How to declare a local constant in C# ?

Question

How to declare a local constant in C# ?

Like in Java, you can do the following :

public void f(){
  final int n = getNum(); // n declared constant
}

How to do the same in C# ? I tried with readonly and const but none seems to work.

Any help would be greatly appreciated.

Thanks.

Answer 1

I'm not sure why readonly and const didn't work for you since these are the keywords you need. You use const if you have a literal (except for array literals) and readonly otherwise:

public void f()
{
    const int answer = 42;
}

private readonly int[] array = new int[] { 1, 2, 3, 4 };
private readonly DateTime date = DateTime.Now;
public void g()
{
    Console.WriteLine(date.ToString());   
}

readonly only works on class level (that is, you can only apply it to fields). Also as a consequence of const requiring a literal, it's inherently static while a readonly field can be either static or instance.

Answer 2

The const keyword is used to modify a declaration of a field or local variable.

From MSDN.

Since C# can't enforce "const correctnes" (like c++) anyway, I don't think it's very useful. Since functions are very narrwoly scoped, it is easy not to lose oversight.

Answer 3

Have a look at the MSDN Documentation about C# constants

Answer 4

In C#, you cannot create a constant that is retrieved from a method.

http://msdn.microsoft.com/en-us/library/e6w8fe1b(VS.71).aspx

Answer 5

In the example you gave, you need to declare the variable as static, because you're initializing it with a method call. If you were initializing with a constant value, like 42, you can use const. For classes, structs and arrays, readonly should work.