Question

I am building a little application in .NET 4 MVC 2 for the office foosball table. I have this repository with a method summing all the values of one column (decimal) using LINQ to SQL. It looks like this:

public decimal GetPlayerRating(int id)
{
    var playerRating = (from r in db.Ratings
                        where r.PlayerID == id
                        select r.Points).Sum();

    return playerRating;
}

The problem is that sometimes when it’s called it returns null because the given player does not have any records in the table. That’s fine — in that case it should just return zero. But I get this error:

The null value cannot be assigned to a
member with type System.Decimal which
is a non-nullable value type.

Description: An unhandled exception
occurred during the execution of the
current web request. Please review the
stack trace for more information about
the error and where it originated in
the code.

Exception Details:
System.InvalidOperationException: The
null value cannot be assigned to a
member with type System.Decimal which
is a non-nullable value type.

How can I tell my method that it is okay to return a null value? Thanks!

Answer 1

Try the null coalese operator

return playerRating ?? 0d;

That will never return a null.

If you really need to return a nullable valuetype, though try this

public decimal? GetPlayerRating(int id)
{ ... }

Adding a ? to the end of a value type like double is rewritten into Nullable < type > (Nullable < double > here)

When dealing with a nullable value type, be sure to check if it has a value (not null)

if ( myNullableType.HasValue ) 
{
    x = myNullableType.Value;
}