NHibernate QueryOver Alias Issue

问题:

I'm using the latest version of NHibernate (3.3.1.4000) from NuGet in .Net 4 targeted project in Visual Web Developer 2010 Express.

When I attempt to follow examples I've seen for defining aliases, I get an exception when setting them up using lambdas (see screenshot).

As you can see I'm getting the error Cannot convert lambda expression to type 'string' because it is not a delegate type.

I have references to the LINQ namespaces in the top of my code:

using System.Linq;
using System.Linq.Expressions;

Any thoughts on what might be causing the problem?

回答1:

In order to use a variable like role in an expression, you have to define it first, like so...

Role roleAlias = null; // <-- these two lines are missing from your code.
Person personAlias = null; 

var x = session.QueryOver<Role>(() => roleAlias)
    .JoinAlias(r => r.People, () => personAlias)
    // ...

ISession.QueryOver<T>(...) has four overloads:

• .QueryOver<T>()
• .QueryOver<T>(Expression<Func<T>> alias)
• .QueryOver<T>(string entityName)
• .QueryOver<T>(string entityName, Expression<Func<T>> alias)

Apparently because it can't figure out what role is, it's assuming you're trying to use the .QueryOver<T>(string entityName) overload, hence the "Cannot convert ... to type 'string'" error message.