Unassigned local variable

public static String toAscii(string hexString)
string binaryString;
for (int i=0;i<hexString.Length;i=i+2)
string tmpstr = (((char)hexString[i]).ToString()+((char)hexString[i+1]).ToString());
int hex = Convert.ToInt16(tmpstr,16);
binaryString += (char)hex;
return binaryString;

What is the problem with this code it is saying unassigned local variable binaryString...???
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use string binaryString=null;

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I'd probably use ...

string binaryString = String.Empty;

because you are appending to it and it is a bit clearer but 6 vs a half dozen.
I would agree, see the problem is the compiler doesn't know whether the for loop would be executed or not, it may and probably with your logic it will, but compiler can't guarentee it, so the string has to be initialized to some value..
Even if the for loop does get executed, the first iteration tries to do

binaryString += ....

where binaryString hasn't been initialised.

So binaryString = String.Empty (and not = null) at the beginning is required.
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