class MyClass
{
public void MyMethod(Type targetType = typeof(MyClass))
{
}
}
Isn't typeof(MyClass)
a compile-time constant?
I am not a IL expert, but seems that it calls a method at L_0005:
return typeof(int);
It´s the same of:
.maxstack 1
.locals init (
[0] class [mscorlib]System.Type typeofvar)
L_0000: ldtoken int32
L_0005: call class [mscorlib]System.Type [mscorlib]System.Type::GetTypeFromHandle(valuetype [mscorlib]System.RuntimeTypeHandle)
L_000a: stloc.0
L_000b: ldloc.0
L_000c: ret
You can see that it isn´ta constant writing type of code:
const Type constType = typeof(int);
That returns a error:
Constant initialize must be compile-time constant
From MSDN - Named and Optional Parameters :
A default value must be one of the following types of expressions:
a constant expression;
an expression of the form new ValType(), where ValType is a value type, such as an enum or a struct;
an expression of the form default(ValType), where ValType is a value type.
typeof
does not necessarily return a compile time constant as it may return different results depending on context.
because it isn't necessarily a constant expression. your example features a typeof on a simple class but what if the class was generic? obviously this isn't constant by far:
class MyClass<T>
{
public void MyMethod(Type targetType = typeof(MyClass<T>))
{
}
}
Isn't
typeof(MyClass)
a compile-time constant?
That particular expression is statically resolvable, yes, but typeof()
is evaluated at execution time (because of generics), so the rule must be that a typeof()
call isn't a compile-time constant.
I do wonder whether it was in C# 1.0, when there was no such argument to be made...
This is old but if someone is looking for a Workaround:
class MyClass
{
public void MyMethod(Type targetType = null)
{
if(targetType == null)
{
targetType = typeof(MyClass);
}
}
}
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