Unexpected behavior of String.Format ()

Question

Given the following code, I expect an empty result or an exception:

String.Format(null, "Hello") 

Instead, the result is the string "Hello". Why is this?

Answer 1

It works because it selects this overload:

 public static String Format( IFormatProvider provider, String format, params Object[] args) { ... } 

A null provider is fine, and no arguments for varargs are suitable either, and so it just prints the string.

Intuitively, we could expect this overload:

 public static String Format(String format, Object arg0) { ... } 

And, of course, if he chose this, we would get an ArgumentNullException .

Answer 2

He chooses overload

 public static string Format(IFormatProvider provider, string format, params object[] args) 

because your second argument is of type string (without the need for conversion). Thus, this overload is closer than overload with two parameters (conversion from string to object is required):

 public static string Format(string format, object arg0) 

You can see the difference by calling:

 String.Format(null, 5); 

In this case, the conversion to the object is selected, and you have an exception (in fact, there is no implicit conversion between int and string ).

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Answer 3

Perhaps he interprets the request as a call to String.Format (provider IFormatProvider, string format, params object [] args) overrides and accepts null as the provider and parameters, but "Hello" as the format, thus returning "Hello".

If you want an empty result to use String.Empty