问题
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.
回答1:
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...
回答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;
}
}
回答3:
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)
回答4:
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.
回答5:
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.
来源：https://stackoverflow.com/questions/13386719/c-sharp-calling-a-struct-constructor-that-has-all-defaulted-parameters