grab linq to sql record by primarykey without knowing its type

Question

how can i grab a record (and eventually delete it) using linq2sql without knowing the type at compile time?

so far i've got

Sub Delete(ByVal RecordType As String, ByVal ID As Integer)
    Dim dummy = Activator.CreateInstance(MyAssembly, RecordType).Unwrap
    Dim tbl = GetTable(dummy.GetType)
    tbl.DeleteOnSubmit(dummy)
End Sub

but of course the dummy is not an actual record, its just a dummy

i don't want to use direct sql (or executecommand) as there's business logic going on at deletion in the datacontext partial class

can this be done somehow?

thank you very much!

EDIT

in response to striplinwarior, i edited my code to:

Sub Delete(ByVal RecordType As ObjectType, ByVal ID As Integer)
    Dim dummy = Activator.CreateInstance(ObjectType.Account.GetType.Assembly.FullName, RecordType.ToString).Unwrap
    SetObjProperty(dummy, PrimaryKeyField(RecordType), ID)
    Dim tbl = GetTable(dummy.GetType)
    tbl.Attach(dummy)
    tbl.DeleteOnSubmit(dummy)
    SubmitChanges()
End Sub

this does fire off the deletion code correclty, but also seems to try to add the record first to the db, as i get a sqlexception that some "not null" fields are empty, which i guess is true about the dummy record, as the only thing this has is the primarykey, else is all empty. so i tried the other code u posted (something i anyways always wanted to have) and that works excellent!

hers my current code:

Function LoadRecord(ByVal RecordType As String, ByVal RecordID As Integer) As Object
    Dim dummy = Activator.CreateInstance(AssemblyName, RecordType).Unwrap
    Dim rowType = dummy.GetType
    Dim eParam = Expression.Parameter(rowType, "e")
    Dim idm = rowType.GetProperty(PrimaryKeyField(RecordType))
    Dim lambda = Expression.Lambda(Expression.Equal(Expression.MakeMemberAccess(eParam, idm), Expression.Constant(RecordID)), eParam)
    Dim firstMethod = GetType(Queryable).GetMethods().[Single](Function(m) m.Name = "Single" AndAlso m.GetParameters().Count() = 2).MakeGenericMethod(rowType)
    Dim tbl = GetTable(rowType)
    Dim obj = firstMethod.Invoke(Nothing, New Object() {tbl, lambda})
    Return obj
End Function
Sub Delete(ByVal RecordType As String, ByVal RecordID As Integer)
    Dim obj = LoadRecord(RecordType, RecordID)
    Dim tbl = GetTable(obj.GetType)
    tbl.DeleteOnSubmit(obj)
    SubmitChanges()
End Sub

Thank You

Answer 1

The only way I can think of is to use the model information from your database mapping to figure out which member represents the primary key:

Dim primaryKey = (From t In db.Mapping.GetTables() _
            Where t.RowType.Type = tableType _
            Let keyMember = (From dm In t.RowType.DataMembers where dm.IsPrimaryKey).FirstOrDefault() _
            Select keyMember.Member.Name).First()

(I'm using LinqPad here: I assume typical LINQ to SQL models have this mapping information available.)

Then use reflection to set the value of that key member on the dummy item you've created. After that, you need to attach the dummy to the table before trying to delete it, passing false as a second parameter to tell LINQ to SQL that you don't actually want to update the object using its current values, but that it should track changes from here on.

tbl.Attach(dummy, false) 
tbl.DeleteOnSubmit(dummy)
db.SubmitChanges()

Does that make sense?

Edit

When you're only deleting an object, you don't necessarily have to get the record from the database. If you set the ID value of the object and then attach it to the context (as shown above), LINQ to SQL will treat it as if it were retrieved from the database. At that point, calling DeleteOnSubmit should tell the context to construct a DELETE statement in SQL based on that object's primary key value.

However, if you need to retrieve the object for some purpose other than deletion, you'll need to construct an expression to represent the query for that object. So, for example, if you were writing the query manually, you would say something like:

Dim obj = tbl.First(Function(e) e.Id = ID)

So to dynamically build the lambda expression inside the parentheses, you might do something like this:

Dim eParam = Expression.Parameter(rowType, "e")
Dim lambda = Expression.Lambda(Expression.Equal(Expression.MakeMemberAccess(eParam, idMember), Expression.Constant(ID)), eParam)

Then you would need to use reflection to invoke the generic First method:

Dim firstMethod = GetType(Queryable).GetMethods().[Single](Function(m) m.Name = "Single" AndAlso m.GetParameters().Count() = 2).MakeGenericMethod(rowType)
Dim obj = firstMethod.Invoke(Nothing, New Object() {tbl, lambda})