c# - where t : class generic constraint and const value declaration -

according c# specification in 10.4 constants:

the type specified in constant declaration must be sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, bool, string, enum-type, or reference-type. each constant-expression must yield value of target type or of type can converted target type implicit conversion (§6.1).

why can't following:

public class genericclass<t>     t : class {     public const t val = null; } 

that should possible, because:

• where t : class means, the type argument must reference type; applies class, interface, delegate, or array type (from msdn)
• it satisfies words specification: the possible value constants of reference-types other string null.

any possible explanation?

possible explanation

consider how clr initializes static members of generic classes or when invokes static constructors on generic types. normally, static initialization occurs when program first loaded; however, generic classes initialize static members first time instance of class created.

bear in mind generic class not single type; every single t gets passed in type declaration creating new type.

now consider const expression, has requirement evaluated @ compile-time. although t constrained class, , therefore can receive value null, variable val not exist in memory until class has been created @ runtime.

for example, consider if const t val valid. elsewhere in code use:

genericclass<string>.val genericclass<object>.val 

edit

although both expressions have value null, former of type string , latter of type object. in order compiler perform substitution, needs know type definitions of constants in question.

constraints may enforced @ compile-time, open generics not converted closed generics until runtime. therefore, genericclass<object>.val cannot stored in compiler's local memory perform substitution because compiler not instantiate closed form of generic class, , not know type instantiate constant expression to.