Let's say I have a class A:
public class A
{
private int value;
public A() => value = 0;
public A(int value) => this.value = value;
}
And I have some method, with a parameter list where some are defaulted:
public void SomeMethod(int i, float f, string s = "", A a = null)
Now is there some way, e.g. through reflection, to be more smart about this parameter list? I would like to be able to do something like the following, so I don't need to check for null everywhere:
public void SomeMethod(int i, float f, string s = "", A a = Default) // should use the empty constructor!
Is this possible with C#?
You could use method overloading
public void SomeMethod(A a, int i, float f, string s = "") { }
public void SomeMethod(int i, float f, string s = "")
{
SomeMethod(new A(), i, f, s);
}
[CONCLUSION]
It is not possible in C#. The default operator returns null for all reference types, and for value types it initializes all data fields using default operator. So in my case, I can actually omit this problem using a struct instead of a class:
public struct A
{
private int value;
public A(int value) => this.value = value;
}
public void SomeMethod(int i, float f, string s = "", A a = default)
And then the input value will be a : A{value = 0}.
A general solution for using a class does not exist in C#, since default values are required to be determinable by compile time and must thus be constant expressions. Only valid constant expressions in C# are primitives, value types, and strings.
Related
I have searched around here for similar problems, but couldn't find a solution for my problem.
MyClass holds several data and does some type casting between different types.
How can i avoid this Error:
A value of type 'string' cannot be used as default parameter
because there are no standard conversions to type Program.MyClass?
I have tried Func and declared multiple function overload to be able to pass multiple argument and handle default parameter. There should be a better way to achieve this. Hopefully you could help.
To clarify my problem i have made this code:
using System;
public class Program
{
public class MyClass {
public string Value { get; set; }
public MyClass()
{
Value = "";
}
public MyClass(string s)
{
Value = s;
}
public override string ToString()
{
return this.Value;
}
}
// Causes CS1750
// A value of type 'string' cannot be used as default parameter
// because there are no standard conversions to type 'Program.MyClass'
public static string test2(string a, MyClass b = " with default text")
{
return a + b;
}
public static string test(string a, string b = " with default text")
{
return a + b;
}
public static void Main()
{
Console.WriteLine(test("test1"));
Console.WriteLine(test("test1", " with my text"));
}
}
That's not quite possible. As the error message states, you require a standard conversion: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/conversions#standard-conversions
All we can do is to define an implicit conversion operator for the class: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/user-defined-conversion-operators
In your case, it would be something like:
public static implicit operator MyClass(string val) => new MyClass(val);
And replace your test method with something like this:
public static string test(string a, MyClass b = null)
{
b = b ?? " with default text";
return a + b;
}
Also note that both your test methods have the same signature if only one argument is provided, so the compiler won't know which one to use, remove the default value for the 2nd test method.
use null for default value, and replace it with default value inside method:
public static string test2(string a, MyClass b = null)
{
if (b == null) return a + " with default text";
return a + b.Value;
}
Add an implicit conversion operator between string and MyClass
public class MyClass
{
public string Value { get; set; }
public MyClass() : this(string.Empty) { }
public MyClass(string s)
{
Value = s;
}
public override string ToString() => this.Value;
public static implicit operator string(MyClass item) => item.Value;
public static implicit operator MyClass(string value) => new MyClass(value)l;
}
You should change 2 steps (In case apply for whole class)
1. Initial default value on Constructor
public MyClass()
{
Value = " with default text";
}
2. Change default params of the method
public static string test(string a, MyClass b)
{
return a + b;
}
BTW, This post is helpful for you.
UPDATED: Set default value for only within function
The code above just suit in case applying for whole class.
If you wanna set default value for only within function, You can try the code below
public static string test2(string a, MyClass b = null)
{
var defaultValue = " with default text";
return a + (b == null ? defaultValue : b.Value);
}
Is there some way I can "overload" a function using delegates? I'd like to have a system in place wherein I can pass an anonymous function in a constructor and save it in a member variable. Data type isn't really an issue, but the passed functions can have one or two parameters. I've tried using (params double[] vals) in the delegate definition, but that complicates the passed anonymous functions and allows more parameters than should be allowed.
