Combine a character constant and a string literal to create another constant

Question

I code in C# primarily these days, but I coded for years in VB.NET. In VB, I could combine a character constant and a string literal to create other constants, which is very handy:

Const FileExtensionSeparatorCharacter As Char = "."c
Const BillingFileTypeExtension As String = FileExtensionSeparatorCharacter & "BIL"

Now I'd like to do the same in C#:

const char FileExtensionSeparatorCharacter = '.';
const string BillingFileTypeExtension = FileExtensionSeparatorCharacter + "BIL";

but this give me a compiler error:

The expression being assigned to 'BillingFileTypeExtension' must be constant

Is there a reason I can't do this in C#?

Answer 1

Is there a reason I can't do this in C#?

Yes, but you're not going to like it. The string concatenation involved in char + string involves implicitly calling ToString() on the char. That's not one of the things you can do in a constant expression.

If you make them both strings, that's fine:

const string FileExtensionSeparator = ".";
const string BillingFileTypeExtension = FileExtensionSeparator + "BIL";

Now that's string + string concatenation, which is fine to occur in a constant expression.

The alternative would be to just use a static readonly field instead:

const char FileExtensionSeparatorCharacter = '.';
static readonly string BillingFileTypeExtension = FileExtensionSeparatorCharacter + "BIL";

Answer 2

I have to assume here that adding a character to a string is not considered a compile time constant, but rather a a run-time operation. If you change the type of FileExtensionSeparatorCharacter to string you will compile just fine.

const string FileExtensionSeparatorCharacter = ".";
const string BillingFileTypeExtension = FileExtensionSeparatorCharacter + "BIL";