Question

I’m trying to check if a variable has been defined as a nullable Guid. eg.

Dim myGuid As Nullable(Of Guid) or Dim myGuid As Guid?

It seems doing a myGuid.GetType returns the underlying type, that is type Guid, not Guid?. So testing myGuid.GetType Is GetType(Guid?) always returns False.

How do I find out if myGuid is a nullable type?

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Ed: I can do the following, which correctly returns True for “Guid?” and False for “Guid”:

Not Nullable.GetUnderlyingType(GetType(Guid?)) Is Nothing

The problem is that I don’t know how to retrieve the nullable type from the variable itself, in order to test it. I’ve only been able to get the underlying (non-nullable) system type.

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Ed: Explanation of why I need to know. 🙂

I’ve written a db helper function. I pass it an object comprised of public members, representing the row data of a table. The members are the table columns.

Using reflection, I loop through these public members to create an INSERT statement for a command object and populate its parameters with the values in those members. So far so good.

But now there’s a table which has a uniqueidentifier column which I must not populate from the row object, as it defaults to “NEWID()” (using SQL Server 2008). Instead of skipping all Guid columns, which would be easy, I only want to skip ones defined in the row data class as “Guid” (non-nullable).

Basically, I’m using the Guid? (Nullable) type to indicate it’s ok to populate that uniqueidentifier column with data. If it’s non-nullable, that tells me to skip it because the column has a NEWID() default value.

Hope that makes sense. If there’s a better way of me knowing which guid columns to skip and which to populate, I’d be eager to know. This is just an approach I thought would be doable.

Answer 1

Found a solution! Seems it’s impossible to determine if a variable alone is nullable but, as long as it’s a member of a class, reflection can be used like this:

The class:

Public Class testclass
  Public intno As Int16
  Public intyes As Int16?
  Public guidno As Guid
  Public guidyes As Guid?
End Class

The code:

Dim oTest As New testclass
For Each f As Reflection.FieldInfo In oTest.GetType.GetFields
  Console.Write(f.Name & " is ")
  If f.FieldType.IsGenericType AndAlso f.FieldType.GetGenericTypeDefinition() Is GetType(Nullable(Of )) Then
    Console.WriteLine("Nullable")
  Else
    Console.WriteLine("Not Nullable")
  End If
Next

The output:

intno is Not Nullable
intyes is Nullable
guidno is Not Nullable
guidyes is Nullable

Thanks to all for your responses.