Question

I am trying to write one linq to sql query and dynamically choose which table to select from at runtime based on an input string provided by the user. Right now I have a selection statement that does this but it just repeats the same query three times, only difference being the selected table. Kinda like this:

if (input == "94")
{
    var query = from i in db.Table94
        select new MyClass(i.A, i.B, i.C);
}

if (input == "95")
{
    var query = from i in db.Table95
        select new MyClass(i.A, i.B, i.C);
}

//more conditional queries

query = query.Where(addtionalFilteringDoneHere);
DataGridView.DataSource = query;

It is easy enough to use conditional Where clauses, but I am looking for something similar for table selections. I tired a couple of things but they didn’t work. Something like:

var query = from i in FetchTable(input)
    select new MyClass(i.A, i.B, i.C);

query = query.Where(addtionalFilteringDoneHere);

public returnType FetchTable(string input)
{
    //need help here
    return db.GetTable<conditionalTable>;
}

I don’t know what the return type would be or even if there a way to do this. Is it possible?

Answer 1

Since you’re trying to create MyClass instances from different tables you’ve got a problem of either creating a dynamic query – which means you throw out EF or LINQ-to-SQL – or you do a basic lookup instead.

I would suggest the basic lookup and that you do something like this:

var fetch = new Dictionary<string, Func<Context, IQueryable<MyClass>>>()
{
    { "94", db => db.Table94.Select(i => new MyClass(i.A, i.B, i.C)) },
    { "95", db => db.Table95.Select(i => new MyClass(i.A, i.B, i.C)) },
};

This creates a strongly-typed query within a general purpose dictionary that you can now easily pull out your MyClass instances.

Something like this:

var query = fetch[input](db).Where(addtionalFilteringDoneHere);
DataGridView.DataSource = query;

You could use fetch[input].Invoke(db) if you like that syntax better.

How does this look as a solution?