Question

Note: I am not trying to determing whether or not a generic parameter is nullable. What I want to know is, if it is nullable, what is the nullable’s underlying type?

Here is what I am trying:

Function CreateTdParameter(Of T)(Name As String, Value As T) As TdParameter

    Dim TdType As TdType
    Dim ValueType As Type = GetType(T)

    If ValueType.IsGenericType Then
        Dim GenType As Type = ValueType.GetGenericTypeDefinition()
        If GenType = GetType(Nullable(Of )) Then
            ValueType = Activator.CreateInstance(GenType.MakeGenericType(New Type() {ValueType}))
            ValueType = Nullable.GetUnderlyingType(ValueType.GetGenericTypeDefinition().UnderlyingSystemType)
        End If
    End If

    If ValueType = GetType(String) Then
        TdType = Teradata.Client.Provider.TdType.VarChar
    ElseIf ValueType = GetType(Integer) Then
        TdType = Teradata.Client.Provider.TdType.Integer
    ElseIf ValueType = GetType(DateTime) Then
        TdType = Teradata.Client.Provider.TdType.Timestamp
    Else
        Throw New NotImplementedException(String.Format("{0} not expected.", Value.GetType))
    End If

    Return CreateTdParameter(Name, Value, TdType, ParameterDirection.Input)

End Function

It results in the following error when a DateTime? is passed through the Value parameter:

GenericArguments[0], 'System.Nullable1[System.DateTime]’, on ‘System.Nullable1[T]' violates the constraint of type 'T'.

It appears that I need to get the underlying type in order to instantiate a nullable of that type using reflection. Not sure how to get around this catch-22.

Can anyone point me to a way that I can tell whether or not a generic type is nullable and, if it is, how I can get its underlying type?

Answer 1

I’ll use C# for the example:

Type underlyingType = Nullable.GetUnderlyingType(typeof(T));
if(underlyingType != null) {
    .. is nullable, with underlying-type as described
}