Kicking around some small structures while answering this post, I came across the following unexpectedly:
The following structure, using an int field is perfectly legal:
struct MyStruct
{
public MyStruct ( int size )
{
this.Size = size; // <-- Legal assignment.
}
public int Size;
}
However, the following structure, using an automatic property does not compile:
struct MyStruct
{
public MyStruct ( int size )
{
this.Size = size; // <-- Compile-Time Error!
}
public int Size{get; set;}
}
The error returned is "The 'this' object cannot be used before all of its fields are assigned to". I know that this is standard procedure for a struct: the backing field for any property must be assigned directly (and not via the property's set accessor) from within the struct's constructor.
A solution is to use an explicit backing field:
struct MyStruct
{
public MyStruct(int size)
{
_size = size;
}
private int _size;
public int Size
{
get { return _size; }
set { _size = value; }
}
}
(Note that VB.NET would not have this issue, because in VB.NET all fields are automatically initialized to 0/null/false when first created.)
This would seem to be an unfortunate limitation when using automatic properties with structs in C#. Thinking conceptually, I was wondering if this wouldn't be a reasonable place for there to be an exception that allows the property set accessor to be called within a struct's constructor, at least for an automatic property?
This is a minor issue, almost an edge-case, but I was wondering what others thought about this...
From C# 6 onward: this is no longer a problem
Becore C# 6, you need to call the default constructor for this to work:
public MyStruct(int size) : this()
{
Size = size;
}
A bigger problem here is that you have a mutable struct. This is never a good idea. I would make it:
public int Size { get; private set; }
Not technically immutable, but close enough.
With recent versions of C#, you can improve on this:
public int Size { get; }
This can now only be assigned in the constructor.
You can fix this by first calling the default constructor:
struct MyStruct
{
public MyStruct(int size) : this()
{
this.Size = size; // <-- now works
}
public int Size { get; set; }
}
Another obscure work-around to this problem is one spotted in the temporary Tuple class in the Managed Extensibility Framework (via Krzysztof Koźmic):
public struct TempTuple<TFirst, TSecond>
{
public TempTuple(TFirst first, TSecond second)
{
this = new TempTuple<TFirst, TSecond>(); // Kung fu!
this.First = first;
this.Second = second;
}
public TFirst First { get; private set; }
public TSecond Second { get; private set; }
(Full source code from Codeplex: Tuple.cs)
I also note that the documentation for CS0188 has been updated to add:
If you see this error when trying to
initialize a property in a struct
constructor, the solution is to change
the constructor parameter to specify
the backing field instead of the
property itself. Auto-implemented
properties should be avoided in
structs because they have no backing
field and therefore cannot be
initialized in any way from the
constructor.
So I take that to mean that the official guidance is to use old-style properties in your structs when you run in to this problem, which is probably less obscure (and more readible) than either of the other two alternatives explored so far.
Related
I had this C# structure that was building in VS2022 literally up until last night when I upgraded from version 17.0.5 to 17.1
internal struct RoutineSettings
{
public bool Show { get; set; } = true;
public ShapeType PreferredShapeType { get; set; } = ShapeType.None;
}
(ShapeType is just an enum).
After the upgrade, I get this error:
error CS8983: A 'struct' with field initializers must include an explicitly declared constructor.
The explanation is straightforward enough. But since this code executed just fine before I am confused.
Is the compiler wrong?
Is it a true language requirement that the compiler was just failing to check for until now?
If the correct answer is #2, it begs the question why is this a requirement? Why is it needed? Because when I "fixed" it, the fix seemed pointless. I just added an empty default constructor:
internal struct RoutineSettings
{
public RoutineSettings() { }
public bool Show { get; set; } = true;
public ShapeType PreferredShapeType { get; set; } = ShapeType.None;
}
And now my code builds again. Why would I need this when it does nothing?
This really appears to have been a compiler bug in the previous version. Up to C# 9.0, structs where not allowed to have a default ctor, and it really was not generated, unlike for classes, where the default ctor was automatically generated when no constructor was defined. This allows to create value types without calling code (by just nullifying the memory). So apparently, they wanted to make this change explicit, because it really makes a difference in the code generated, whether the default constructor exists or not. Do note that in your example, the default constructor is not empty, because any field initializers are implicitly added to the constructor code. So in C# 10, when the constructor is declared, it is generated, otherwise it is left away from the generated IL.
