I'm using a type converter to convert values from a Sql Server int column to instances of a custom C# type.

In the type converter's "ConvertFrom()", I have

if (value.GetType() == typeof(int))
    return SomeClass.Parse((int) value);
else if (value.GetType() == typeof(DBNull))
    return null;
else
...

In the "ConvertTo()", I have

if (value == null)
    return DBNull.Value;

The section that is causing the exception is in an entity's get accessor that returns the custom type.

if(valueToReturn == null)
    {
        valueToReturn = TypeDefaultValue.GetDefaultValue(typeof(SomeClass));
    }
return (SomeClass)valueToReturn;

However, GetDefaultValue() is returning "new object()" because "defaultValueType.UnderlyingSystemType.FullName" isn't any of the cases listed. The resulting "(SomeClass)valueToReturn" downcast fails.

In response to this exception, I initially changed my type converter to return a "default" value of my custom type when ConvertFrom() gets a DBNull.Value (like "0" with Int32). Since my type is a reference type though, null is more natural, and I'd like to use that.

Returning null instead of "new object()" in "GetDefaultValue" would fix this particular issue. However, I don't know what side effects that would have. Also, there would be the usual concerns with backwards compatibility.

Is the only solution to go back to a "default" value to represent database nulls? Thanks!

What is the purpose of SomeClass. Are you wanting null int values to be seen as DBNull. You would have SomeClass have a property like _isnull, so your default constructor should set that value to true and then when you return the default SomeClass you can check your object to see if it is null and proceed if not retrieve the int value from the object. I may be missing the question, but I would prefer to return a SomeClass object set with _isnull to true over returning null, that way your cast to some class would work also.

I would prefer to return a SomeClass object set with _isnull to true over returning null, that way your cast to some class would work also.

Casting a null value to any reference type is valid and just results in null.

Are you wanting null int values to be seen as DBNull.

My explanation was poor. The problem stems from the int database column being nullable. When a row has a non-null value for that column, the value is converted to an instance of SomeClass. However, when the database value is null, I'd like null to be returned by the type converter. That is what the code snippet from my implementation of ConvertFrom() is doing. That part is okay but later the null value causes a problem with the code gen'ed get accessor for a property of type SomeClass.

TypeDefaultValue.cs is generated by a template which uses an include template you can adjust. The template is called typeDefaultValueInclude.template and is located in SharedTemplates\C# (or VB.NET).

Create a copy of that template, then add your types to the list of cases. Then create a copy of the templateset config you're using and change the template ID binding so it uses the copy of the template instead (i.e. your filename instead of typeDefaultValueInclude.template). Regenerate the code et viola.