Converting lambda expression into an expression tree

Question

From C# in depth:

Not all lambda expressions can be converted to expression trees. You can’t convert a lambda with a block of statements ( even just one return statement ) into an expresion tree --> it has to be in the form that just evaluates a single expression.

Since Linq-to-Object statements don't get converted into expression tree objects, the lambda expressions used with Linq-to-Object operators can contain a block of statements

        string[] count = { "one ", "two ", "three ", "four ", "five ", "six " };
        IEnumerable<int> result = count.Select(item =>
                               {
                                   Console.WriteLine(item);
                                   return item.Length;
                               });
        foreach (int i in result);

OUTPUT:

one two three four five six

I haven't yet started learning Linq-To-Sql or Linq-To-Entities, but I assume lambda expressions used with LINQ statements that operate on IQueryable<T> can only ever contain a single expression, due to restristion that only a single expression can be converted into an expression tree?

Thank you

Answer 1

It's not just that they can only contain a single expression - it's can't be a statement lambda at all. Even a block like this:

var result = query.Select(item => { return item.Length; });

would be invalid for LINQ to SQL. It would have to be expressed as:

var result = query.Select(item => item.Length);

(I've just noticed that that's the bit of the book you quoted. Oh well - it shows I'm consistent :)

Note that as of .NET 4, expression trees themselves do have the ability to contain blocks, but the C# compiler can't translate statement lambdas into that kind of expression tree. There are other restrictions too, but they rarely cause problems.

This is specified in section 4.6 of the C# 4 spec:

Not all anonymous functions can be represented as expression trees. For instance, anonymous functions with statement bodies and anonymous functions containing assignment expressions cannot be represented. In these cases, a conversion still exists, but will fail at compile-time.

Answer 2

You are correct, they can only ever contain a single expression.