Correct overload selection

Question

I have the code

//this runs 
string[] s_items = {"0","0"};
string s_output = string.Format("{0}{1}",s_items);

//this one throws a exception
int[] i_items = {0,0};          
string i_output = string.Format("{0}{1}",i_items);

why does the first one run and the second throw a exception? Why selects

int[] the Format(String, Object) overload

string[] the Format(String, Object[]) overload

Answer 1

A string[] can be converted to an object[] because they're both arrays of reference types. And all references are "equal". This is one of the nasty (array) conversions that is built into the C# language from day 1 and should not have existed but we didn't have generics and proper co/contravariance rules from day 1.

An int[] cannot be converted to an object[] because ints, the things actually contained in the first array, are not references.

Answer 2

From the msdn documentation,

This is a problem of compiler overload resolution. Because the compiler cannot convert an array of integers to an object array, it treats the integer array as a single argument, so it calls the Format(String, Object) method.

See more here.