Option Strict On and DBNull.Value - vb.net

Question

I've started converting an application of mine to use Option Strict On. I've been doing the CStr,Ctype etc, and it's been going well.
SQLCommand.Parameters.AddWithValue("#TERMINATE", If(IsNothing(txtEarningsTerminated.DateValue), DBNull.Value, txtEarningsTerminated.DateValue))
Then I hit this. The txtEarningsTerminated.DateValue is a custom masked text box. When it's value is Nothing I don't want to store anything in the database. However, it states
Cannot infer a common type, and Option Strict On does not allow 'Object' to be assumed.
When I change DBNull.Value to "" or nothing, the error goes away. However as Nothing, it fails during runtime stating
The parameterized query '(#CONTROL int,#CLIENTCODE nvarchar(10),#NoBill int,#TERMINATE nv' expects the parameter '#TERMINATE', which was not supplied.
I want to put a NULL in the database. This value can be a date and then become a NULL.
How do I translate this so as to not produce an error under Option Strict On?

Answer 1

The reason is because operator If(condition, executeAndReturnIfTrue, executeAndReturnIfFalse) expects that both true and false expressions will return same type.
In your case you return DbNull type if true and String(or some other type you didn't mentioned) if result is false.
If create SqlParameter more explicitly, then you can use next approach:
Dim value As Object = Nothing
If txtEarningsTerminated.DateValue Is Nothing Then
value = DbNull.Value
Else
value = xtEarningsTerminated.DateValue
End If
Dim param As SqlParameter = New SqlParameter With
{
.ParameterName = "#TERMINATE",
.SqlDbType = SqlDbType.VarChar, 'Or use your correct type
.Value = value
}
As mentioned in the comments using AddWithValue will force ADO.NET to decide the sql type automatically for your value, which can lead in different problems. For example everytime you run same query with different values your query plan will be recompiled every time you change value.
You can create extension method for string
Public Module ExtensionsModule
Public Function DbNullIfNothing(this As String) As Object
If this Is Nothing Then Return DBNull.Value
Return this
End Function
End Module
Then use it instead of your "inline If method"
Dim value As String = Nothing
Dim param As SqlParameter = New SqlParameter With
{
.ParameterName = "#TERMINATE",
.SqlDbType = SqlDbType.VarChar,
.Value = value.DbNullIfNothing()
}

Answer 2

So the answer was obvious and in the error. I just had to encase the DBNull.Value in a Ctype Object
CType(DBNull.Value, Object)
And then the code would compile just fine.

Answer 3

Here's one way to do it (assuming you're using stored procedures in your database):
In the stored procedure you're calling, set the date input parameter to equal null, making it an optional field with a default value of null (#terminate DATETIME = NULL).
That way, if you don't include that parameter in your procedure call, the procedure won't fail.
Set the default value of the date field to DateTime.MinValue so it always has some value. Then you can test to see if it is equal to a date other than DateTime.MinValue. If it is a valid date value, add the line to include the date parameter to the procedure call. If it is equal to DateTime.MinValue, don't add the parameter to the call.