(new[] { typeof(string), typeof(int) }),
            Expression.Constant("def"),
            Expression.Constant(456));
var lambda = LambdaExpression.Lambda(exp);
object myObj = lambda.Compile().DynamicInvoke();
Console.WriteLine(myObj);

If you don't have to create many instances of this type, Activator.CreateInstance will do just as well (it's faster for a few instances, but slower for many). This code prints { Name = ghi, Num = 789 }:

Type anonType = new { Name = "abc", Num = 123 }.GetType();
object myObj = Activator.CreateInstance(anonType, "ghi", 789);
Console.WriteLine(myObj);