Problem with LINQ to SQL query in C#

Question

1.00/5 (2 votes)

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Hello everyone

I’m using LINQ to SQL in C#.
There is a Context named “db” which is concluded of three tables “hvl85, hvl86, hvl87”.
There are similar fields with various records in all three tables.
I want all records of each table to be shown in DATAGRIDVIEW based on the chosen number whenever the user selects one of these numbers: 85, 86 or 87.
I have decided to write just one Query.
I have written these codes as you see below but I encounter an error.
Please help me. Thank you.

What I have tried:

C#

Table<hvl85> table85 = db.GetTable<hvl85>();
Table<hvl86> table86 = db.GetTable<hvl86>();
Table<hvl87> table87 = db.GetTable<hvl87>();

Table<itable> mytable;

switch (number)
{
    case 85:
        mytable = table85;
        break;
    case 86:
        mytable = table86;
        break;
    case 87:
        mytable = table86;
        break;
}
var queryAllRecords = from q in mytable select q;
dataGridView1.DataSource = queryAllRecords;

Posted 22-Feb-19 9:46am

Pouria Polouk

Updated 26-Feb-19 8:05am

Maciej Los

v2

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Comments

Patrice T 22-Feb-19 15:54pm

'but I encounter an error.'
Which one ?

2 solutions

Solution 1

You don't tell us what the error is, but the most likely is that it's a compilation error saying that a variable has not been initialized:

Table<itable> mytable;

switch (number)
{
case 85:
mytable = table85;
break;
case 86:
mytable = table86;
break;
case 87:
mytable = table86;
break;
}
var queryAllRecords = from q in mytable select q;

The compiler can see a route through that code where no value is assigned to mytable : specifically if number is not 85, 86, or 87.
Add a default case to your switch which handles that eventuality, and set an initial value.

Posted 22-Feb-19 11:10am

OriginalGriff

Comments

Pouria Polouk 23-Feb-19 0:20am

Thank you for your attention.
Despite writing the Default part, there are three errors related to the tables:

Cannot implicitly convert type 'System.Data.Linq.Table<windowsapp.hvl85>' to 'System.Data.Linq.Table<system.data.linq.itable>'
Cannot implicitly convert type 'System.Data.Linq.Table<windowsapp.hvl86>' to 'System.Data.Linq.Table<system.data.linq.itable>'
Cannot implicitly convert type 'System.Data.Linq.Table<windowsapp.hvl87>' to 'System.Data.Linq.Table<system.data.linq.itable>'

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Richard Deeming · Accepted Answer · 2019-02-26T07:49:00

Quote:
Cannot implicitly convert type 'System.Data.Linq.Table<windowsapp.hvl85>' to 'System.Data.Linq.Table<system.data.linq.itable>'

The Table<T> class doesn't seem to be an interface or delegate type, so it cannot be covariant[^] on its generic type parameter. Even if hvl85 implements ITable, you won't be able to assign a Table<hvl85> instance to a Table<ITable> variable.

Instead, you'll need to use the System.Collections.IEnumerable interface[^] to store the query:

C#

System.Collections.IEnumerable queryAllRecords;
switch (number)
{
    case 85:
    {
        queryAllRecords = db.GetTable<hvl85>();
        break;
    }
    case 86:
    {
        queryAllRecords = db.GetTable<hvl86>();
        break;
    }
    case 87:
    {
        queryAllRecords = db.GetTable<hvl87>();
        break;
    }
    default:
    {
        queryAllRecords = null;
        break;
    }
}

dataGridView1.DataSource = queryAllRecords;