"Cannot implicitly convert type 'System.Guid?' to 'System.Guid'." - Nullable GUID

Question

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'.

Answer 1

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.

Answer 2

OK; how is myGuidColumn defined, and how is myGuid defined?

If myGuid is Guid? and myGuidColumn is Guid, then the error is correct: you will need to use myGuid.Value, or (Guid)myGuid to get the value (which will throw if it is null), or perhaps myGuid.GetValueOrDefault() to return the zero guid if null.

If myGuid is Guid and myGuidColumn is Guid?, then it should work.

If myGuidColumn is object, you probably need DBNull.Value instead of the regular null.

Of course, if the column is truly nullable, you might simply want to ensure that it is Guid? in the C# code ;-p

Answer 3

same as Greg Beech's answer

myNewRow.myGuidColumn = (object)myGuid ?? DBNull.Value

Answer 4

You have to cast null to a nullable Guid, this how it worked for me :

myRecord.myGuidCol = (myGuid == null) ? (Guid?)null : myGuid.Value

Answer 5

Try System.Guid.Empty where you want it to be null

Answer 6

If you want to avoid working with nullable GUIDs in your c# code (personally, I often find it cumbersome to work with nullable types) you could somewhere early assign Guid.Empty to the .NET data which is null in the db. That way, you don't have to bother with all the .HasValue stuff and just check if myGuid != Guid.Empty instead.

Answer 7

or:

    internal static T CastTo<T>(object value)
    {
        return value != DBNull.Value ? (T)value : default(T);
    }

Answer 8

If you are into extension methods...

/// <summary>
/// Returns nullable Guid (Guid?) value if not null or Guid.Empty, otherwise returns DBNull.Value
/// </summary>
public static object GetValueOrDBNull(this Guid? aGuid)
{
  return (!aGuid.IsNullOrEmpty()) ? (object)aGuid : DBNull.Value;
}

/// <summary>
/// Determines if a nullable Guid (Guid?) is null or Guid.Empty
/// </summary>
public static bool IsNullOrEmpty(this Guid? aGuid)
{
  return (!aGuid.HasValue || aGuid.Value == Guid.Empty);
}

Then you could say: myNewRow.myGuidColumn = myGuid.GetValueOrDBNull();

NOTE: This will insert null when myGuid == Guid.Empty, you could easily tweak the method if you want to allow empty Guids in your column.

Answer 9

You can use a helper method:

public static class Ado {
    public static void SetParameterValue<T>( IDataParameter parameter, T? value ) where T : struct {
        if ( null == value ) { parameter.Value = DBNull.Value; }
        else { parameter.Value = value.Value; }
    }
    public static void SetParameterValue( IDataParameter parameter, string value ) {
        if ( null == value ) { parameter.Value = DBNull.Value; }
        else { parameter.Value = value; }
    }
}

Answer 10

Guid? _field = null;
if (myValue!="")//test if myValue has value
{
_field = Guid.Parse(myValue)
}