Passing lambda expression as parameter

Question

I've got a few tables that all have the same column domainID which basically just controls what data gets displayed on which website, as they share a database.

So when I go to databind a table to a control I would need to create a large switch to handle the different LINQ queries. I would like to create a utility method which takes the table type as a parameter and then return a where clause based on a column in passed table.

public static IEnumerable<T> ExecuteInContext<T>(
               IQueryable<T> src)
        {
             int domain = 1;//hard coded for example

             return src.Where(x => x.DomainID == domain);//Won't work, has to be a way to do this.            
        }

I'm stuck on the return code. You can't simply construct a where clause like I currently am because it doesn't know what table i'm talking about.

I'm trying to call that first method like this:

using (DataClasses1DataContext db = new DataClasses1DataContext())
        {

            var q = Utility.ExecuteInContext(db.GetTable<item>());

            Repeater1.DataSource = q;
            Repeater1.DataBind();
        }

I hope this explains what I'm trying to do.

Edit: BrokenGlass's answer solved my problem. I would like to add that you need to open up your .dbml.cs file and extend the table/class with your interface. I also wanted to point out that the project wouldn't build if my column was nullable, it said it wasn't the same return type as my interface.

BrokenGlass · Accepted Answer · 2011-04-08 19:42:40Z

3

You have to restrict your T to a class that has a property of DomainID - you can add these interface implementations in partial classes that extend your data model.

public interface IFoo
{
    int DomainId { get; set; }
}
..

public static IQueryable<T> ExecuteInContext<T>(IQueryable<T> src) where T: IFoo
{
  int domain = 1;//hard coded for example
  return src.Where(x => x.DomainID == domain);
}

answered Apr 8, 2011 at 19:42

BrokenGlass

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4 Comments

The Muffin Man Over a year ago

When I try to call this method and set the parameter as my item table it says there is no explicit reference conversion from item to utility.IFoo

BrokenGlass Over a year ago

does your item table class implement IFoo ? This is necessary for this to work. You might have to add this in a partial class, I work with Linq to Entities - hopefully the same is possible in Linq to SQL, if not then ignore this answer

The Muffin Man Over a year ago

I'm not sure how to do that. What I currently have is the second code block in my question.

The Muffin Man Over a year ago

I got it to work. I created a new partial class for each table class that inherits IFoo. Thanks for the base code.

Akash Kava · Accepted Answer · 2011-04-08 19:44:28Z

2

Expression pe = Expression.Parameter(typeof(T));
Expression prope = Expression.Property(pe, "DomainID");
Expression ce = Expression.Equals(prope, 
    Expression.Constant((int)1);

Expression<Func<T,bool>> exp =
Expression.Lambda<Func<T,bool>>(
    ce, pe);

return query.Where(exp);

answered Apr 8, 2011 at 19:44

Akash Kava

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2 Comments

BrokenGlass Over a year ago

this looks scary but interesting - so this approach would work regardless of what type T is if it has a property DomainID ?

Akash Kava Over a year ago

Yes, if you use reflector, this is what exactly compiler does.

Dan J · Accepted Answer · 2011-04-08 19:42:04Z

0

You should be able to cast your generic parameter to the intended type...

public static IEnumerable<T> ExecuteInContext<T>(IQueryable<T> src)
{
    int domain = 1;//hard coded for example

    return src.Where(x => ((T)x).DomainID == domain);
}

But you realize you've created a generic method that assumes its type parameter will always expose a specific property? If you're going to do that, you should apply a generic type constraint such that T is always derived from a type that has that property...

For example:

public static IEnumerable<T> ExecuteInContext<T>(IQueryable<T> src) where T : IMyDomainObject

answered Apr 8, 2011 at 19:42

Dan J

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2 Comments

The Muffin Man Over a year ago

The queries that will be called this method will always have a table which contains the domainID column, if that's what you mean.

The Muffin Man Over a year ago

It doesn't allow me to cast the parameter to the intended type.

Haplo · Accepted Answer · 2011-04-08 19:43:29Z

0

I'm not sure if I understand what you mean, but maybe you want to add a where clause:

public static IEnumerable<T> ExecuteInContext<T>(IQueryable<T> src)     

     where T: MyType //MyType exposing your DomainId   
    {             
       int domain = 1;//hard coded for example            
        return src.Where(x => x.DomainID == domain);//Won't work, has to be a way to do this.                    
    }

answered Apr 8, 2011 at 19:43

Haplo

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