Invalid Cast on field

[KevinYanuk]

Hello;

I am trying to compare a double field:

if ((double)_feature.get_Value(_feature.Fields.FindField("MYFIELD")) > 8.33)

And I am getting an invalid cast error.

I used the same logic before in my code, and I can't quite figure out why this is not letting me do this.


Thanks

[GeorgeFaraj]

What do you get when you do this?

_feature.get_Value(_feature.Fields.FindField("MYFIELD")).ToString()

[AlexanderGray]

If the value in the field is null, you will not be able to cast it to double.  When there is a null value in a field, .net returns a dbnull.value which cannot be cast to a double even if the field type is double.  Perhaps this is the problem.

[KevinYanuk]

What do you get when you do this?

_feature.get_Value(_feature.Fields.FindField("MYFIELD")).ToString()

cannot perform the operator on "object" and "double"

If the value in the field is null, you will not be able to cast it to double.  When there is a null value in a field, .net returns a dbnull.value which cannot be cast to a double even if the field type is double.  Perhaps this is the problem.

Ah, I believe this is probably the case.  I will have to check tomorrow, but I'm pretty sure this is my problem.  Thanks!

[GeorgeFaraj]

cannot perform the operator on "object" and "double"

Sure you can call ToString() on objects and doubles.

[AlexanderGray]

ToString will fail if the object is dbnull value.  Happens all the time, you have to check for that every time, it is a real pain.  If the type returned was a AO type, I would write an extension to check for that but I hesitate on writing extensions for type object.

[GeorgeFaraj]

ToString will fail if the object is dbnull value.  Happens all the time, you have to check for that every time, it is a real pain.  If the type returned was a AO type, I would write an extension to check for that but I hesitate on writing extensions for type object.

Indeed. You should already be checking for null values before calling any methods on it.

[NeilClemmons]

A simple solution to this problem is to create a class for reading and writing values to database fields.  Put the class into a common code library that you can reference in all of your projects.

This is how you might write a method on that class for reading doubles:

        Public Shared Function DatabaseToDouble(ByVal value As Object) As Double
            If value Is DBNull.Value Then
                Return 0
            Else
                Return Convert.ToDouble(value)
            End If
        End Function

Here's how you would call it:
Dim d As Double = whateverYouNamedThatClass.DatabaseToDouble(feature.Value(fieldIndex))

You would add a method to the class for each data type you need (string, int, boolean, etc).

[AlexanderGray]

The code assumes that nulls are to be treated the same as zero in all cases.  That is a decision you have to make to start with.  In my work, there are cases where that assumption is incorrect so that is something to think about.  You may also return Double.NaN but then you still have to check it before using it.

Putting a not null constraint on the field with a default value of zero would avoid this problem to start with if you have that sort of control over the database.

[NeilClemmons]

All you need to do is overload the method to accept a default value that allows you to specify the value you want returned in cases where the database value is null.

        Public Shared Function DatabaseToDouble(ByVal value As Object, ByVal defaultValue As Double) As Double
            If value Is DBNull.Value Then
                Return defaultValue
            Else
                Return Convert.ToDouble(value)
            End If
        End Function