So I've created two empty methods to hold the two types. As an example:
public class OpWrapper
{
public int operands; //the number of operands this operator needs.
public int precedence; //the precedence this operator gets when calculating.
public bool rightAssoc; //whether or not this operator is right associative (true) or left associative (false).
public delegate double evalDelegate(double a, double b);
public delegate double calcDelegate(double a);
public evalDelegate eval; //method used for two value inputs. Assigned in constructor.
public calcDelegate calc; //method used for single value input. Assigned in constructor.
//constructor initializes all variables.
public OpWrapper(int o, int p, evalDelegate f, bool a = false)
{
operands = o;
precedence = p;
rightAssoc = a;
eval = new evalDelegate(f);
}
//overloaded constructor assigns the proper method.
public OpWrapper(int o, int p, calcDelegate f, bool a = false)
{
operands = o;
precedence = p;
rightAssoc = a;
calc = new calcDelegate(f);
}
public double evaluate(params double[] values)
{
//do stuff
if (operands == 1)
{
return calc(values[0]);
}
else
{
return eval(values[0], values[1]);
}
//more stuff
}
}
Ultimately, what I'd like to do is more like this:
public class OpWrapper
{
public int operands; //the number of operands this operator needs.
public int precedence; //the precedence this operator gets when calculating.
public bool rightAssoc; //whether or not this operator is right associative (true) or left associative (false).
public delegate double evalDelegate(double a, double b);
public delegate double calcDelegate(double a);
public ???????? calc; //method that does the passed function.
//constructor initializes all variables.
public OpWrapper(int o, int p, evalDelegate f, bool a = false)
{
operands = o;
precedence = p;
rightAssoc = a;
eval = new evalDelegate(f);
}
//overloaded constructor assigns the proper method.
public OpWrapper(int o, int p, calcDelegate f, bool a = false)
{
operands = o;
precedence = p;
rightAssoc = a;
calc = new calcDelegate(f);
}
public double evaluate(params double[] values)
{
//do stuff
if (operands == 1)
{
return calc(values[0]);
}
else
{
return calc(values[0], values[1]);
}
//more stuff
}
}
I'm not tremendously familiar with C#, yet, but surely there is a way to do something like this without having to define an instance of one delegate or the other that will just go unused.
I'll throw my hat into the ring...
Here is how you would use a Func.
public class OpWrapper
{
public int operands; //the number of operands this operator needs.
public int precedence; //the precedence this operator gets when calculating.
public bool rightAssoc; //whether or not this operator is right associative (true) or left associative (false).
public object func;
//constructor initializes all variables.
public OpWrapper(int p, Func<double, double> f, bool a = false)
{
//No need to pass in o, we can infer from context that its a single parameter
operands = 1;
precedence = p;
rightAssoc = a;
func = f;
}
//overloaded constructor assigns the proper method.
public OpWrapper(int p, Func<double, double, double> f, bool a = false)
{
//No need to pass in o, we can infer from context that its a double parameter
operands = 2;
precedence = p;
rightAssoc = a;
func = f;
}
public double evaluate(params double[] values)
{
if (values.Length != operands)
throw new InvalidOperationException("Invalid number of operands");
//do stuff
if (operands == 1)
{
return ((Func<double, double>)func)(values[0]);
}
else
{
return ((Func<double, double, double>)func)(values[0], values[1]);
}
//more stuff
}
}
Note that I removed "o" from the call, and that I used casting to choose the right operation (and checked if the proper number of operands was provided).
.Net provides some useful delegate types out of the box; namely Action for a void returning method and Func for one retaining an argument. These provide type safety over anonymous delegates and provide a clean approach to what you need which appears to be something like the Command or the Strategy pattern.
You can also use expressions to declare delegates inline as follows:
public void InvokeAction(Action invoke)
{
invoke();
}
InvokeAction(() => Console.WriteLine(...));
=> essentially means 'into' and if you had arguments you would declare them before the arrow:
(arg1, arg2) => ...
