Convert.ToString returns string.empty instead of null

Question

I've found out a strange behaviour of Convert.ToString and I would like to understand, why it does behave like this.

Have a look at following code pieces:

string obj = null;
var str = Convert.ToString(obj);
Console.WriteLine(str); // CORRECT: returns null;

all good so far, but:

DBNull obj = DBNull.Value;
var str = Convert.ToString(obj);
Console.WriteLine(str); // ???: returns string.Empty ("")

and

object obj = null;
var str = Convert.ToString(obj);
Console.WriteLine(str); // ???: returns string.Empty ("")

It looks to me like a bug, because when i do a conversion to a string and the input is NULL the result should be default of a string, which is also NULL.

Answer 1

Convert.ToString has a String overload that does nothing:

Returns the specified string instance; no actual conversion is performed.

and its Object overload is defined thus:

The string representation of value, or String.Empty if value is null.

It might be a bit surprising, but there’s no reason to use Convert.ToString on a String expression in the first place.

Answer 2

That's documented behaviour

Convert.ToString Method (Object)

The string representation of value, or String.Empty if value is null.

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Convert.ToString Method (String)

value is returned unchanged.