Question

Why does expr1 compile but not expr2?

Func<object> func = () => new object();
Expression<Func<object>> expr1 = () => new object();
Expression<Func<object>> expr2 = func; //Cannot implicitly convert type 'System.Func<object>' to 'System.Linq.Expressions.Expression<System.Func<object>>'

Answer 1

A Func<T> is a delegate (a pointer to a function), while an Expression<Func<T>> is an expression tree (a tree-like data structure that describes an operation). It follows that you cannot assign one to the other, as they are completely dissimilar.

When you assign a lambda function directly to a Func<T> the compiler compiles the code for your function and assigns a pointer to the compiled code to e.g. func.

On the other hand, when you assign a lambda function directly to an Expression<Func<T>> the compiler builds up the expression tree (which is simply an instance of a reference type) and assigns a reference to that object to e.g. expr1. This is just a convenience that the compiler provides to you, giving you a much more attractive option than manually constructing the expression tree yourself in code (which of course is also completely possible).