Why can't you use null as a key for a Dictionary?

Question

Apparently, you cannot use a null for a key, even if your key is a nullable type.

This code:

var nullableBoolLabels = new System.Collect         


        

Answer 1

I've just been reading up on this; and as Eric replied, I now believe this is incorrect, not all strings are automatically Interned, and I needed to override the equality operation.

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This bit me when I converted a dictionary from using a string as a key to an array of bytes.

I was in the vanilla C mindset of a string simply being an array of characters, so it took me a while to figure out why a string built by concatenation worked as a key for lookups while a byte array built in a loop did not.

It's because internally .net assigns all strings that contain the same value to the same reference. (it's called 'Interning')

so, after running:

{
string str1 = "AB";
string str2 = "A";
str1 += "C";
str2 += "BC";
}

str1 and str2 actually point to the exact same place in memory! which makes them the same onject; which allows a dictionary to find an item added with str1 as a key by using str2.

while if you:

{
char[3] char1;
char[3] char2;
char1[0] = 'A';
char1[1] = 'B';
char1[2] = 'C';
char2[0] = 'A';
char2[1] = 'B';
char2[2] = 'C';
}

char1 and char2 are distinct references; if you use char1 to add an item to a dictionary, you cannot use char2 to look it up.