How does string.Format handle null values?

In the following code below, why do the two string.Format calls not behave the same way? In the first one, no exception is thrown, but in the second one an ArgumentNullException is thrown.

static void Main(string[] args)
{
    Exception e = null;
    string msgOne = string.Format("An exception occurred: {0}", e);
    string msgTwo = string.Format("Another exception occurred: {0}", null);
}

Could someone please help me understand the difference between the two?

I'm guessing here, but it looks to be the difference of which overloaded call you're hitting; String.Format has multiple.

In the first example, it would make sense you're hitting String.Format(string,object).

In the second example by providing null you're most likely hitting String.Format(string,params object[]) which, per the documentation, would raise an ArgumentNullException when:

format or args is null.

If you're running .NET4, try using named parameters:

String.Format("Another exception occured: {0}", arg0: null);

Why is it hitting the params object[] overload? Probably because null isn't an object, and the way params works is that you can pass either each value as a new object in the call or pass it an array of the values. That is to say, the following are one in the same:

String.Format("Hello, {0}! Today is {1}.", "World", "Sunny");
String.Format("Hello, {0}! Today is {1}.", new Object[]{ "World", "Sunny" })

So it's translating your statement call to something along the lines of:

String format = "Another exception occured: {0}";
Object[] args = null;
String.Format(format, args); // throw new ArgumentNullException();

In your first example, you are hitting Format(String, Object), which looks like this when disassembled:

 public static string Format(string format, object arg0)
 {
    return Format(null, format, new object[] { arg0 });
 }

Note the new object[] around that.

The second one, you are apparently hitting the Format(string, object[]) usage, at least that is the one being invoked when I perform the same test. Disassembled, that looks like this:

 public static string Format(string format, params object[] args)
 {
     return Format(null, format, args);
 }

So all of these actually get funneled to Format(IFormatProvider, string, object[]). Cool, let's look at the first few lines there:

public static string Format(IFormatProvider provider, string format, params object[] args)
{
    if ((format == null) || (args == null))
    {
        throw new ArgumentNullException((format == null) ? "format" : "args");
    }
...
}

...welp, there's your problem, right there! The first invocation is wrapping it in a new array, so it's not null. Passing in null explicitly doesn't make it do that, due to the specific instance of Format() that's calling.

How does string.Format handle null values?

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