Why does C# limit the set of types that can be declared as const?

From the C# specification, chapter 10.4 - Constants:
(10.4 in the C# 3.0 specification, 10.3 in the online version for 2.0)

A constant is a class member that represents a constant value: a value that can be computed at compile time.

This basically says that you can only use expressions that consists solely of literals. Any calls to any methods, constructors (that cannot be represented as pure IL literals) cannot be used, as there is no way for the compiler to do that execution, and thus compute the results, at compile time. Also, since there is no way to tag a method as invariant (ie. there is a one-to-one mapping between input and output), the only way for the compiler to do this would be to either analyze the IL to see if it depends on things other than the input parameters, special-case handle some types (like IntPtr), or just disallow every call to any code.

IntPtr, as an example, though being a value type, is still a structure, and not one of the built-in literals. As such, any expression using an IntPtr will need to call code in the IntPtr structure, and this is what is not legal for a constant declaration.

The only legal constant value type example I can think of would be one that is initialized with zeroes by just declaring it, and that's hardly useful.

As for how the compiler treats/uses constants, it will use the computed value in place of the constant name in the code.

Thus, you have the following effect:

• No reference to the original constant name, class it was declared in, or namespace, is compiled into the code in this location
• If you decompile the code, it will have magic numbers in it, simply because the original "reference" to the constant is, as mentioned above, not present, only the value of the constant
• The compiler can use this to optimize, or even remove, unnecessary code. For instance, if (SomeClass.Version == 1), when SomeClass.Version has the value of 1, will in fact remove the if-statement, and keep the block of code being executed. If the value of the constant is not 1, then the whole if-statement and its block will be removed.
• Since the value of a constant is compiled into the code, and not a reference to the constant, using constants from other assemblies will not automagically update the compiled code in any way if the value of the constant should change (which it should not!)

In other words, with the following scenario:

1. Assembly A, contains a constant named "Version", having a value of 1
2. Assembly B, contains an expression that analyzes the version number of assembly A from that constant and compares it to 1, to make sure it can work with the assembly
3. Someone modifies assembly A, increasing the value of the constant to 2, and rebuilds A (but not B)

In this case, assembly B, in its compiled form, will still compare the value of 1 to 1, because when B was compiled, the constant had the value 1.

In fact, if that is the only usage of anything from assembly A in assembly B, assembly B will be compiled without a dependency on assembly A. Executing the code containing that expression in assembly B will not load assembly A.

Constants should thus only be used for things that will never change. If it is a value that might or will change some time in the future, and you cannot guarantee that all other assemblies are rebuilt simultaneously, a readonly field is more appropriate than a constant.

So this is ok:

• public const Int32 NumberOfDaysInAWeekInGregorianCalendar = 7;
• public const Int32 NumberOfHoursInADayOnEarth = 24;

while this is not:

• public const Int32 AgeOfProgrammer = 25;
• public const String NameOfLastProgrammerThatModifiedAssembly = "Joe Programmer";

Edit May 27th 2016

OK, just got an upvote, so I re-read my answer here and this is actually slightly wrong.

Now, the intention of the C# language specification is everything I wrote above. You're not supposed to use something that cannot be represented with a literal as a const.

But can you? Well, yes....

Let's take a look at the decimal type.

public class Test
{
    public const decimal Value = 10.123M;
}

Let's look at what this class looks like really when looked at with ildasm:

.field public static initonly valuetype [mscorlib]System.Decimal X
.custom instance void [mscorlib]System.Runtime.CompilerServices.DecimalConstantAttribute::.ctor(int8, uint8, uint32, uint32, uint32) = ( 01 00 01 00 00 00 00 00 00 00 00 00 64 00 00 00 00 00 ) 

Let me break it down for you:

.field public static initonly

corresponds to:

public static readonly

That's right, a const decimal is actually a readonly decimal.

The real deal here is that the compiler will use that DecimalConstantAttribute to work its magic.

Now, this is the only such magic I know of with the C# compiler but I thought it was worth mentioning.

Does anyone have a theory on the rationale for this limitation?

If it's allowed to be just a theory, my theory is that const values of primitive types can be expressed at literal opcode parameters in the MSIL ... but the values of other, non-primitive types cannot, because MSIL doesn't have the syntax to express the value of a user-defined type as a literal.

I believe that the concept of const is actually syntactic sugar in C#, and that it just replaces any uses of the name with the literal value

What does the compiler do with const objects in other languages?

You can use readonly for mutable types that me be evaluated at runtime. See this article for the differences.