Optional parameters “must be a compile-time constant”
c#, c++-cli, constants, default-value
Solution
The const keyword in C++ declares constants that are not all that const, it can be arbitrarily cast away with a const_cast<>.
You'll need to use a C++/CLI specific keyword to declare a .NET constant, the kind whose value is stored in the metadata but doesn't also have underlying storage. Use the literal keyword:
literal double Invalid = -1;
Which solves your problem, the C# compiler is happy with that one. Do note the iffiness of public constants. Your C# compiler will compile the value into the generated IL directly, substituting "Invalid" with the literal value. This will turn out poorly when you update your C++/CLI assembly with a bug fix that changes the literal but don't recompile the C# code. Public constants are okay for manifest constants, like Math::Pi.
Problem
I have a similar problem the issue described in this SO question In my case however the optional param default values is: 1. Defined in a separate C++/CLI dll. 2. Is already defined in that lib as "public static const" When I try to use the value as a default parameter value from C# I get "must be a compile time constant". Is there a way to share a common const value between (C++/cli) library and (C#) app? C++/CLI lib: ``` namespace MyCPlusPlusCLILib { public ref class CPPCLIClass { public: static const double Invalid = -1; } ``` C# code: ``` public MyMethod(double fish = MyCPlusPlusCLILib.CPPCLIClass.Invalid) { } // C# compiler error "Must be a compile time const" ``` OR ``` const double MyConstDouble = MyCPlusPlusCLILib.CPPCLIClass.Invalid; // C# compiler error "Must be a compile time const" ```