17

I ran into this issue today when creating a struct to hold a bunch of data. Here is an example:

public struct ExampleStruct
{
    public int Value { get; private set; }

    public ExampleStruct(int value = 1)
        : this()
    {
        Value = value;
    }
}

Looks fine and dandy. The problem is when I try to use this constructor without specifying a value and desiring to use the defaulted value of 1 for the parameter:

private static void Main(string[] args)
{
    ExampleStruct example1 = new ExampleStruct();

    Console.WriteLine(example1.Value);
}

This code outputs 0 and does not output 1. The reason is that all structs have public parameter-less constructors. So, like how I'm calling this() my explicit constructor, in Main, that same thing occurs where new ExampleStruct() is actually calling ExampleStruct() but not calling ExampleStruct(int value = 1). Since it does that, it uses int's default value of 0 as Value.

To make matters worse, my actual code is checking to see that int value = 1 parameter is within a valid range within the constructor. Add this to the ExampleStruct(int value = 1) constructor above:

if(value < 1 || value > 3)
{
    throw new ArgumentException("Value is out of range");
}

So, as it stands right now, the default constructor actually created an object that is invalid in the context I need it for. Anyone know how I can either:

  • A. Call the ExampleStruct(int value = 1) constructor.
  • B. Modify how the default values are populated for the ExampleStruct() constructor.
  • C. Some other suggestion/option.

Also, I am aware that I could use a field like this instead of my Value property:

public readonly int Value;

But my philosophy is to use fields privately unless they are const or static.

Lastly, the reason I'm using a struct instead of a class is because this is simply an object to hold non-mutable data, should be fully populated when it is constructed, and when passed as a parameter, should not be able to be null (since it is passed by value as a struct), which is what struct's are designed for.

Improve this question
5
  • That's not necessarily the full reason to use a struct, you can still do all that with a class and just throw ArgumentNullException. The main reasons to use strcut are outlined by the MSDN: msdn.microsoft.com/en-us/library/ms229017.aspx Commented Nov 14, 2012 at 20:27
  • 1
    Some day I should blog about all the CRAZY examples there are with the use of structs! thanks for this one! Commented Nov 14, 2012 at 20:34
  • notice that with every call you are making a copy of your data since its a struct.. Commented Nov 14, 2012 at 20:35
  • Instead of having the range 1-3, have the range 0-2 and have a property which modifies the return value to be in the correct range ;) Commented Nov 14, 2012 at 20:45
  • @Porges: you could do that, but that would be pretty messy and would have to be well documented. I prefer the nullable initialized field instead with a default on proeprty accessor. Commented Nov 14, 2012 at 21:12

5 Answers 5

Reset to default
29

Actually, MSDN has some good guidance on struct

Consider defining a structure instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects.

Do not define a structure unless the type has all of the following characteristics:

It logically represents a single value, similar to primitive types (integer, double, and so on).

It has an instance size smaller than 16 bytes.

It is immutable.

It will not have to be boxed frequently.

Notice that they are considerations for considering a struct, and its never a "this should always be a struct". That is because the choice to use a struct can have performance and usage implications (both positive and negative) and should be chosen carefully.

Notice in particular that they don't recommend struct for things > 16 bytes (then the cost of copying becomes more expensive than copying a reference).

Now, for your case, there is really no good way to do this other than to create a factory to generate a struct for you in a default state or to do some sort of trick in your property to fool it into initializing on first use.

Remember, a struct is supposed to work such that new X() == default(X), that is, a newly constructed struct will contain the default values for all fields of that struct. This is pretty evident, since C# will not let you define a parameterless constructor for a struct, though it is curious that they allow all arguments to be defaulted without a warning.

Thus, I'd actually suggest you stick with a class and make it immutable and just check for null on the methods that it gets passed to.

public class ExampleClass
{
    // have property expose the "default" if not yet set
    public int Value { get; private set; }

    // remove default, doesn't work
    public ExampleStruct(int value)
    {
        Value = value;
    }
}

However, if you absolutely must have a struct for other reasons - but please consider the costs of struct such as copy-casts, etc - you could do this:

public struct ExampleStruct
{
    private int? _value;

    // have property expose the "default" if not yet set
    public int Value
    {
        get { return _value ?? 1; }
    }

    // remove default, doesn't work
    public ExampleStruct(int value)
        : this()
    {
        _value = value;
    }
}

Notice that by default, the Nullable<int> will be null (that is, HasValue == false), thus if this is true, we didn't set it yet, and can use the null-coalescing operator to return our default of 1 instead. If we did set it in the constructor, it will be non-null and take that value instead...

