How can make a variable (not class member) "read only" in C#

Question

I'm new to the C# world and I can't find a method to declare read only variable in C# (something like declaring "const" variable in c++). Is there one?

I'll give you an example:

...
int f() { return x; } // x is not const member
...
void g() {
    int readOnlyVar = f(); // is there a method to declare readOnlyVar as read only or const

    // Some code in which I want to restrict access to readOnlyVar to read only 
}

Answer 1

There isn't an identical analogue.

The readonly keyword allows the variable value to be mutated, but only in a constructor.

The const keyword means the value cannot mutate and needs to be a compile time constant and can only be one of the following types: sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, bool, string, an enum-type, or a reference-type. (C# 4.0 spec §10.4).

And in c#, readonly only applies to fields and cannot be applied to local variables.

Answer 2

No, there exists no solution for your code-example.

const in C# is for compile-time constants and since your variable gets its value from a function, it's not known at compile-time.

The readonly keyword does kind of what you're looking for, but that's only for member variables in classes (and only allows variables to be set in the constructor of the class).

But, on the other hand, why would you ever need it? If your function is very long, it should be refactored into smaller functions. If it's not very long, then why would you need to enforce a rule like this? Just don't assign to readOnlyVar is probably my best suggestion for you I'm afraid.

Answer 3

There are two ways to set a variable as read only.

public class ClassA
{
  private const int I = 5;
  private readonly int J = 5;
}

The const keyword will set the value at compile time. The readonly keyword will set the value at construction. So, if you need different values for each instance, use readonly. Otherwise use const.