C# System.Data.StrongTypingException：列的值'<columnname> ' 在 '<tablename> ' 表是 LINQ 查询中的 DBNull</tablename></columnname>

Question

I need to manage 3 databases with many tables and NULL values that can be almost everywhere. For some cases I've found a solution, but now I need help.

The code so far:

  public DataTable queryResults()
    {

    stockDataSet stockDS = new stockDataSet();
    extensionStockDataSet extensionStockDS = new extensionStockDataSet();      administrationDataSet administrationDS = new administrationDataSet();

    stockDataSetTableAdapters.sparePartsTableAdapter sparePartsAdapter = new stockDataSet TableAdapters.sparePartsTableAdapter();
    sparePartsAdapter.Fill(stockDS.spareParts);

    administrationDataSetTableAdapters.usersTableAdapter usersAdapter = new administrationDataSetTableAdapters.usersTableAdapter();
    usersAdapter.Fill(administrationDS.users);

    extensionStockDataSetTableAdapters.dataPartsTableAdapter dataPartsAdapter = new extensionStockDataSetTableAdapters.dataPartsTableAdapter();
    dataPartsAdapter.Fill(extensionStockDS.dataParts);


    var query = from sp in stockDS.spareParts

                join ad in administrationDS.users
                on sp.idUser equals ad.idUser

                join ext in extensionStockDS.dataParts
                on sp.idStock equals ext.idStock

                into joined 
                from j in joined.DefaultIfEmpty()


                where sp.idTypePart != 7

                orderby sp.idUser

                select new { namePart = sp.namePart,
                             idPartInExtStock = (j == null ? 0 : j.idPart),
                             notes = sp.notes,
                             parNo = ad.parno,
                             descr = ma.description,
                             idBrand = (j == null || j.idBrand == null ? "NOT AVAILABLE”: j.idBrand),
                             idModel = (j == null || j.idModel == null ? " NOT AVAILABLE " : j.idModel),
                             idVersion = (j == null || j.idVersion == null ? "NOT AVAILABLE" : j.idVersion)

                             };


    DataTable results = new DataTable();

    results = LINQToDataTable(query); //converts LINQ results to DataTable with System.Reflection

    return results;

    }

Going double click on every xsd DataSet file in Explore Soluzion panel in Visual Studio -> select a field of any table -> Properties panel -> NullValue -> (Null) I can allow the Null value for every String, that's what I've done for

notes = sp.notes
parNo = ad.parno
descr = ma.description

This can't be done every data type, eg for System.Int32 and System.DateTime types: if you try to set for them NullValue -> (Null) or NullValue -> (Empty) , it returns the message:

Property value not valid

So, for idPartInExtStock that is a System.Int32 type, I've found this solution:

idPartInExtStock = (j == null ? 0 : j.idPart)

that is working, and I've implemented it (just for try) also for String fields

 idBrand = (j == null || j.idBrand == null ? "NOT AVAILABLE”: j.idBrand),
 idModel = (j == null || j.idModel == null ? " NOT AVAILABLE " : j.idModel),
 idVersion = (j == null || j.idVersion == null ? "NOT AVAILABLE" : j.idVersion)

Now I'm getting another Null problem with the line:

where sp.idTypePart != 7

it returns at runtime the folliwing error from the stockDataSet.Designer.cs :

System.Data.StrongTypingException: Value for column idTypePart in spareParts table is DBNull

Unfortunately I can't change values in the databases, I need to manage them as they are.

My questions:

1. how can I solve this System.Data.StrongTypingException: Value of column idTypePart in spareParts table is DBNull , considering that sp.idTypePart is System.Int32 type, the cast on it doesn't work and is not possible to set the (Null) or (Empty) value for it?

So far I've tried:

1A. (int?)sp.idTypePart != 7 ==> it says 'The cast is redundant' , so this is useless

1B. sp?.idTypePart != 7 ==> compiles, runs, but returns again System.Data.StrongTypingException: Value for column idTypePart in spareParts table is DBNull

1C. where (sp.idTypePart.= 7 && sp.idTypePart != null) ==> returns the warning 'expression value is always true' , so this is useless

2. Isn't there some method to avoid all the definitions of these exeptions for manage Null values in every field and for every data type?

Answer 1

Ok, for now the problem seems solved. I mean the "read data" part, I can extract the information correctly and success my query.

What I've done is simply going:

double click on xsd stockDataSet in Explore Solution panel -> select idTypePart field in spareParts table Properties panel ->

so

(1.) : Datatype -> System.String (instead of the default System.Int32 preset)

and then

(2.) : NullValue -> (Null)

I've done this even before yesterday, but for some reason that I can't figure out this wouldn't work at time.

Anyway now the reading problem is solved, and this answer is for that question.

I wonder about what will happen when I'll write data, since this field in the database is an int, while in the dataset is a string.