How Can I Create a Null Tag With Web Service Return Value

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C#

.NET

webservice

Hi,
I have a class and a WebMethod in my C# Web Service Project like this:

public class Results
{
    public int result1;
    public int result2;
    public string str1;
}

[WebMethod]
public Results getResults(int value1, int value2)
{
    Results results = new Results();
    results.result1 = value1 + value2;
    if (value1 > value2)
    {
        results.result2 = value1 - value2;
        results.str1 = "ok";
    }
    return results;
}

When I give values; value1:5 value2:3 then this XML created:

<Results>
<result1>8</result1>
<result2>2</result2>
<str1>ok</str1>
</Results>

And when I give values; value1:5 value2:8 then this XML created:

<Results>
<result1>13</result1>
<result2>0</result2>
</Results>

result2 tag returns 0 value and str1 tag is not visible. C# automaticly creates tag with value of 0 for integers, and no tags for null strings.
How can I get a response like this????:

<Results>
<result1>13</result1>
<result2></result2>
<str1></str1>
</Results>

I want to create all tags with values. If value is null, can i create null tag?
(<str1></str1> or <str1/>)

Thanks.