CS0133 "The expression being assigned to 'identifier' must be constant" - what's the reason behind that?

Question

With a lot of C++ background I've got used to writing the following:

const int count = ...; //some non-trivial stuff here
for( int i = 0; i < count; i++ ) {
   ...
}

and I expected that the same would work fine in C#. However...

byte[] buffer = new byte[4];
const int count = buffer.Length;

produces error CS0133: The expression being assigned to 'count' must be constant.

I don't get it. Why is that invalid? int is a value type, isn't it? Why can't I assign a value and make the variable unchangeable this way?

Answer 1

const is meant to represent a compile-time constant... not just a read-only value.

You can't specify read-only but non-compile-time-constant local variables in C#, I'm afraid. Some local variables are inherently read-only - such as the iteration variable in a foreach loop and any variables declared in the fisrt part of a using statement. However, you can't create your own read-only variables.

If you use const within a method, that effectively replaces any usage of that identifier with the compile-time constant value. Personally I've rarely seen this used in real C# code.

Answer 2

You can't assign a variable number to a const. It is a compile time constant.

From the C# reference on const:

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

Answer 3

Because const in C# is a lot more const than const in C++. ;)

In C#, const is used to denote a compile-time constant expression. It'd be similar to this C++ code:

enum {
  count = buffer.Length;
}

Because buffer.Length is evaluated at runtime, it is not a constant expression, and so this would produce a compile error.

C# has a readonly keyword which is a bit more similar to C++'s const. (It's still much more limited though, and there is no such thing as const-correctness in C#)