是否应引用null变量引发异常

Question

The query below consists of multiple INNER JOIN clauses.

The Student table has a lot of foreign keys which are substituted by names in the query. I am using LEFT OUTER JOIN query below. Please note MarkID in the Students can be null . That means marksGroup.DefaultifEmpty() may return null as well. Therefore if I put only m instead of m != null ? m.MarkName : "-- Not marked --" m != null ? m.MarkName : "-- Not marked --" in the last line, the code IMO should throw an exception when m is null because I am referring to this variable but it didn't. On a example which I build this query on, a compiler thrown exception. Please see the code:

var result = from st in dbContext.Students where st.DepartmentID == 17
             join d in dbContext.Departments on st.DepartmentID equals d.DepartmentID
             join sv in dbContext.SoftwareVersions on st.SoftwareVersionID equals sv.SoftwareVersionID
             join stat in dbContext.Statuses on st.StatusID equals stat.StatusID
             join m in dbContext.Marks on st.MarkID equals m.MarkID into marksGroup
             from m in marksGroup.DefaultIfEmpty()
             select new
             {
                 student = st.StudentName,
                 department = p.DepartmentName,
                 software = sv.SoftwareVersionName,
                 status = st.StatusName,
                 marked = m != null ? m.MarkName : "-- Not marked --"
             };

What am I missing. I am still beginner in terms of C# and LINQ .

Answer 1

When you execute a LINQ statement against a SQL backend (I assume you have Entity Framework here) the whole statement is translated into SQL and executed by the database engine.

So this is what the .Net runtime does:

• Translate the statement (which is an Expression ) into SQL
• Send the SQL to the database through ADO.Net.
• Receive the SQL result set (essentially an array of rows, where a row is itself an array of values).
• Create objects from these rows.

In your case, the objects to be created can safely be defined by the part

select new
{
    student = st.StudentName,
    department = p.DepartmentName,
    software = sv.SoftwareVersionName,
    status = st.StatusName,
    marked = m.MarkName
}

The only thing the .Net runtime is concerned with is to create an instance of an anonymous type and set its properties from one row in the result set. In some rows, the value for marked may be null , so marked will set to null in that particular object. The .Net runtime is not dealing with a MarksGroup object there.

Of course in LINQ to objects this would be totally different. There the null check is absolutely necessary to prevent null object references.