I have a class and I would like to include an "Empty" constant member similar to Point.Empty in System.Drawing. Is that possible?
Here's a simplified version of what is giving an error:
public class TrivialClass
{
public const TrivialClass Empty = new TrivialClass(0);
public int MyValue;
public TrivialClass(int InitialValue)
{
MyValue = InitialValue;
}
}
The error given is: TrivialClass.Empty is of type TrivialClass. A const field of a reference type other than string can only be initialized with null.
If it matters, I'd like to use it like this:
void SomeFunction()
{
TrivialClass myTrivial = TrivialClass.Empty;
// Do stuff ...
}
You can use static readonly for these types. Constants can only be initialised with literal values (e.g. numbers, strings).
public class TrivialClass
{
public static readonly TrivialClass Empty = new TrivialClass(0);
public int MyValue;
public TrivialClass(int InitialValue)
{
MyValue = InitialValue;
}
}
After looking up the definition. Point.Empty is also static readonly. Reference here.
Just a quick shoot, but I would suggest that Point.Empty is a static member, not a constant:
public class TrivialClass
{
public static readonly TrivialClass Empty = new TrivialClass(0);
...
}
Related
I have two constructors which feed values to readonly fields.
public class Sample
{
public Sample(string theIntAsString)
{
int i = int.Parse(theIntAsString);
_intField = i;
}
public Sample(int theInt) => _intField = theInt;
public int IntProperty => _intField;
private readonly int _intField;
}
One constructor receives the values directly, and the other does some calculation and obtains the values, then sets the fields.
Now here's the catch:
I don't want to duplicate the
setting code. In this case, just one
field is set but of course there may
well be more than one.
To make the fields readonly, I need
to set them from the constructor, so
I can't "extract" the shared code to
a utility function.
I don't know how to call one
constructor from another.
Any ideas?
Like this:
public Sample(string str) : this(int.Parse(str)) { }
If what you want can't be achieved satisfactorily without having the initialization in its own method (e.g. because you want to do too much before the initialization code, or wrap it in a try-finally, or whatever) you can have any or all constructors pass the readonly variables by reference to an initialization routine, which will then be able to manipulate them at will.
public class Sample
{
private readonly int _intField;
public int IntProperty => _intField;
private void setupStuff(ref int intField, int newValue) => intField = newValue;
public Sample(string theIntAsString)
{
int i = int.Parse(theIntAsString);
setupStuff(ref _intField,i);
}
public Sample(int theInt) => setupStuff(ref _intField, theInt);
}
Before the body of the constructor, use either:
: base (parameters)
: this (parameters)
Example:
public class People: User
{
public People (int EmpID) : base (EmpID)
{
// Add more statements here.
}
}
I am improving upon supercat's answer. I guess the following can also be done:
class Sample
{
private readonly int _intField;
public int IntProperty
{
get { return _intField; }
}
void setupStuff(ref int intField, int newValue)
{
//Do some stuff here based upon the necessary initialized variables.
intField = newValue;
}
public Sample(string theIntAsString, bool? doStuff = true)
{
//Initialization of some necessary variables.
//==========================================
int i = int.Parse(theIntAsString);
// ................
// .......................
//==========================================
if (!doStuff.HasValue || doStuff.Value == true)
setupStuff(ref _intField,i);
}
public Sample(int theInt): this(theInt, false) //"false" param to avoid setupStuff() being called two times
{
setupStuff(ref _intField, theInt);
}
}
Here is an example that calls another constructor, then checks on the property it has set.
public SomeClass(int i)
{
I = i;
}
public SomeClass(SomeOtherClass soc)
: this(soc.J)
{
if (I==0)
{
I = DoSomethingHere();
}
}
Yeah, you can call other method before of the call base or this!
public class MyException : Exception
{
public MyException(int number) : base(ConvertToString(number))
{
}
private static string ConvertToString(int number)
{
return number.toString()
}
}
Constructor chaining i.e you can use "Base" for Is a relationship and "This" you can use for same class, when you want call multiple Constructor in single call.