Also note that there are a bunch of pitfalls when using this feature. The default ctor is still not executed when allocating an array of the struct:
[Fact]
public void StructCtorIsExecuted()
{
var r = new RoutineSettingsWithDefaultCtor();
Assert.Equal(10, r.PreferredShapeType);
RoutineSettingsWithDefaultCtor[] array = new RoutineSettingsWithDefaultCtor[10];
Assert.Equal(0, array[0].PreferredShapeType); // <-- When allocating an array, the default ctor is NOT executed
}
internal struct RoutineSettingsWithDefaultCtor
{
public RoutineSettingsWithDefaultCtor()
{
PreferredShapeType = 10;
}
public bool Show { get; set; } = true;
public int PreferredShapeType { get; set; } = 2;
}
}
some more problems are described here: https://davidshergilashvili.space/2021/09/05/c-10-struct-type-can-define-default-constructor/
That's not a bug, but rather a new design choice for C#:
https://github.com/dotnet/sdk/issues/23971
Solution:
Define a public parameter-less constructor and it will solve the issue.
For example, before:
public struct Abc
{
public int Num1 = 5;
}
After:
public struct Abc
{
public int Num1 = 5;
public Abc() {} // <--- Parameter-less default constructor
}
I am asking a beginner level question. Though I am working in MVC but I am really confused with a simple concept and that is "Properties". There are lot of questions that
I have already gone through but there is surely a doubt in mind and did'nt able to clear it up.
Actually c# properties used for getting and setting the value to the private fields.
Like
Public class MyClass
{
private int number;
public int Number{
get{ return this.number;}
set{ number=Value }
}
}
class Program
{
static void Main()
{
MyClass example = new MyClass();
example.Number = 5; // set { }
Console.WriteLine(example.Number); // get { }
}
}
Now , the value is assigned to property also and to the variable also. Right?
Now , here is my doubt::
When we create property in model for MVc structure, we only have
public int Number{get;set;}
If this is okay to work with then why we are creating unnecessorily one more field of private access specifier. If encapsulation is the reason for that or hiding the data then why not in model in MVC?
Actually, in the above class example can I only use
Console.WriteLine(example.number);
after declaring it public?
Then what's the use of creating property over here?
Properties can be used to a store and retrieve values from a backing field (number in your case) directly as in your first sample. But property getters and setters are ordinary blocks of code that you can use as you want. So you don't have to assign a backing field, but can derive the value of a property also from another property in a getter, e.g.
public int NumberTimesTwo
{
get
{
return Number * 2;
}
}
However, as a common scenario is to have a property retrieve and assign the value of a backing field, there is a shortcut that you can use:
public int Number { get; set; }
In this case, the compiler automatically creates a private backing field that the property retrieves in the getter and assigns in the setter, so the code is equivalent to the following, but less to type:
private int _number;
public into Number
{
get
{
return _number;
}
set
{
_number = value;
}
}
As the backing field is also private, you cannot access it from outside of the class directly.
private int myVar;
public int MyProperty
{
get { return myVar; }
set { myVar = value; }
}
You are implementing Encapsulation by using MyProperty, which is public to access myVar which is private and is accessible only in the block where defined, that is, your class and not outside it.
Btw, in what way does this QA not answer your question? Try going through this for further reference.
Simple question I imagine, but what is the difference between these lines of code:
Code 1
public int Temp { get; set; }
and
Code 2
private int temp;
public int Temp { get { return temp; } }
My understand was that an automatic property as per Code 1 would perform the exact same function as Code 2?
I'm reading Head First C# and I'm finding it hard to understand why it's using two different ways of doing the same thing?
The primary difference between your Code1 and Code2 is that in #1, the property is settable.
You can achieve the same thing using automatic properties, because the setter can be private:
public int Temp { get; private set; }
Automatic properties was added in C#3, and is really just syntactic sugar for the longer version using a field. If you don't need to access the field directly, there is no reason not to use automatic properties. Automatic properties are equivalent to using a field - the compiler generates the field for you, it is just not accessible in code.
The first one is a writable property.
It's equivalent to
private int temp;
public int Temp {
get { return temp; }
set { temp = value; }
}
(except that you can't use the backingfield directly), but it requires 1 line of code instead of five.
When writing classes with 5 or 6 simple properties, auto-properties can make the classes much shorter.
You can make read-only auto-properties by writing
public int Temp { get; private set; }
The "automagic" property is just a "short-hand" notation:
public int Temp { get; set; }
is just a lot simpler to type than
public int Temp
{
get { return _temp; }
set { _temp = value; }
}
but functionally equivalent. Just a nice "shorthand" to improve your productivity, but no additional or magic functionality, really.
If your second example had both a getter and a setter, they would be functionally equivalent.
