Set a Default Value of Functions Optional Argument to Null

Question

How can you set a function optional argument to null?  For example, I want to set the optional argument intCode equal to Null.  These arguments are used to send to a stored procedure as parameters.

Public Function pubfnc_SetCommentCode(ByVal strFieldName As String, ByVal lngResultID As Int32, Optional ByVal intCode As Int32 = DBNull.Value) As String

Currently, I'm getting an error for DBNull.Value; "Constant expression is required."

Thanks, Ryan

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Ryan

Answer 1

How can you set a function optional argument to null?  For example, I want to set the optional argument intCode equal to Null.  These arguments are used to send to a stored procedure as parameters.

 

Public Function pubfnc_SetCommentCode(ByVal strFieldName As String, ByVal lngResultID As Int32, Optional ByVal intCode As Int32 = DBNull.Value) As String

Currently, I'm getting an error for DBNull.Value; "Constant expression is required."

Thanks, Ryan

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Ryan

Answer 2

In C# you would use an int? parameter with a default value null, no idea how to do in VB though.

This forum is for Windows Forms class library (classes under the System.Windows.Forms namespace), you should post VB language questions to the VB language forum.

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Answer 3

Optional parameters, by definition, must be supplied a default value. DBNull.Value is not sufficient as it means "no value", which conflicts with  the optional parameter rule.

So, one solution would be to supply actual hard-coded values for your Int or Date or String (Some date that indicates not a valid date, like #01/01/1900# and  perhaps an empty string ("")

In your case, you are using int - int is not a reference type so it cannot be assign null - it should be assigned a constant (choose a negative number for example instead of NULL)  like intCode as Int32 = -1

But, realy the better solution would be to not use Optional Parameters at  all and instead use Overloading, which would allow you to write a version of  your method that doesn't take the last parameter at all...

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Ali Hamdar (alihamdar.com - www.ids.com.lb)

Answer 4

have you tried to set it to Nothing instead of DBNull ?

there are some articles online tht might help. Here's one:

http://stackoverflow.com/questions/2086281/how-can-i-create-a-nullable-optional-numeric-integer-double-parameter-in-vb-net

Answer 5

Because DBNull.Value is a field, you can't use it as a default parameter value. You can only use constants, which can be defined at compile time. Anything which isn't known until runtime cannot be used as a default value.

Really, the best option is just to create an overload:

 

Public Function pubfnc_SetCommentCode(ByVal strFieldName As String, ByVal IngResultID As Int32, ByVal intCode As Int32?)
 ' Do your stuff here
End Function

Public Function pubfnc_SetCommentCode(ByVal strFieldName As String, ByVal IngResultID As Int32)
 pubfnc_SetCommentCode(strFieldName, IngResultID, Nothing)
End Function

EDIT: Or I guess just change Int32 to Int32? and DBNull.Value to Nothing in your original code.

 

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Answer 6

It is a value type so will always have a value.

The suggestion of using a specific value which the function translates in DbNull is proabably the way to go.

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Reed Kimble - "When you do things right, people won't be sure you've done anything at all"

Answer 7

Ali and Reed

thanks for the reminder. I forgot an Integer parameter must have a default value.

I've generally stayed away from Optional parameters and used overloaded methods instead.

Answer 8

Hi Ryan.

Your could pass a value for intCode as a STRING perhaps?

>>

Public Class Form1

  Public Function pubfnc_SetCommentCode(ByVal strFieldName As String, ByVal lngResultID As Int32, Optional ByVal intCode As String = Nothing) As String

    Dim returnString As String = ""
    'Your code here:>>

    Return returnString

  End Function

End Class

 

or use:>>

Public Class Form1

  Public Function pubfnc_SetCommentCode(ByVal strFieldName As String, ByVal lngResultID As Int32, Optional ByVal intCode As String = Microsoft.VisualBasic.vbNullString) As String

    Dim returnString As String = ""
    'Your code here:>>

    Return returnString

  End Function

End Class

 

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Regards,

John

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Answer 9

Tim,

What does Int32? mean?

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Ryan

Answer 10

Int32? means the Nullable Type in .Net (http://msdn.microsoft.com/en-us/library/ms235245.aspx), then this varible can get Nothing value or Int32 value.

And we cannot set the DBNull.Value to the optinal argument, also the DBNull cannot be casted to Int32, by reflector tool, you could view the method ToInt32 throws one exception:

Private Function System.IConvertible.ToInt32(ByVal provider As IFormatProvider) As Integer Implements IConvertible.ToInt32
 Throw New InvalidCastException(Environment.GetResourceString("InvalidCast_FromDBNull"))
End Function

So Tim's reply is the answer.

 

Sincerely,

 

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Bob Bao [MSFT]
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Please remember to mark the replies as answers if they help and unmark them if they provide no help.

Answer 11

Yes, as I mentioned in my post - Overloading is always a better solution...

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Ali Hamdar (alihamdar.com - www.ids.com.lb)

Answer 12

Like Bob said, Int32? is just a shorthand way of writing Nullable<Int32>. See the link he provided for details on the Nullable class.

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Answer 13

Plese do not duplicate your threads...  You may request that a moderator move the thread to a different forum if you feel like you would get better results from the Language forum instead of the General forum.

-EDIT-

Sorry I just realized another moderator moved your second post here instead of merging it with the first.  Let me know if you want this moved back to the general forum (though this is probably more of a language related question).

 

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Reed Kimble - "When you do things right, people won't be sure you've done anything at all"