passing entity type as parameter in linq

Question

How would I go about passing an entity type as a parameter in linq?

For e.g. The method will receive the entity name value as a string and I would like to pass the entity name to the below linq query. Is it possible to make the linq query generic ?

public ActionResult EntityRecords(string entityTypeName)
{
    var entityResults = context.<EntityType>.Tolist();
    return View(entityResults);
}

I would like to pass the Entity type as a parameter and return all property values.

Also, is it possible to filter results based the some property?

Answer 1

Assuming your context class is looking like this:

public class MyContext : DbContext
{
    public DbSet<Entity1> Entity1 { get; set; }
    public DbSet<Entity2> Entity2 { get; set; }

    // and so on ...
}

simplest solution is to write method that looks like

private List<object> Selector(string entityTypeName)
{
  if (entityTypeName == "Entity1") 
    return context.Entity1.ToList();

  if (entityTypeName == "Entity2")
    return context.Entity2.ToList()

  // and so on  

  // you may add a custom message here, like "Unknown type"
  throw new Exception(); 
}

But we don't want to hardcode this stuff, so let create Selector dynamically with Linq.Expressions

Define a Func field within your controller:

private readonly Func<string, List<object>> selector;

Now you can create a factory for this member:

private Func<string, List<object>> SelectByType()
{
    var myContext = Expression.Constant(context);
    var entityTypeName = Expression.Parameter(typeof(string), "entityTypeName");

    var label = Expression.Label(typeof(List<object>));
    var body = Expression.Block(typeof(MyContext).GetProperties()
        .Where(p => typeof(IQueryable).IsAssignableFrom(p.PropertyType) && p.PropertyType.IsGenericType)
        .ToDictionary(
            k => Expression.Constant(k.PropertyType.GetGenericArguments().First().Name),
            v => Expression.Call(typeof(Enumerable), "ToList", new[] {typeof(object)}, Expression.Property(myContext, v.Name))
        )
        .Select(kv =>
            Expression.IfThen(Expression.Equal(kv.Key, entityTypeName),
              Expression.Return(label, kv.Value))
        )
        .Concat(new Expression[]
        {
            Expression.Throw(Expression.New(typeof(Exception))),
            Expression.Label(label, Expression.Constant(null, typeof(List<object>))),
        })
    );

    var lambda = Expression.Lambda<Func<string, List<object>>>(body, entityTypeName);
    return lambda.Compile();
}

and assign Func with it (somewhere in constructor)

selector = SelectByType();

Now you can use it like

public ActionResult EntityRecords(string entityTypeName)
{
    var entityResults = selector(entityTypeName);
    return View(entityResults);
}