Expressions and Action/Func have all but taken over from anonymous delegates in modern .Net coding.
If you have a property of type Action on a class you call it as a method directly.
public Action Calc { get; set; }
Calc = () => Console.WriteLine(...);
Calc();
this will help you. In this i have just initialise your calc variable as object which is base type of all types (int,class,delegates etc) and in evaluate method i have cast it from object to its appropriate type.
public class OpWrapper
{
public int operands; //the number of operands this operator needs.
public int precedence; //the precedence this operator gets when calculating.
public bool rightAssoc; //whether or not this operator is right associative (true) or left associative (false).
public delegate double evalDelegate(double a, double b);
public delegate double calcDelegate(double a);
public object calc; //method that does the passed function.
//constructor initializes all variables.
public OpWrapper(int o, int p, evalDelegate f, bool a = false)
{
operands = o;
precedence = p;
rightAssoc = a;
calc = new evalDelegate(f);
}
//overloaded constructor assigns the proper method.
public OpWrapper(int o, int p, calcDelegate f, bool a = false)
{
operands = o;
precedence = p;
rightAssoc = a;
calc = new calcDelegate(f);
}
public double evaluate(params double[] values)
{
//do stuff
if (operands == 1)
{
return (calc as calcDelegate)(values[0]);
}
else
{
return (calc as evalDelegate)(values[0], values[1]);
}
//more stuff
}
}
I have this class:
public class SmartTable : DataTable
{
public string this[int Row, int Column] { ... }
public string this[int Row, string Column] { ... }
}
and i want to add an implicit operator on THIS[,]
then i could use:
string s = smartT[a,b];
or
int i = smartT[a,b];
I googled this but even I don't know how to search it.
I tried (based on IntelliSense) declare something like:
public static implicit operator int[int r, int c](...) {...}
or
public static implicit operator int (SmartTable sm, int a, int b)
and don't work.
Thanks
=== Edit ===
This is a DataTable, and a table have strings, integers, ...
I want to avoid put Convert.To--(...) every time i use this table...
If I try to put a Field on a int, is because it's a integer field...
A solution I'm using is create iGet(int C, int R), sGet(...), dGet(...)
If you can change your SmartTable design to return or work with a custom class instead of the primitive string type, then you can add your own implicit conversions to int or string.
public class SmartTable : DataTable
{
//dummy/hard-coded values here for demonstration purposes
public DataValue this[int Row, int Column] { get { return new DataValue() {Value="3"}; } set { } }
public DataValue this[int Row, string Column] { get { return new DataValue() {Value="3"}; } set { } }
}
public class DataValue
{
public string Value;
public static implicit operator int(DataValue datavalue)
{
return Int32.Parse(datavalue.Value);
}
public static implicit operator string(DataValue datavalue)
{
return datavalue.Value;
}
}
And some usage:
string s = smartT[0, 0];
int i = smartT[0, 0];
Console.WriteLine(s);//"3"
Console.WriteLine(i);//3
Note that this kinda flies in the face of using implicit operators. For example, if your DataValue.Value is non-convertable to int (for example, if it were "Hello World!") it would throw an exception which is generally against the best practices and unexpected for developers leveraging your API.
I created a struct
public struct MyCalender : IComparable<MyCalender>
{
public int CompareTo(PersianDate other)
{
return DateTime.Compare(this, other);
}
.
.
.
.
.
}
I new two object of this in a other UserControl, and i want compare they.
I use this code but i get error.
