Using null coalescing in foreach statement

Question

Hey All

Trying to figure out how to get the null coalescing operator to work in a foreach loop.

I'm checking to see what a string ends with and based on that, route it to a certain method. Basically what I want to say is....

foreach (String s in strList)
{
    if s.EndsWith("d") ?? Method1(s) ?? Method2(s) ?? "Unknown file type";
}

In attempting to do this, of course you get the "Operator ?? cannot be used on type bool and type string." I know there is other ways to do it, just want to see how it can be done with null coalescing.

Have a good weekend.

@Richard Ev: Oh yes of course. Switch, if else, etc. Was just curious how it could be handled

@Jon Skeet: After reading your comments it hit me, this is just bad! I am interested in two file extensions basically. If a file ends with "abc" for instance, send to method 1, if the file ends with "xyz" send to method 2. But what if a file ends with an extension of "hij"...boom, you're done.

Thanks to Brian and GenericTypeTea as well for the thoughful input

I'm content calling it closed.

Answer 1

I think the compiler gave you the appropriate answer, you can't.

Null coalescing is essentially this if statement:

if(x == null)
  DoY();
else
  DoZ();

A boolean value cannot be null, so you can't coalesce it like that. I'm not sure what your other methods return, but it seems like you want a simple || operator here.

Answer 2

It looks like you want to use the normal ternary operator, not null coalescing. Something like:

(s.EndsWith("d") ? Method1(s) : Method2(s)) ?? "Unknown file type";

This is equivalent to:

string result;
if (s.EndsWith("d"))
  result = Method1(s);
else
  result = Method2(s);
if (result == null)
  result = "Unknown file type";
return result;

Answer 3

I think you want a combination of the conditional (ternary) operator and the null coalescing operator:

foreach (String s in strList)
{
    string result = (s.EndsWith("d") ? Method1(s) : Method2(s)) 
        ?? "Unknown file type";
}

In simple english, this will do the following:

If s ends with d, then it will try Method1.
If s does not end with d then it will try Method2.
Then if the outcome is null, it will use "Unknown file type"
If the outcome is not null, it will use the result of either A or B

Answer 4

You should first use the ?? null coalescing operator to guard against a null s reference. Then use the ? ternary operator to choose between Method1 and Method2. Finally use the ?? null coalescing operator again to provide the default value.

foreach (string s in strList)
{
    string computed = s;
    computed = computed ?? String.Empty;
    computed = computed.EndsWith("d") ? Method1(s) : Method2(s);
    computed = computed ?? "Unknown file type";
}