Using LINQ to map dynamically (or construct projections) - linq

Question

I know I can map two object types with LINQ using a projection as so:
var destModel = from m in sourceModel
select new DestModelType {A = m.A, C = m.C, E = m.E}
where
class SourceModelType
{
string A {get; set;}
string B {get; set;}
string C {get; set;}
string D {get; set;}
string E {get; set;}
}
class DestModelType
{
string A {get; set;}
string C {get; set;}
string E {get; set;}
}
But what if I want to make something like a generic to do this, where I don't know specifically the two types I am dealing with. So it would walk the "Dest" type and match with the matching "Source" types.. is this possible? Also, to achieve deferred execution, I would want it just to return an IQueryable.
For example:
public IQueryable<TDest> ProjectionMap<TSource, TDest>(IQueryable<TSource> sourceModel)
{
// dynamically build the LINQ projection based on the properties in TDest
// return the IQueryable containing the constructed projection
}
I know this is challenging, but I hope not impossible, because it will save me a bunch of explicit mapping work between models and viewmodels.

Answer 1

You have to generate an expression tree, but a simple one, so it's not so hard...
void Main()
{
var source = new[]
{
new SourceModelType { A = "hello", B = "world", C = "foo", D = "bar", E = "Baz" },
new SourceModelType { A = "The", B = "answer", C = "is", D = "42", E = "!" }
};
var dest = ProjectionMap<SourceModelType, DestModelType>(source.AsQueryable());
dest.Dump();
}
public static IQueryable<TDest> ProjectionMap<TSource, TDest>(IQueryable<TSource> sourceModel)
where TDest : new()
{
var sourceProperties = typeof(TSource).GetProperties().Where(p => p.CanRead);
var destProperties = typeof(TDest).GetProperties().Where(p => p.CanWrite);
var propertyMap = from d in destProperties
join s in sourceProperties on new { d.Name, d.PropertyType } equals new { s.Name, s.PropertyType }
select new { Source = s, Dest = d };
var itemParam = Expression.Parameter(typeof(TSource), "item");
var memberBindings = propertyMap.Select(p => (MemberBinding)Expression.Bind(p.Dest, Expression.Property(itemParam, p.Source)));
var newExpression = Expression.New(typeof(TDest));
var memberInitExpression = Expression.MemberInit(newExpression, memberBindings);
var projection = Expression.Lambda<Func<TSource, TDest>>(memberInitExpression, itemParam);
projection.Dump();
return sourceModel.Select(projection);
}
(tested in LinqPad, hence the Dumps)
The generated projection expression looks like that :
item => new DestModelType() {A = item.A, C = item.C, E = item.E}