ExpressionTree rewrite - MakeMemberAccess() for Navigation Properties

问题

Related vaguely to a previous question

Note : I'm using a derivative of the ExpressionTree visitor as explained here

In my VisitMemberAccess method I currently create MemberExpressions using something along the lines of:

// `mapping` is a class used to map EntityA's members to EntityB's members
return Expression.MakeMemberAccess(Visit(m.Expression), mapping.TargetMemberInfo);

For the most part, this works.

Given some test classes...

public class EntityA
{
    public long Id { get; set; }
    public string Name { get; set; }
}

public class EntityB
{
    public long MyId {get; set; }
    public string MyName { get; set; }
}

The code will correctly map (EntityA x) => x.Id to (EntityB x) => x.MyId which is great and works lovely. My problem comes when you introduce navigation properties:

public class EntityB
{
    public long MyId {get; set; }
    public EntityBDetails NavigationProperty { get; set; }
}    

public class EntityBDetails
{
    public string MyName { get; set; }
}

Given the above trivial case, I would want (EntityA x) x => x.Name to map to (EntityB x) x => x.NavigationProperty.Name. And therelies the problem, I have no idea what to supply to MakeMemberAccess to make this work... I can compare mapping.TargetMemberInfo.DeclaringType == mapping.TargetMemberInfo.ReflectedType to determine whether there is a navigation property involved, but how do I create the necessary MemberExpression?

Thanks in advance!

NB: The code base I'm working on is VB; C# tends to get better/faster answers on SO so I've converted by hand. Let me know if I've made silly typo's/etc

回答1:

I think it could help to translate the C# code into English, and then translate that into an expression-creating code:

The expression x.NavigationProperty.Name actually means “access property NavigationProperty on x and then access property Name on the result. Now, the code:

ParameterExpression x = …;
var navigationProperty = typeof(EntityB).GetProperty("NavigationProperty");
var name = typeof(EntityBDetails).GetProperty("Name");

var navigationPropertyAccess = Expression.MakeMemberAccess(x, navigationProperty);
var nameAccess = Expression.MakeMemberAccess(navigationPropertyAccess , name);

来源：https://stackoverflow.com/questions/7334668/expressiontree-rewrite-makememberaccess-for-navigation-properties

标签

.net-3.5

expression-trees