l33t l33t - 13 days ago 7
C# Question

Convert Expression<Func<T, U>> to Expression<Func<object, object>>

I have a

LambdaExpression
which is of type
Expression<Func<T, string>>
. The design of the code currently does not allow me to keep
T
which means that I am forced to use the slower
DynamicInvoke
instead of
Invoke
.

Since I know the type of
T
I want to convert the expression so that it accepts a
T
object, allowing me to use
Invoke
. How?

Here's a good start

class Program
{
class MyClass
{
public string MyProperty => "Foo";
}

static LambdaExpression GetExpression(Expression<Func<MyClass, object>> expr)
{
return expr;
}

static void Main(string[] args)
{
var e1 = GetExpression(t => t.MyProperty);
var e2 = Expression.Lambda<Func<object, object>>(e1, e1.Parameters);

object myClass = new MyClass();
string s1 = (string)e1.Compile().DynamicInvoke(myClass);
object s2 = e2.Compile().Invoke(myClass);
}
}

hvd hvd
Answer

The non-expression version would look like

Func<object, object> Convert<T>(Func<T, object> f) {
  return o => f((T)o);
}

This is what you need to do in the expression version as well. You're right, Expression.Convert can do that.

Expression<Func<MyClass, object>> e1 = t => t.MyProperty;
var p = Expression.Parameter(typeof(object));
var e2 = Expression.Lambda<Func<object, object>>(
    Expression.Invoke(e1, Expression.Convert(p, typeof(MyClass))), p);

Note: as @xanatos rightly notes, for converting e.g. Expression<Func<T, int>> to Expression<Func<object, object>>, although C# supports an implicit boxing conversion from int to object, expression trees don't. If this is relevant to the question, another Expression.Convert is needed.