C# Nullable Type question [duplicate]

This question already has answers here: Closed 12 years ago.

Possible Duplicate:

Nullable types and the ternary operator. Why won’t this work?

for example:

int? taxid;
if (ddlProductTax.Selecte开发者_高级运维dValue == "") {
  taxid = null; } 
else {
  taxid = Convert.ToInt32(ddlProductTax.SelectedValue);
} //Correct

But

int? taxid;
taxid = (ddlProductTax.SelectedValue == "" ? null : Convert.ToInt32(ddlProductTax.SelectedValue)); //Error

It error say and int32 cannot implicit convert.

The ( ? truepart : falsepart); is not short of (if ..else..) ?

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The last two operands of the ternary operator should both yield the same type.

Cast either side to int?:

taxid = ddlProductTax.SelectedValue == "" ?
                                 (int?)null
                                 : Convert.ToInt32(ddlProductTax.SelectedValue); 

You can see the exact behavior in the specification:

• 7.12 Conditional operator
• 6.1 Implicit conversions

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Duplicate
For why it works. Nullable types and the ternary operator: why is `? 10 : null` forbidden?

Here's the fix:

string x = "";
int? taxid;
taxid = (x == "" ? null : (int?) Convert.ToInt32(x)); // add the int? cast
Console.WriteLine(taxid);

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Apply this correction and it should work.

int? taxid; 
taxid = (ddlProductTax.SelectedValue == "" ? null : new int?(Convert.ToInt32(ddlProductTax.SelectedValue))); //Now it works.

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Here is a little helper method

taxid = GetNullableInt32(ddlProductTax.SelectedValue);

public static int? GetNullableInt32(string str)
{
        int result;
        if (Int32.TryParse(str, out result))
        {
            return result;
        }
        return null;
}

---

I think that it's down to the way in which the expressions are evaluated. With the ? : construct both of the results must evaulate to the same type and here there is no implicit conversion between the null value and an Int32.

Try:

taxid = (ddlProductTax.SelectedValue == "" )? Convert.ToInt32(null) : Convert.ToInt32(ddlProductTax.SelectedValue);