Linq Join with Dynamic Expression

问题:

I'm attempting to do a dynamic join in linq. Meaning that I only know at runtime what field the join will occur on.

I've done the following:

var itemParam = Expression.Parameter(typeof(E), "obj");
var entityAccess = Expression.MakeMemberAccess(Expression.Parameter(typeof(E), "obj"), typeof(E).GetMember(Field).First());
var lambda = Expression.Lambda(entityAccess, itemParam);
var q = dbSet.Join(context.Acl, lambda, acl => acl.ObjectID, (entity, acl) => new { Entity = entity, ACL = acl });

However this throws at compile time, even though lambda appears to be the right syntax telling me that it cannot convert from LambdaExpression to Expression<System.Func<E, int>>.

How do I get it to create the right expression dynamically that uses my field (i.e. property "Field" above in the typeof(E).GetMember(Field).First()) line?

回答1:

Use Expression.Lambda<TDelegate>, so that you end up with the line

// obj => obj.Field
var lambda = Expression.Lambda<Func<E, int>>(entityAccess, itemParam);

Update

As per your comment, the reason the expression fails is because you are using two different parameters. You define itemParam, but then do not use it in Expression.MakeMemberAccess

Try the following instead:

// obj
var itemParam = Expression.Parameter(typeof(E), "obj");

// obj.Field
var entityAccess = Expression.MakeMemberAccess(itemParam, typeof(E).GetMember(Field).First());