Question

I’ve got a C# method that takes a bunch of parameters, all of which have default values. One of the parameters is a List. I can’t figure out how to specify that the List should default to empty. Here’s what it looks like:

    public static void execute(
        String condition = "Unnamed condition",
        List<String> messages,
        Object actual    = null,
        Object expected  = null)

I can’t quite figure out how to specify that messages should be empty by default. When I enter:

...
List<String> messages = new List<String> ()
...

it complains that “default parameter value for ‘messages’ must be a compile-time constant”.

Any ideas?

Answer 1

Because default parameter values must be compile-time expressions, the only acceptable default parameter value for reference types is null.

You can get around this with an overload, though:

 public static void execute(String condition = "Unnamed condition")
 {
     execute(condition, new List<String>(), null, null);
 }

Or constructing a list if the argument is null. If you must need a list and want to treat all null as an empty list, this would also handle if they called with null explicitly.

 public static void execute(String condition = "Unnamed condition",
        List<String> messages = null, Object actual = null,
        Object expected = null)
{
    // if you really want this to be empty if null, can check and assign.
    if (messages == null)
    {
        messages = new List<String>();
    }

    // your other logic
}

Or, if messages is only used in one place, you can use the null-coallescing operator to substitute an empty enumeration:

 public static void execute(String condition = "Unnamed condition",
        List<String> messages = null, Object actual = null,
        Object expected = null)
{
    // assuming you are using messages once for iteration or something...
    foreach(var msg in messages ?? Enumerable.Empty<String>())
    ...
}

Though obviously a simple if-guard can be more efficient. Really depends if you want to treat it as an empty enumerable or as an empty list or just bypass logic…