Cannot use ref or out parameter in lambda expressions

Question

Why can't you use a ref or out parameter in a lambda expression?

I came across the error today and found a workaround but I was still curious why this is a compile-time error.

Here's a simple example:

    private void Foo()
    {
        int value;
        Bar(out value);
    }

    private void Bar(out int value)
    {
        value = 3;
        int[] array = {1, 2, 3, 4, 5};
        int newValue = array.Where(a => a == value).First();
    }

Answer 1

Under the hood, the anonymous method is implemented by hoisting captured variables (which is what your question body is all about) and storing them as fields of a compiler generated class. There is no way to store a ref or out parameter as a field. Eric Lippert discussed it in a blog entry. Note that there is a difference between captured variables and lambda parameters. You can have "formal parameters" like the following as they are not captured variables:

delegate void TestDelegate (out int x);
static void Main(string[] args)
{
    TestDelegate testDel = (out int x) => { x = 10; };
    int p;
    testDel(out p);
    Console.WriteLine(p);
}

Answer 2

There are certain restrictions when you are using a lambda expression with a parameter with ref and out keyword. When your variable is passed with ref or out keyword, you must explicitly specify the parameter type because the compiler cannot infer the type of the variable.

From CodeProject: Exploring Lamba Expressions in C#

Answer 3

Lambdas have the appearance of changing the lifetime of variables that they capture. For instance the following lambda expression causes the parameter p1 to live longer than the current method frame as it's value can be accessed after the method frame is no longer on the stack

Func<int> Example(int p1) {
  return () => p1;
}

Another property of captured variables is that changes to the variable are also visible outside the lambda expression. For example the following prints 42

void Example2(int p1) {
  Action del = () => { p1 = 42; }
  del();
  Console.WriteLine(p1);
}

These two properties produce a certain set of effects which fly in the face of a ref parameter in the following ways

• ref parameters may have a fixed lifetime. Consider passing a local variable as a ref parameter to a function.
• Side effects in the lambda would need to be visible on the ref parameter itself. Both within the method and in the caller.

These are somewhat incompatible properties and are one of the reasons they are disallowed in lambda expressions.