Small c# challenge [modified]

Giorgi Dalakishvili

Let's say you have this class:

class myClass
{
public double? Foo;
}

and this piece of code:

myClass temp=new myClass()
SqlDataReader rd = GetDataSomeHow();

if(rd.IsDBNull(0))
temp.Foo = null;
else
temp.Foo = rd.GetDouble(0);

Rewrite the above statement without if. [Edit] One line solution Sorry if it's too easy for you :) Here is the solution in case you can't solve it: Solution[^]

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modified on Thursday, October 23, 2008 11:23 AM

James R Twine · modified on Thursday, October 23, 2008 11:23 AM

Tertiary operator seems like too easy a solution... Peace!

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Marc Clifton · modified on Thursday, October 23, 2008 11:23 AM

What BS. A tertiary operator is just like an if. [edit] And the fact that nullable types didn't resolve the mismatch between null and DBNull pisses me off to no end.[/edit] [edit2] And yeah, you can really get rid of the if this way:

object Foo;
...
temp.Foo=rd[0];

:laugh: Marc

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Chris Losinger · modified on Thursday, October 23, 2008 11:23 AM

try
{
temp.Foo = rd.GetDouble(0);
}
catch (Exception e)
{
temp.Foo = null;
}

that's assuming GetDouble will throw an exception on null.

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Ennis Ray Lynch Jr · modified on Thursday, October 23, 2008 11:23 AM

temp.Foo = rd[0] == DbNull.Value ? null : (double)rd[0]

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Ennis Ray Lynch Jr

Me too.

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Ennis Ray Lynch Jr

you could also rewrite the query

SELECT
isnul(foo, '')
FROM
wherever

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cpkilekofp

Marc Clifton wrote:

And yeah, you can really get rid of the if this way: object Foo;...temp.Foo=rd[0];

That's what I would have thought from the C# 2.0 spec. But I have to ask: have you tried it, and observed it to work?

cpkilekofp

Ennis Ray Lynch, Jr. wrote:

you could also rewrite the query SELECT isnul(foo, '')FROM wherever

Oh, yes, I ALWAYS want a VARCHAR(0) returned when I'm expecting a double :laugh:

Ennis Ray Lynch Jr

I wasn't paying any attention. I just squeezed in an emergency production db fix using that exact code so I naturally used the ''.

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James Simpson

You could also write a set of extension methods for DbDataReader eg public static double? GetDoubleOrDefault(this DbDataReader src, int index) { return GetDoubleOrDefault(src, index, 0); } public static double? GetDoubleOrDefault(this DbDataReader src, int index, double? def) { return this.IsDbNull(index) == true ? def : this.GetDouble(index) } then your example might be: temp.Foo = rd.GetDoubleOrDefault(0); or temp.Foo = rd.GetDoubleOrDefault(0, 0);

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Simon P Stevens

I like that. Keeps the code nice and readable.

Simon

PIEBALDconsult · modified on Thursday, October 23, 2008 11:23 AM

I wouldn't use the tertiary operator in this case either, most times I use it to avoid a local variable: System.Console.Writeline ( rd [ 0 ] == System.DBNull.Value ? "null" : rd [ 0 ].ToString() ) ; In this case I'd initialize Foo to null in the class (shouldn't it be anyway?) and then:

if ( rd [ 0 ] != System.DBNull.Value )
{
temp.Foo = (double) rd [ 0 ] ;
}

But this kind of stuff belongs encapsulated in something, perhaps pass the DataReader to the constructor of myClass and hide the details there. This is why my wrapper for ExecuteScalar requires the user to pass in a value to use in case of null.

    public virtual T
    ExecuteScalar<T>
    (
        T IfNull
    )
    {
        lock ( this.cmd )
        {
            **T    result    = IfNull ;**
            bool wasclosed = this.IsClosed ;
            
            try
            {
                object temp ;
                
                if ( wasclosed )
                {
                    this.Open() ;
                }

                temp = this.cmd.ExecuteScalar() ;

                **if ( ( temp != null ) && ( temp != System.DBNull.Value ) )
                {
                    result = (T) temp ;
                }**
            }
            catch ( System.Exception err )
            {
                throw ( WrapDataException ( "ExecuteScalar" , this.cmd , err ) ) ;
            }
            finally
            {
                if ( wasclosed )
                {
                    this.Close() ;
                }
            }

            return ( result ) ;
        }
    }

Giorgi Dalakishvili

rd[0] returns object, not double.

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Giorgi Dalakishvili

In that case how will you know if the database contained zero or it was null and isnull turned it into zero?

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Ennis Ray Lynch Jr

That is why I usually use the ternary operator. However, if you are using .NET 1.0 or 1.1 my solution is valid for your question.

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Giorgi Dalakishvili

What if you have several properties in your class? Code will get very ugly.

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Giorgi Dalakishvili

Type of conditional expression cannot be determined because there is no implicit conversion between '<null>' and 'double' Giorgi Dalakishvili #region signature My Articles / My Latest Article[^] / My blog[^] #endregion

modified on Thursday, October 23, 2008 11:33 AM

Ennis Ray Lynch Jr · modified on Thursday, October 23, 2008 11:33 AM

Follow the subthread I have already responded to this, it was a mistake on my part and should have been isnull(foo, 0) or more fully isnull(foo, cast(0 as float))

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Giorgi Dalakishvili

Sorry, some characters got swallowed. The code will not compile.

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