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

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);