MyCalender value = new MyCalender(2010,11,12);
MyCalender value2 = new MyCalender(2010,11,12);
if (value < value2) ==> geterror
IComparable exposes CompareTo. < and > must be overloaded separately:
class Foo : IComparable<Foo>
{
private static readonly Foo Min = new Foo(Int32.MinValue);
private readonly int value;
public Foo(int value)
{
this.value = value;
}
public int CompareTo(Foo other)
{
return this.value.CompareTo((other ?? Min).value);
}
public static bool operator <(Foo a, Foo b)
{
return (a ?? Min).CompareTo(b) < 0;
}
public static bool operator >(Foo a, Foo b)
{
return (a ?? Min).CompareTo(b) > 0;
}
}
I edited the code so that it does not fail when comparing against null. To keep it brief I used a shortcut that works unless value is Int32.MinValue for a proper Foo. Strictly speaking you'd have to check for null explicitly to get the contract right:
By definition, any object compares greater than (or follows) null, and
two null references compare equal to each other.
Besides, implementing IComparable<T> means that CompareTo(T value) takes a parameter of T. Therefore MyCalendar : IComparable<MyCalender> should implement a method CompareTo(MyCalendar other) rather than PersianDate (or implement IComparable<PersianDate>).
You should either use CompareTo method that you already implemented instead of > in the line you posted or you need to overload > and < operators for your specific class. For instance:
public static bool operator >(MyCalendar c1, MyCalendar c2)
{
return c1.CompareTo(c2) > 0;
}
public static bool operator <(MyCalendar c1, MyCalendar c2)
{
return c1.CompareTo(c2) < 0;
}
But keep in mind that you have to overload both of them.
if comparing just a datetime object,
would something like
DateTime A = DateTime.Now, B = DateTime.Now.AddMinutes(1);
var isqual = A.Date.CompareTo(B.Date);
do the trick?
or something like:
class Calender
{
public DateTime datetime { get; set;}
}
class DateComparer : Calender, IComparable<Calender>
{
public int CompareTo(Calender other)
{
return other.datetime.Date.CompareTo(this.datetime.Date);
}
}
I want some idea to how implicitly convert nullable "?" variables to district ones.
given this example
int? x = 5;
int y = x; //this gonna fail, !!!
i need some way to override = parameter, but unfortunately the = parameter is not overloadable... any suggestions
I'm using C#
You have two options, access the value directly (if you know for sure it's not null):
int y = x.Value;
or, use the null coalescing operator:
int y = x ?? 0; // 0 if null...
It is possible to implement an implicit cast operator, but only to or from types you define. For example, doing something like this..
public class NullableExtensions
{
public static implicit operator int(int? value)
{
return value ?? default(int);
}
}
.. will return a CS0556 compile error because the cast doesn't include the user-defined type.
The closest you could do is define your own Nullable type that does contain an implicit cast operator:
public struct ImplicitNullable<T> where T: struct
{
public bool HasValue { get { return this._value.HasValue; } }
public T Value { get { return this._value.Value; } }
public ImplicitNullable(T value) : this() { this._value = value; }
public ImplicitNullable(Nullable<T> value) : this() { this._value = value; }
public static implicit operator ImplicitNullable<T>(T value) { return new ImplicitNullable<T>(value); }
public static implicit operator ImplicitNullable<T>(Nullable<T> value) { return new ImplicitNullable<T>(value); }
public static implicit operator T(ImplicitNullable<T> value) { return value._value ?? default(T); }
public static implicit operator Nullable<T>(ImplicitNullable<T> value) { return value._value; }
private Nullable<T> _value { get; set; }
// Should define other Nullable<T> members, especially
// Equals and GetHashCode to avoid boxing
}
Note that although it's possible to write this code, it will likely lead to hard to trace bugs. I would recommend using an explicit cast, or throwing an exception when the value is null.
Afterwards, you can cast to and from as expected:
static void Main()
{
int myInt = 1;
int? nullableInt = 2;
ImplicitNullable<int> implicitInt;
// Convert from int or int?
implicitInt = myInt;
implicitInt = nullableInt;
// Convert to int or int?
myInt = implicitInt;
nullableInt = implicitInt;
}
Wait, I'm so confused...
Why don't you just use GetValueOrDefault?
I'm assuming this is C#.
You need to either cast, or use .value:
int? x = 5;
int y;
if(x.HasValue)
y = x.Value;
else
throw new//... handle error or something