asp.netsql-servervb.netlabeldatabound

How do you handle NULL values in DataBound controls?

I am trying to limit the characters displayed in ASP.NET Label controls that have their Text values databound to a SQL Database. If the field in the database is not NULL, I do not have any trouble. If they are NULL, I get the exception "Object reference not set to an instance of an object."

My code is as follows:

<asp:Label ID="LabelDescriptionLabel" runat="server" Text='<%# IIf(Not IsDBNull(Item.LabelDescription), IIf(Item.LabelDescription.ToString.Length > 30, Left(Item.LabelDescription, 30) & "...", Item.LabelDescription), Item.LabelDescription)%>' />

I have also tried the following, without any luck:

<asp:Label ID="LabelDescriptionLabel" runat="server" Text='<%# IIf(Not IsNothing(Item.LabelDescription), IIf(Item.LabelDescription.ToString.Length > 30, Left(Item.LabelDescription, 30) & "...", Item.LabelDescription), Item.LabelDescription)%>' />

I am using ASP.NET with VB.NET. I have tried using the CSS "text-overflow: ellipsis" method as well, but I had some table formatting issues that I have not resolved.

Is there a way to handle these NULL values and prevent run-time exceptions?

Thanks!

Solution

  • One problem you are having is that the old IIF is a function which evaluates all the parts before it decides what to return. 

    IIF(Not IsDBNull(Item.LabelDescription), TruePart, FalsePart)

    It does not matter if the expression is true or not, the TruePart and FalsePart statements will always be executed. Since they include references to something which may be DBNull/Nothing/null, you get an error.

    Something else though is that Nothing is not the same as DBNull, and if Item is an Object which can be Nothing/null, then testing the description of Nothing/null will be problematic.

    The If Operator has been available since abut 2008. The structure is the same but it only evaluates the test condition and the True or False expression which applies:

    If( Condition, TruePart, FalsePart )

    The "short circuiting" prevents the expression/Part which does not apply from being evaluated. As long as the rest of the code is valid, you should be able to just clip one of the Is from the IIF functions.

    Since you want to the object Nothing I would use the Nothing keyword rather than the VB IsNothing function:

    If(Item IsNot Nothing), 
        If(Item.LabelDescription.ToString.Length > 30, x, y), y)

    The If operator is not the same as the If/Then/Else statement block.