Can I initialize a const string from a const char in C#?

Question

I am trying to do the following in a way or another:

const char EscapeChar = '\\';
const string EscapeString = EscapeChar.ToString(); // or ("" + EscapeChar)

This does't compile. Is there another way to make it work?

Answer 1

From the C# Language Specification (§ 17.3 and 14.16):

17.3 Constants

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

and

14.16 Constant expressions

A constant expression is an expression that shall be fully evaluated at compile-time. Where an expression is required to be constant this is indicated in the grammar by using constant-expression. [...] The following constructs are permitted in constant expressions:

• Literals (including the null literal)
• References to const members of class and struct types.
• References to members of enumeration types.
• Parenthesized sub-expressions, which are themselves constant expressions.
• Cast expressions, provided the target type is one of the types listed above.
• The predefined checked and unchecked, +, –, !, and ~ unary operators.
• The predefined +, –, *, /, %, <<, >>, &, |, ^, &&, ||, ==, !=, <, >, <=, and >= binary operators, provided each operand is of a type listed above.
• The ?: conditional operator.
• sizeof expressions, provided the unmanaged-type is one of the types specified in §14.5.12.
• default value expressions, provided the type is one of the types listed above, or the type is a reference type or a type parameter that is known to be a reference type (§25.7).

The following conversions are permitted in constant expressions:

• Identity conversions
• Numeric conversions
• Enumeration conversions

An other way to achieve what you want is to use a static readonly member. Readonly members are evaluated at runtime, not at compile time. Therefore you can use the ToString() method.

private static readonly EscapeString = EscapeChar.ToString();

Note: Because a readonly field can be initialized either at the declaration or in the constructor of a class, readonly fields can have different values depending on the constructor used.

Here is a good article about the differences between const and readonly members.

Answer 2

I don't see any way of doing it, which I agree is a bit of a pity - but do you really need it to be a const instead of static readonly? The latter will have almost the same semantics.