VB.NET Nullables

I'm experiencing unpredicted effects with nullables in VB.net. The object in question has a property defined:

Public Property Value As Int32?

When I try to coalesce the value using IIf, I get a null exception

cmd.Parameters.AddWithValue("@HOValue", IIf(headOffice.Value.HasValue, headOffice.Value .Value, DBNull.Value))

In C#, I know there's no implicit conversi开发者_JAVA技巧on for nullables, hence you can't use ??, but why is the first part of the IIf being evaluated in VB.NET?

The reson for this is that Iif is a function, so both the true value and false value are evaluated before the condition.

Instead, use If i.e.:

 cmd.Parameters.AddWithValue("@HOValue", If(headOffice.Value.HasValue, headOffice.Value.Value, DBNull.Value)) ' Assuming you've already checked that headOffice.Value IsNot Nothing

Iff is a function, i.e. its arguments are evaluated before it is executed. When headOffice.Value is null, then headOffice.Value.Value cannot be evaluated here.