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I have an extension for string arrays which returns an empty String if the array is null, if the array length is == 1 it returns the first element otherwise element at index position is returned, no boundaries checking.

public static Strings IndexOrDefault(this String[] arr, Int32 index) { if (arr == null) return String.Empty; return arr.Length == 1 ? arr[0] : arr[index]; }

Now, I need to extend this functionality to other arrays, so I figured this extension method could do the job

public static T IndexOrDefault<T>(this T[] arr, Int32 index) { if (arr == null) return default(T); return arr.Length == 1 ? arr[0] : arr[index]; }

And everything was fine except default(String) is not String.Empty but null. So I'm returning nulls now for Strings, which was the first requirement...

Is there a replacement for default there that could return an empty generic value instead of a null?

Edit 2: (first one was the definition)

I'm using this approach now, which it works, but it's not as elegant as I wanted (well, the method is not elegant either, but the solution could :) )

public static T IndexOrDefault<T>(this T[] arr, Int32 index) { if (arr == null) return default(T); return arr.Length == 1 ? arr[0] : arr[index]; } public static String IndexOrDefault(this String[] arr, Int32 index) { if (arr == null) return String.Empty; return arr.Length == 1 ? arr[0] : arr[index]; }

Two different methods, one for String and the generic one for the rest.

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    Returning anything but null from a method named WhateverOrDefault for a reference type would be confusing, unintuitive, and honestly an outright lie. Your method says one thing, you are doing another. null is the default for string, not String.Empty. Also, why swallow an error by returning array[0] if the length is 1? If the caller is passing an invalid index then they should know about it, not spend time (potentially) trying to find a hidden bug. Commented Jul 4, 2011 at 23:33
  • Yeah you're right, this code was ported from asp (with server side JS) and it needs some refactoring here and there.. but, somebody has to pay for that :) Commented Jul 4, 2011 at 23:46

7 Answers 7

Reset to default
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There is no generic default other than null for reference types and some variant of 0 for value types

You can easily provide one though

public static T IndexOrDefault<T>(this T[] arr, Int32 index, T theDefaultValue) { if (arr == null) return theDefaultValue; return arr.Length == 1 ? arr[0] : arr[index]; } public static T IndexOrDefault<T>(this T[] arr, Int32 index, Func<T> defaultFactory) { if (arr == null) return defaultFactory(); return arr.Length == 1 ? arr[0] : arr[index]; }
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5 Comments

Good answer. Two suggestions - first, don't name your parameter default as it's a reserved word. Second, make the T defaultValue an optional parameter so that you have a nicer API and only have to specify the default value when you need to override it.
Also, something to note is that if you pass in the default value for an object type, you have to new one up even if you aren't going to need it which may be costly. I assume this is why you allowed for the defaultFactory alternative. Another approach is to use the coalescing operator ?? in the caller and leave the default override out of the method implementation entirely. This would be a more consistent and idiomatic option that better matches the usage of the rest of the framework's built-in ...OrDefault() methods.
@mattmc3 Thanks for the 'default' tip. Correct, the defaultFactory implementation is there for situations where creating a new T() is costly or not frequently needed. I didn't include a default value for the parameter because I'm not sure which version of c# the OP is using.
@mattmc3 the problem with using the ?? operator is that it is possible for the array to have null stored in the index. In that situation you would have no way of knowing whether the null coming out of the function was because the array is null or because the null value was located at the specified index
This solution would imply to change all the code I already have because of the third parameter, that's not applicable here. Nice answer by the way, thanks!
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or the first element if the array does not have an element in the index position

Not quite. The method returns the first element if the length of the array is one, but it doesn't check the index at all. What you describe would rather be something like this:

public static Strings IndexOrDefault(this String[] arr, Int32 index) { if (arr == null || arr.Length == 0) return String.Empty; if (index >= arr.Length) return arr[0]; return arr[index]; }

The generic version would be:

public static T IndexOrDefault<T>(this T[] arr, Int32 index) { if (arr == null || arr.Length == 0) return default(T); if (index >= arr.Length) return arr[0]; return arr[index]; }

Is there a replacement for default there that could return an empty generic value instead of a null?

No, there is no way of getting an empty value for a generic type. Most types doesn't have any empty value at all. An int for example can't be empty.

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You're also right, thanks for the comment, I'll rewrite the definition, the original code did not check that and it was ported as it is.
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No, null is the default value for string. You'll need to either add code that tests specifically for when T is a string, or use a different approach.

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You could try returning a new T() - but you'll need to add a where T : new constraint on your method declaration.

Otherwise, default of string is null. So the next option is to create a string overload... I know it’s not the best option but you may manage with it.

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2 Comments

Note, thus approach won't work for value types, eg. Int... So personally, I'd rather test if T is String and explicitly create a String.Empty to return.
I'm not near a compiler, but can't you do a second override where T : struct for value types?
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You could check for the "special" type string. Also your code is not doing what you said it would do - fixed some edge conditions for you:

public static T IndexOrDefault<T>(this T[] arr, Int32 index) { if (arr == null || arr.Length == 0) return default(T); return arr.Length <= index ? arr[0] : arr[index]; }
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What happens when you return default of T?

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you can see it here (msdn.microsoft.com/en-us/library/xwth0h0d.aspx) but summarizing: for reference values default(T) is null, for numeric values is 0 for structs all values are set to 0 or null depending on if they're reference or numeric types
Hmmm.. Maybe he could use an activator.createinstance() call, casting it to T to pass back?
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It is a little bit complex but you can use

 public static T IndexOrDefault<T>(this T[] arr, Int32 index) { if (arr == null) { if (typeof(T) == typeof(string)) return (T)(object)string.Empty; return default(T); } return arr.Length == 1 ? arr[0] : arr[index]; }
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