class BaseClass
{
public BaseClass():this(10)
{
}
public BaseClass(int val)
{
}
}
class Program
{
static void Main(string[] args)
{
new BaseClass();
ReadLine();
}
}
When you inherit a class from a base class, you can invoke the base class constructor by instantiating the derived class
class sample
{
public int x;
public sample(int value)
{
x = value;
}
}
class der : sample
{
public int a;
public int b;
public der(int value1,int value2) : base(50)
{
a = value1;
b = value2;
}
}
class run
{
public static void Main(string[] args)
{
der obj = new der(10,20);
System.Console.WriteLine(obj.x);
System.Console.WriteLine(obj.a);
System.Console.WriteLine(obj.b);
}
}
Output of the sample program is
50 10 20
You can also use this keyword to invoke a constructor from another constructor
class sample
{
public int x;
public sample(int value)
{
x = value;
}
public sample(sample obj) : this(obj.x)
{
}
}
class run
{
public static void Main(string[] args)
{
sample s = new sample(20);
sample ss = new sample(s);
System.Console.WriteLine(ss.x);
}
}
The output of this sample program is
20
Error handling and making your code reusable is key. I added string to int validation and it is possible to add other types if needed. Solving this problem with a more reusable solution could be this:
public class Sample
{
public Sample(object inputToInt)
{
_intField = objectToInt(inputToInt);
}
public int IntProperty => _intField;
private readonly int _intField;
}
public static int objectToInt(object inputToInt)
{
switch (inputToInt)
{
case int inputInt:
return inputInt;
break;
case string inputString:
if (!int.TryParse(inputString, out int parsedInt))
{
throw new InvalidParameterException($"The input {inputString} could not be parsed to int");
}
return parsedInt;
default:
throw new InvalidParameterException($"Constructor do not support {inputToInt.GetType().Name}");
break;
}
}
Please, please, and pretty please do not try this at home, or work, or anywhere really.
This is a way solve to a very very specific problem, and I hope you will not have that.
I'm posting this since it is technically an answer, and another perspective to look at it.
I repeat, do not use it under any condition. Code is to run with LINQPad.
void Main()
{
(new A(1)).Dump();
(new B(2, -1)).Dump();
var b2 = new B(2, -1);
b2.Increment();
b2.Dump();
}
class A
{
public readonly int I = 0;
public A(int i)
{
I = i;
}
}
class B: A
{
public int J;
public B(int i, int j): base(i)
{
J = j;
}
public B(int i, bool wtf): base(i)
{
}
public void Increment()
{
int i = I + 1;
var t = typeof(B).BaseType;
var ctor = t.GetConstructors().First();
ctor.Invoke(this, new object[] { i });
}
}
Since constructor is a method, you can call it with reflection. Now you either think with portals, or visualize a picture of a can of worms. sorry about this.
In my case, I had a main constructor that used an OracleDataReader as an argument, but I wanted to use different query to create the instance:
I had this code:
public Subscriber(OracleDataReader contractReader)
{
this.contract = Convert.ToString(contractReader["contract"]);
this.customerGroup = Convert.ToString(contractReader["customerGroup"]);
this.subGroup = Convert.ToString(contractReader["customerSubGroup"]);
this.pricingPlan= Convert.ToString(contractReader["pricingPlan"]);
this.items = new Dictionary<string, Member>();
this.status = 0;
}
So I created the following constructor:
public Subscriber(string contract, string customerGroup) : this(getSubReader(contract, customerGroup))
{ }
and this method:
private static OracleDataReader getSubReader(string contract, string customerGroup)
{
cmdSubscriber.Parameters[":contract"].Value = contract + "%";
cmdSubscriber.Parameters[":customerGroup"].Value = customerGroup+ "%";
return cmdSubscriber.ExecuteReader();
}
notes: a statically defined cmdSubscriber is defined elsewhere in the code; My main constructor has been simplified for this illustration.
In case you need to run something before calling another constructor not after.
public class Sample
{
static int preprocess(string theIntAsString)
{
return preprocess(int.Parse(theIntAsString));
}
static int preprocess(int theIntNeedRounding)
{
return theIntNeedRounding/100;
}
public Sample(string theIntAsString)
{
_intField = preprocess(theIntAsString)
}
public Sample(int theIntNeedRounding)
{
_intField = preprocess(theIntNeedRounding)
}
public int IntProperty => _intField;
private readonly int _intField;
}
And ValueTuple can be very helpful if you need to set more than one field.
NOTE: most of the solutions above does not work for structs.
Unfortunately initializing struct fields in a method called by a constructor is not recognized by the compiler and will lead to 2 errors:
in the constructor: Field xxxx must be fully assigned...
in the method, if you have readonly fields: a read-only field cannot be assigned except in a constructor.
These can be really frustrating for example when you just need to do simple check to decide on which constructor to orient your call to.
I've got a class like this with some custom attributes. I'm not sure whether i actually have to implement these.
