LINQ to SQL, How to merge multiple IQueryable Lists of different subtype?

Question

Consider the following code

IQueryable<FooBar> fooBarQuery = _fooBar;
IQueryable<FooBaz> fooBazQuery = _fooBaz;
IQueryable<IFoo> mergedQuery = _fooBar.Cast<IFoo>().Concat(_fooBaz.Cast<IFoo>());

The above will merged the _fooBar and _fooBaz. However, the object type in the merged query list will all change to FooBar. If I flip the concat (_fooBaz concat to _fooBar), the object type will change to FooBaz.

I believe, Due to this Subsequent concat on mergedQuery will throw this error: "Types in Union or Concat have members assigned in different order."

Can anybody help me how to merge multiple IQueryable lists of different subtypes?

Answer 1

A solution is to use intermediate objects:

interface IFoo
{
    int PorpertyA { get; }
    int PorpertyB { get; }
    ...
}

class FooImpl : IFoo
{
    public int PorpertyA { get; set; }
    public int PorpertyB { get; set; }
    ...
}

IQueryable<IFoo> mergedQuery =
    fooBarQuery.Select(x => new FooImpl {
        PropertyA = x.PropertyA,
        PropertyB = x.PropertyB
        ...
    })
    .Union(fooBazQuery.Select(x => new FooImpl {
        PropertyA = x.PropertyA,
        PropertyB = x.PropertyB
        ...
    });

Answer 2

The code you provided itself merge the two IQueryable lists of different subtypes. May be I don't understand the problem statement correctly. I have used the following code.

        List<FooBar> lstFooBar = new List<FooBar>() { new FooBar() { r = 1 } };
        List<FooBaz> lstFooBaz = new List<FooBaz>() { new FooBaz() { z = 2 } };

        IQueryable<FooBar> fooBarQuery = lstFooBar.AsQueryable<FooBar>();
        IQueryable<FooBaz> fooBazQuery = lstFooBaz.AsQueryable<FooBaz>();

        IQueryable<IFoo> mergedQuery = fooBazQuery.Cast<IFoo>  
                                       ().Concat(fooBarQuery.Cast<IFoo>());

Where the classes implementation was as follows:

 interface IFoo
  {
    int A { get; set; }
   }

class FooBar : IFoo
{
    public int r { get; set; }
    public int A { get; set; }
}

class FooBaz : IFoo
{
    public int z { get; set; }
    public int A { get; set; }
}

See in the following attachment the two members of the merged query are of different type , not of same type. If there is any understanding issue then please let me know.