<p>In my database, in one of the table I have a GUID column with allow nulls. I have a method  with a Guid? parameter that inserts a new data row in the table. However when I say myNewRow.myGuidColumn = myGuid I get the following error: "Cannot implicitly convert type 'System.Guid?' to 'System.Guid'." </p>

The ADO.NET API has some problems when it comes to handling nullable value types (i.e. it simply doesn't work correctly). We've had no end of issues with it, and so have arrived at the conclusion that it's best to manually set the value to null, e.g.
```
```
myNewRow.myGuidColumn = myGuid == null ? (object)DBNull.Value : myGuid.Value
```
```
It's painful extra work that ADO.NET should handle, but it doesn't seem to do so reliably (even in 3.5 SP1). This at least works correctly.
We've also seen issues with passing nullable value types to SqlParameters where the generated SQL includes the keyword
```
DEFAULT
```
instead of
```
NULL
```
for the value so I'd recommend the same approach when building parameters.