Setting the default value of a C# Optional Parameter

c# default-value optional-parameters
Jaxidian picture Jaxidian · Apr 28, 2010 · Viewed 25.7k times · Source

Whenever I attempt to set the default value of an optional parameter to something in a resource file, I get a compile-time error of

Default parameter value for 'message' must be a compile-time constant.

Is there any way that I can change how the resource files work to make this possible?

public void ValidationError(string fieldName, 
                            string message = ValidationMessages.ContactNotFound)

In this, ValidationMessages is a resource file.

Answer

Jon Skeet picture Jon Skeet · Apr 28, 2010

One option is to make the default value null and then populate that appropriately:

public void ValidationError(string fieldName, string message = null)
{
    string realMessage = message ?? ValidationMessages.ContactNotFound;
    ...
}

Of course, this only works if you don't want to allow null as a genuine value.

Another potential option would be to have a pre-build step which created a file full of const strings based on the resources; you could then reference those consts. It would be fairly awkward though.

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