Signum Framework aims to minimize type mismatches between C# and SQL when generating tables, but some differences remain—especially around null handling.

C# (since version 8) supports nullable reference types, allowing you to explicitly mark reference types as nullable (string?). Accessing a member on a null reference throws a NullReferenceException.

SQL, however, propagates null values implicitly. This can cause issues when projecting SQL results into C# value types.

For example:

Database.Query<BugEntity>().Select(a => a.Fixer!.Id).ToList();

This query returns a List<PrimaryKey>. If Fixer is null for any BugEntity, the query will throw an exception, since PrimaryKey is an struct and cannot represent null.

If you project a non-nullable value from a potentially null relationship, you will get a FieldReaderException:

Data is Null. This method or property cannot be called on Null values.
Ordinal: 2
ColumnName: FixerId
Row: 0
Calling: row.Reader.GetInt32(2)
Projector:
    row => new <>f__AnonymousType0<int,string,int,string>(Id = row.Reader.GetInt32(0), Description = row.Reader.GetString(1), FixerId = row.Reader.GetInt32(2), FixerName = row.Reader.GetString(3))
Command:
    SELECT bdn.Id, bdn.Description, bdn.FixerId, ddn.Name
    FROM BugEntity AS bdn
    LEFT OUTER JOIN DeveloperEntity AS ddn
      ON (bdn.FixerId = ddn.Id)

• Ordinal: Column position
• ColumnName: Name of the column
• Row: Row number
• Calling: Expression accessing the value
• Projector: C# expression generating the result
• Command: SQL query executed

1. Check ColumnName to see why it is null.
2. If null is not valid, filter out such results:

   .Where(b => b.Fixer != null)

3. If null is valid, project to a nullable type using an explicit cast to a nullable type:

   .Select(b => new {
       b.Id,
       b.Description,
       FixerId = (PrimaryKey?)b.Fixer!.Id,
       FixerName = b.Fixer.Name
   })

   Note: The null-propagating operator (?.) is still not supported in expression trees (such as those used by the Signum LINQ provider), until then you will need to use !. with explicit casts.

• The null-propagating operator (?.) is not supported in expression trees, use (!.) instead. In Signum LINQ queries the behavioud is the same (LEFT OUTER JOIN).
• If an expression using !. returns a value type (int, decimal, DateTime...) you need to explicitly cast to the nullable version to make place for the null value, otherwise you will get a FieldReaderException.

The current status is confusing... but is the only alternative we have given the state of the C# compiler. More infor here.