我可以从C＃中的const char初始化一个const字符串吗？

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.

Answer 3

C#.Net const requires its value initialised at compile time. That is the reason, your code is not compiling. You can use readonly field to assign run time value.

However, following code will work:

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

Answer 4

The only ways I can think of (both not ideal) are:

const string EscapeString = "\\";
private static readonly EscapeString = EscapeChar.ToString();

Or you could just stick with the char version and ToString() it whenever you need the string version :)