As it stands now, the first is publicly gett-able but can't be set publicly. You could also achieve the same thing using auto properties:
public int Temp { get; private set; }
And in case you're curious, automatic properties still get a backing private field. That bit is just handled by the compiler for you so that life is easier.
As for the reason why I would use property with a backing field is when I want to do something else when getting or setting the property. For example, a validation routine embedded into the property itself, or caching, etc...
Otherwise, for simple get and set, I'd use the automatic property format. It's more compact and involves less coding which I think it's a good thing.
I have read that there are good reasons to use properties instead of fields in c# on SO. So now I want to convert my code from using fields to using properties.
For an instance field of a class, I can set a default value. For example:
int speed = 100;
For the equivalent property, which I think is:
int Speed { get; set; }
My understanding is that the Speed property will be initialised to zero when the class is instantiated. I have been unable to find out how to set a default value to easily update my code. Is there an elegant way to provide a default value for a property?
It seems like there should be an elegant way to do this, without using a constructor, but I just can't find out how.
Best bet is to do a normal old-fashioned field-backed property, like:
private int _speed = 100;
public int Speed { get { return _speed; } set { _speed = value; } }
I think easiest way to set default value:
public sealed class Employee
{
public int Id { get; set; } = 10;
}
The design-pattern I use, which is used throughout Microsoft's Windows.Forms controls and other .NET Classes. Moreover, from my understanding, the initialization outside of the contructor allows just-in-time compiler to optimze the class code.
public class Foo {
public static const int DefaultSpeed = 100;
private int _speed = DefaultSpeed;
[DefaultValue(DefaultSpeed)]
public int Speed { get { return _speed; } set { _speed = value; } }
}
public class Foo {
public static Color DefaultForecolor { get {return SystemColors.WindowText; }}
private Color _forecolor = DefaultForecolor;
[DefaultValue(DefaultForeColor)]
public Color Forecolor { get { return _forecolor; } set { _forecolor = value; } }
}
you must set the default value for the property in the constructor. There's no other way to do it besides this for automatic properties since the fields for the automatic props are declared at compile time and replaced within getter/setter. However in explicit properties, you can initialize the field the property uses to read or write as Joe mentioned in his answer.
The constructor is the only way to set the initial value of an auto property.
CciSharp supports the DefaultValue attribute, which allows placing default values on auto properties. Whether CciSharp qualifies as an "elegant" solution, however, is a matter of opinion (it acts as a post-compiler that edits the IL in the binary).
Given a struct like this:
public struct SomeStruct
{
public SomeStruct(String stringProperty, Int32 intProperty)
{
this.StringProperty = stringProperty;
this.IntProperty = intProperty;
}
public String StringProperty { get; set; }
public Int32 IntProperty { get; set; }
}
Of course, a compiler error is generated that reads The 'this' object cannot be used before all of its fields are assigned to.
Is there a way to assign values to the backing fields or the properties themselves, or do I have to implement properties the old-fashioned way with my own explicit backing fields?
Prior to C# 6, you need to use the "this" constructor in this scenario:
public SomeStruct(String stringProperty, Int32 intProperty) : this()
{
this.StringProperty = stringProperty;
this.IntProperty = intProperty;
}
Doing this calls the default constructor and by doing so, it initializes all the fields, thus allowing this to be referenced in the custom constructor.
Edit: until C# 6, when this started being legal; however, these days it would be much better as a readonly struct:
public readonly struct SomeStruct
{
public SomeStruct(string stringProperty, int intProperty)
{
this.StringProperty = stringProperty;
this.IntProperty = intProperty;
}
public string StringProperty { get; }
public int IntProperty { get; }
}
If a structure is going to have, and its users will expect it to have, all three of the following characteristics:
Its entire state is encapsulated in some particular fixed set of readable members
An instance may be readily created in which those members have any combination of values which are valid for their respective types.
A default instance of the type should have all of those members set to the default values of their respective types.
the type should expose its members as fields. The above requirements mean a struct won't be able to do anything an exposed-field struct can't do, and will also mean that if code doesn't hit any threading hazards the struct will be able to do anything an exposed-field struct can do, albeit perhaps more slowly and with more threading hazards.
If a struct Foo has fields f1 and f2, and a constructor which sets those fields in that order, and if fooExpr is some sort of expression of type Foo [perhaps a variable, field, array reference, property, or whatever] the statement:
myFoo.f2 = someValue;
will only be allowed in cases where one could legally say
myFoo = new Foo(myFoo.f1, someValue);
and in all circumstances where the first form is allowed and the behavior of the second form has defined semantics, they will behave identically. Thus, trying to "encapsulate" the properties of Foo doesn't really accomplish anything except make the code more cumbersome to write, less clear, and slower to execute.