Using IFF statement in C#

You wont be able to store System.DBNull.Value in a string variable.

hongjun

then whatz the solution for this. I want to pass System.dbnull.value to the stored procedure if value not passed in the variable.

Also, String.Empty is a constant that represents an empty string. If you want to check if a particular string is empty you can use string.IsNullOrEmpty(strName);
So it would be:
if (string.IsNullOrEmpty(strname))
    // Insert System.DbNull
else
    // Insert strname

I couldnt find such thing string.IsNullorEmpty ...

see this link
http://jstawski.com/archive/2006/12/08/String.Empty_2C00_-null_2C00_-Length_2C00_-or-String.IsEmptyOrNull-_3F00_.aspx

kumar_jac's is actually a pretty good solution when combined with hiteshgoldeneye's idea of only inserting it when you are building the query. You will need to alter it to cast str to an object though:

public object StringEmptyToDBNull(string str)
{
    return (str == string.Empty) ? DBNull.Value : (object)str;
}

Sorry, you'll also want to check if the string is null, just for robustness:

public static object StringEmptyToDBNull(string str)
{
     return (str == null || str == string.Empty) ? DBNull.Value : (object)str;
}

(Also note that it can be static)