C# Create default value for class, e.g. through reflection

Question

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#?

The closest thing to a default in c# is the default operator which would still be null in case of a reference type. Why can't you allow for a null and do the check once in the method's implementation? — Lennart Stoop
– Lennart Stoop, Commented Sep 23, 2019 at 13:54
Is that Method "Client-Facing"? If so, you need to validate parameters anyway. If it is actually OK to pass null , then you could use overloading as in Innat3's answer or use the Null-Object Pattern or just do var localA = a ?? new A(); — Fildor
– Fildor, Commented Sep 23, 2019 at 14:16
@LennartStoop Because I have a similar pattern many places in the code base, and having 0 as default int parameter is way better than having null for a class reference. — alexpanter
– alexpanter, Commented Sep 24, 2019 at 8:19

Innat3 · Accepted Answer · 2019-09-23 14:05:00Z

1

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);
}

answered Sep 23, 2019 at 14:05

Innat3

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alexpanter · Accepted Answer · 2019-09-24 10:50:41Z

0

[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.

answered Sep 24, 2019 at 10:50

alexpanter

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