Why the following Conditional Operator works strangely in StringBuilder containing Nullable type? in C#?

Question

StringBuilder htmlResp=new StringBuilder();
int? cuID= 1;
string cuName="Tom";
string cuEmpID="ZXCV";

htmlResp .Append( "<option value=\"" + cuID.Value + "\">" + cuName+" ("+cuEmpID==""? "-":cuEmpID+")"+  "</option>");
html.Resp.ToString();

I cannot understand why the above code(modified from the actual codes) always gives me weird result:"ZXCV)" instead of "Tom (ZXCV)".

Does anybody knows the reason behind and can provide a reference if possible?

Answer 1

You are missing some brackets around your conditional expression.

Try this instead:

string text = "<option value=\"" + cuID.Value + "\">" + cuName + " (" + (cuEmpID == "" ? "-" : cuEmpID) + ")" + "</option>";
htmlResp.Append(text);

As to why the missing brackets caused that to happen... That's an interesting question!

To answer it, let me simplify the original code a bit:

string text = ">>>" + cuEmpID == "" ? "-" : cuEmpID + "<<<"; // Gives "ZXCV<<<"

What's happening is that the conditional expression is using ">>>" + cuEmpID == "" as the condition. This is not equal to "" so the right-hand side of the conditional expression is used, namely the cuEmpID + "<<<" part, which gives the output we're seeing.

You should really simplify the expression, for example:

string normalisedEmpID = cuEmpID == "" ? "-" : cuEmpID;

string text = string.Format
(
    "<option value=\"{0}\">{1} ({2})</option>",
    cuID.Value,
    cuName,
    normalisedEmpID
);

Answer 2

A good example of "don't make expressions too complex".

 "<option value=\"" + cuID.Value + "\">" + cuName 
        + " ("+cuEmpID==""? "-":cuEmpID+")"+  "</option>");

is compiled as

 ("<option value=\"" + cuID.Value + "\">" + cuName+" ("+cuEmpID ) == "" 
      ? "-"
      : cuEmpID+")"+  "</option>");

A simple 'fix' would be

 "<option value=\"" + cuID.Value + "\">" + cuName + " (" 
       + ((cuEmpID == "") ? "-" : cuEmpID ) 
       + ")" + "</option>"

but for readability, use some intermediate variables. ?: should never be nested inside other expressions.