Improve this answer
Sign up to request clarification or add additional context in comments.

3 Comments

Thanks, James. I ended up changing it to a class. Methods that have ExampleClass as a parameter have it defaulted to ExampleClass example = null. I put documentation stating that if it is set to null, it defaults to example ?? new ExampleClass(). Since it is a class now, it ends up calling the proper constructor of public ExampleClass(int value = 1).
Good tip on the ? and null-coalescing operator. This provided me with a solution for Size.Empty where Height and Width could have a default value of Double.NaN ...thus truly an empty Size.
Regarding performance related to structs, there have been some updates to the language since this was last edited: devblogs.microsoft.com/premier-developer/…
2

I don't think it's good design to have a struct ExampleStruct such that

default(ExampleStruct)

i.e. the value where all instance fields are zero/false/null, is not a valid value of the struct. And as you know, when you say

new ExampleStruct()

that's exactly the same as default(ExampleStruct) and gives you the value of your struct where all fields (including "generated" fields from auto-properties) are zero.

Maybe you could do this:

public struct ExampleStruct
{
  readonly int valueMinusOne;

  public int Value { get { return valueMinusOne + 1; } }

  public ExampleStruct(int value)
  {
    valueMinusOne = value - 1;
  }
}
Improve this answer

5 Comments

"I don't think it's good design to have a struct ExampleStruct such that" [...]" Actually, it's not only bad design, it's not even possible; the language won't let you do anything else.
why not have an empty constructor setting it to 1?
@CarloV.Dango Try doing that and see for yourself. You'll get a compiler error.
C# using structs is like coding c++ yikes!
@Servy I know the default is always the value where everything is zero. I'm saying it's bad to consider this default value "invalid". My extended version of the answer tries to translate the value such that the default 1 is stored internally as zero. Somewhat like the DateTime struct where default(DateTime) (with Ticks field zero) represents 0001-01-01.
1

I guess the compiler is actually picking the automatic default ctor for structs here http://msdn.microsoft.com/en-us/library/aa288208(v=vs.71).aspx, rather than using your ctor with the default values.

added references: http://csharpindepth.com/Articles/General/Overloading.aspx (Optional parameters section)

Improve this answer

4 Comments

7.5.3.2 Better function member in the c# spec.
I'm simply saying that you should have included that in your answer. it's clearly right, it's just not well explained.
Fair enough. Btw, same goes for classes, only that you have to add the default ctor explicitly.
The exact wording in that spec: Otherwise if all parameters of MP have a corresponding argument whereas default arguments need to be substituted for at least one optional parameter in MQ then MP is better than MQ.
1

Although some languages (CIL if nothing else) will allow one to define a struct constructor such that new T() has fields set to something other than their "all zeroes" defaults, C# and vb.net (and probably most other languages) take the view that since things like array elements and class fields must always be initialized to default(T), and since having them be initialized to something which didn't match new T() would be confusing, structures shouldn't define new T() to mean something other than default(T).

I would suggest that rather than trying to jinx a default parameter in a parameterized constructor, you simply define a static struct property which returns your desired default value, and replace every occurrence of new ExampleStruct() with ExampleStruct.NiceDefault;.

2015 Addendum

It appears that C# and VB.NET may ease the prohibition against defining parameterless struct constructors. This may cause a statement like Dim s = New StructType() to assign s a value which is different from the value given to new array items of type StructType. I'm not terribly keen on the change, given that new StructType is often used in places where something analogous to C#'s default(StructType) would be more appropriate if it existed. VB.NET would allow Dim s As StructType = Nothing, but that seems rather hokey.

Improve this answer

Comments

0

why do you have public ExampleStruct(int value = 1) : this() ?

shouldn't it be public ExampleStruct(int value = 1)? I think the :this() is creating the no-parameter constructor.

Improve this answer

2 Comments

It would be worthwhile for you to understand more clearly how : this() relates to calling the default constructor.
Note that his struct uses auto-properties. They introduce "special" backing fields. Therefore the user-defined constructors have to chain this() to make things compile.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.