Exception in static constructor

Question

I've dug around SO for an answer to this, and the best one I can find so far is here, however that is geared toward instances with static constructors; I'm only using the class statically.

My code:

public static class MailHelper {

    private static string mailHost;

    static MailHelper() {

        var mailSettings = ConfigurationManager.GetSection("MailSettings") as NameValueCollection;
        if (null == mailSettings) {
            throw new ConfigurationErrorsException("Missing Mail Settings in the configuration file");
        }

        mailHost = ConfigurationManager.AppSettings["mailHost"];
        if (null == mailHost) {
            throw new ConfigurationErrorsException("Missing mailHost setting in the configuration file");
        }

    }

    public static void SendMail(MailMessage Message) {
        ...
    }

}


try {
    MailHelper.SendMail(Message);
}
catch (ConfigurationErrorsException exc) {
    ...
}

//  ???    
MailHelper.SendMail(Message);


.

So if the static constructor throws an exception the first time it's called, what happens the second time I try to access the static SendMail() method?

PS: Sorry if you don't like Stroustrup's version of K&R brace styling, but don't edit my post just to change the braces to your preferred Allman style. Thanks.

Answer 1

Once a type initializer has failed once, it is never retried. The type is dead for the lifetime of the AppDomain. (Note that this is true for all type initializers, not just for types with static constructors. A type with static variables with initializer expressions, but no static constructors, can exhibit subtle differences in the timing of the type initializer execution - but it'll still only happen once.)

Demonstration:

using System;

public sealed class Bang
{
    static Bang()
    {
        Console.WriteLine("In static constructor");
        throw new Exception("Bang!");
    }

    public static void Foo() {}
}

class Test
{
    static void Main()
    {
        for (int i = 0; i < 5; i++)
        {
            try
            {
                Bang.Foo();
            }
            catch (Exception e)
            {
                Console.WriteLine(e.GetType().Name);
            }
        }
    }
}

Output:

In static constructor
TypeInitializationException
TypeInitializationException
TypeInitializationException
TypeInitializationException
TypeInitializationException

As you can see, the static constructor is only called once.

Answer 2

The other two answers are good answers to your direct question - here's a metaanswer - you should be throwing the exception in the method when you detect that the configuration elements are not populated, rather than in the constructor. IMHO, "not configured" is a valid configuration state for those elements at the constructor phase, just not at SendMail time. That will sidestep this whole problem.

Answer 3

From the Microsoft Documentation (Static Constructors (C# Programming Guide)):

If a static constructor throws an exception, the runtime will not invoke it a second time, and the type will remain uninitialized for the lifetime of the application domain in which your program is running. Most commonly, a TypeInitializationException exception is thrown when a static constructor is unable to instantiate a type or for an unhandled exception occurring within a static constructor. For implicit static constructors that are not explicitly defined in source code, troubleshooting may require inspection of the intermediate language (IL) code.