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

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

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

This doc should help: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/const

A constant expression is an expression that can be fully evaluated at compile time.

Answer 2

Declare your local variable as an iteration variable. Iteration variables are readonly (You didn't ask for a pretty solution).

public void f() 
{
  foreach (int n in new int[] { getNum() }) // n declared constant
  {
    n = 3; // won't compile: "error CS1656: Cannot assign to 'n' because it is a 'foreach iteration variable'"
  }
}

Answer 3

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 4

There is a sort of workaround that requires ReSharper. You can't get readonly locals, but you can at least detect mutated ones and color them differently.

Use the Fonts and Colors item Resharper Mutable Local Variable Identifier.

For me, I have locals colored grey, and then I chose a bold white for the mutated variables (this is with a dark theme). This means that any variable that is written to more than once shows up bright compared to regular ones. You can then do what you can to try to avoid having a mutated variable, or if the method really does require one then it will at least be highlighted.

Answer 5

As of 2018-10-02, it isn't possible to have a readonly local in c#, but there is an open proposal for that feature that has ongoing discussion.

This article provides a useful summary.

Answer 6

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.

Answer 7

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.