[AttributeUsage(AttributeTargets.Field)]
private class IsValue : Attribute { }
[AttributeUsage(AttributeTargets.Field)]
private class IsRep : Attribute { }
[AttributeUsage(AttributeTargets.Class)]
private class IsConstant : Attribute { }
public static class Constants
{
[IsConstant]
public static class EulerGamma
{
[IsValue]
public const double Value = 0.5772156649015;
[IsRep]
public const string Str = "γ";
}
[IsConstant]
public static class EulerNumber
{
[IsValue]
public const double Value = 2.718281828459;
[IsRep]
public const string Str = "e";
}
[IsConstant]
public static class Pi
{
[IsValue]
public const double Value = 3.1415926535898;
[IsRep]
public const string Str = "π";
}
[IsConstant]
public static class GoldenRatio
{
[IsValue]
public const double Value = 1.6180339887499;
[IsRep]
public const string Str = "φ";
}
}
Let's say this is in some class "MyMathClass", where I'd like to implement a method like this:
string ValueOrString(double x)
This method would return string representation of the constant if the number passed is equal to the constant, else it would return the original number.
So, if i passed exactly 3.1415926535898 this method would give me the string "π".
If is passed for example 2.5315621321 this would return me "2.5315621321" (string).
Would you please help me out?
I would create a class that uses a dictionary:
public static class Constants
{
static Dictionary<double, string> constantNames;
static Constants()
{
Constants.constantNames = new Dictionary<double, string>();
Constants.constantNames.Add(3.1415926535898, "π");
Constants.constantNames.Add(2.718281828459, "e");
}
public static string ValueOrString(double value)
{
if (constantNames.ContainsKey(value))
{
return constantNames[value];
}
else
{
return value.ToString();
}
}
}
When the function string ValueOrString(double value) is called, you can check if the provided value exists in the dictionary. If it exists, you retrieve the name of the constant from it. Otherwise, you return the value as a string.
I have a static Class and within it I have multiple public static attributes. I treat this class as my global class.
However now I need to treat this class as a variable so that I can pass it to a method of another class for processing..
I can't instantiate this class.. So in effect I can only assign the variables inside this class.
Is my understanding correct or am I missing something?
public static class Global
{
public const int RobotMax = 2;
// GUI sync context
public static MainForm mainForm;
public static SynchronizationContext UIContext;
// Database
public static Database DB = null;
public static string localDBName = "local.db";
public static Database localDB = null;
public static Database ChangeLogDB = null;
public static string changeLogDBName = "ChangeLog.db";
}
Let say I have a class like this, and I need to somehow keep a copy of this in another class maybe
public static class Global_bk
{
public const int RobotMax = 2;
// GUI sync context
public static MainForm mainForm;
public static SynchronizationContext UIContext;
// Database
public static Database DB = null;
public static string localDBName = "local.db";
public static Database localDB = null;
public static Database ChangeLogDB = null;
public static string changeLogDBName = "ChangeLog.db";
}
I need to copy the contents from Global to Global_bk.
And after that I need to compare the contents of the two classes in a method like
static class extentions
{
public static List<Variance> DetailedCompare<T>(T val1, T val2)
{
List<Variance> variances = new List<Variance>();
FieldInfo[] fi = val1.GetType().GetFields();
foreach (FieldInfo f in fi)
{
Variance v = new Variance();
v.Prop = f.Name;
v.valA = f.GetValue(val1);
v.valB = f.GetValue(val2);
if (!v.valA.Equals(v.valB))
variances.Add(v);
}
return variances;
}
}
class Variance
{
string _prop;
public string Prop
{
get { return _prop; }
set { _prop = value; }
}
object _valA;
public object valA
{
get { return _valA; }
set { _valA = value; }
}
object _valB;
public object valB
{
get { return _valB; }
set { _valB = value; }
}
}
So on my main form, how do I go about calling the compare method and passing the static Global class inside?
example: extentions.DetailedCompare(Global, Global_bk) ? Of course this would give me an error because I cant pass a type as a variable.
Please help me, this is driving me nuts...
How about the singleton pattern ? You can pass reference to shared interface (IDoable in exable below) and still have just one instance.
I.E.:
public interface IDoable {
int Value { get; set; }
void Foo();
}
public static class DoableWrapper {
private MyDoable : IDoable {
public int Value { get;set; }
public void Foo() {
}
}
private static IDoable s_Doable = new MyDoable();
public static IDoable Instance {
get { return s_Doable; }
}
}
Singleton is the way to go here. You can do it like this:
internal class SomeClass
{
private static SomeClass singleton;
private SomeClass(){} //yes: private constructor
public static SomeClass GetInstance()
{
return singleton ?? new SomeClass();
}
public int SomeProperty {get;set;}
public void SomeMethod()
{
//do something
}
}
The GetInstance Method will return you a SomeClass object that you can edit and pass into whatever you need.
