Get an entity from a table using reflection from an abstract type

Question

Ok, so I have an abstract class called Product. I have 3 tables called “Items, Sets, and Packages” that implement the product. The product has a public property that provides the primary key of the object.

I have a form where I transfer the product. I would like to pull this product from a new data file without having to write a large switch reflecting its type in order to get its proper table.

I wanted to do something like this, but bit-bit will not accept foo.

public BuilderInclusionsForm(Product p) : this() { Type foo = p.GetType(); product = db2.GetTable(p.GetType()).Cast<foo>().SingleOrDefault(a => a.ProductID == p.ProductID); 

or that:

 public BuilderInclusionsForm(Product p) : this() { Type foo = p.GetType(); product = db2.GetTable(p.GetType()).OfType<foo>().SingleOrDefault(a => a.ProductID == p.ProductID); 

Answer 1

Thanks to Mr. Skeet, a vibrant member of my team pointed to the following solution.

 public BuilderInclusionsForm(Product p) : this() { IEnumerable<Product> ps = db2.GetTable(p.GetType()).Cast<Product>(); product = ps.SingleOrDefault(a => a.ProductID == p.ProductID); } 

Sorry to waste your time. Please don't rubbish pick up my miserable ass of John. = O.D.

Answer 2

No, because the type argument must be known at compile time so that it appears in the source code.

You can either make BuilderInclusionsForm generic in the product type, or write a generic method like this:

 private static T FindProduct<T>(T product) where T : Product { return db2.GetTable(typeof(T)) .OfType<T>() .SingleOrDefault(a => a.ProductID == p.ProductID); } 

and then call it with reflection:

 public BuilderInclusionsForm(Product p) : this() { MethodInfo method = typeof(BuilderInclusionsForm).GetMethod("FindProduct", BindingFlags.Static | BindingFlags.NonPublic); MethodInfo concrete = method.MakeGenericMethod(new Type[] { p.GetType() }); product = (Product) concrete.Invoke(null, new object[] { p }); } 

(Obviously, you can cache the open form of the method.)

Not bad, but it should work. I suspect that it would be best to create a BuilderInclusionsForm, but you can always have a helper class:

 public static class BuilderInclusionsForm { public static BuilderInclusionsForm<T> Create<T>(T product) where T : Product { return new BuilderInclusionsForm<T>(product); } } 

which allows you to use type inference.