You can access the members with classname.membername.
internal static class SomeClass
{
public static int SomeProperty {get;set;}
public static void SomeMethod()
{
//do something
}
}
static void main()
{
SomeClass.SomeProperty = 15;
SomeClass.SomeMethod();
}
The only way you are going to obtain a variable with the "class" information is using reflection. You can get a Type object for the class.
namespace Foo {
public class Bar
{
}
}
Type type = Type.GetType("Foo.Bar");
Otherwise, if you are really describing a class "instance" then use an object and simply instantiate one.
C# offers no other notation for class variables.
I have the following class but I can't seem to get the desired results with C#.
public class AppOsType {
public static class IOS {
public static int IOS()
{
return 100;
}
public static string ToString()
{
return "iOS";
}
}
... // additional values
}
I want to get the following results:
AppOsType.IOS // returns 100
AppOsType.IOS.ToString() // returns "iOS"
But I'm getting an error saying AppOsType.IOS is a type when i do the following:
Assert.AreEqual(100, AppOsType.IOS);
What am I missing?
Edit: left out static.
Not sure why all the static and inner class stuff is needed for, why don't you keep it simple and define an enum:
public enum AppOsType
{
IOS = 100
}
Then use
var ios = AppOsType.IOS;
var number = (int)ios;
var name = ios.ToString();
If you need to return a translated string based on enum, you could add a dictionary:
var translations = new Dictionary<AppOsType, string>()
{
{ AppOsType.IOS, "iOs" }
}
and then
var ios = AppOsType.IOS;
var number = (int)ios;
var name = translations[ios];
If you really need this nested static class inside the AppOsType class then you need to change something because a method cannot have the same name of the class and a constructor cannot return values. (Think to the fact as if the return value of the constructor is already defined to be the instance of the class)
void Main()
{
Console.WriteLine(AppOsType.IOS.Version);
Console.WriteLine(AppOsType.IOS.ToString());
}
public class AppOsType
{
// .... other members here ?? ...
public static class IOS
{
public static readonly int Version;
static IOS()
{
// In the static constructor you could set the readonly
// static property
Version = 100;
}
public static string ToString()
{
return "iOS";
}
}
}
Its becaus IOS is a class. AppsOsType.IOS points to the static class. If your Method public int IOS() would be static, you can access it using AppOsType.IOS.IOS()
If you don't want an enum for some reason, to make it work with desired syntax, you need a public property (not a ctor or method) :
public class AppOsType {
public static class IOS {
public static int IOS
{
get { return 100; }
}
public static string ToString()
{
return "iOS";
}
}
}
You are using the same name IOS so change this
AppOsType.IOS // returns 100 [error]
to
AppOsType.IOS.IOS // might returns 100
I'm trying to create a class which takes value a as a parameters in it's constructor.
It has a private member variable which stores this value. The value should not be changed afterwards.
Here's what I have, it works but I don't think it's the best solution out there:
internal class Foo
{
private int a;
public int A
{
get
{
return this.a;
}
}
public Foo(int a)
{
this.a = a;
}
}
So this way you can not access a from outside of the class, and A-property only has a get method. However, you can still change a from inside the class, and using a property which only returns one variable and nothing else feels stupid.
Am I doing this right, or is there a way to improve my code/more proper way to do this?
Additionally declare your private field readonly and you're there!
public class Foo
{
public Foo(int bar)
{
this.bar = bar;
}
public int Bar
{
get
{
return bar;
}
}
private readonly int bar;
}
“In C# 6 and later, you can initialize auto-implemented properties similarly to fields”. Just like you can initialize a readonly field in a constructor, you can initialize a get-only auto-implemented property in a constructor. Thus, the following now compiles:
public class Class1
{
public int A { get; }
public Class1(int a)
{
A = a;
}
}
…and the following yields an error:
public class Class1
{
public int A { get; }
public Class1(int a)
{
A = a;
}
public void Mutate()
{
// Class1.cs(11,9,11,10): error CS0200: Property or indexer 'Class1.A' cannot be assigned to -- it is read only
A++;
}
}
I like it—you get the terseness of field initialization with the interface/OOP-friendliness of properties.
internal class Foo
{
private readonly int _a;
public int A
{
get
{
return _a;
}
}
public Foo(int a)
{
_a = a;
}
}
This should do it.