1) Expression.Field(Expression ex, string name); “ex” is the expression which should be null (see my previous post); so I should pass null and the name of the field; well it doesn’t work – throws exception: “ex” can’t be null.

2) Expression.Field(Expression ex, FieldInfo field). Using this I pass null for ex and the FieldInfo for my field, which I get using reflection;

So I figured: when the member expression refers to a static member, its Expression property is always null. This means the only info regarding the target’s declaring type is kept in the Member property.

In a more general context, there is no way to create an expression to access a static member. Please tell me that I’m wrong…

class SomeClass {
public static readonly int Field = 6;
}

Say there’s some method GetStaticField(Expression<Func> fieldExpression);
then I would call
GetStaticField(() => SomeClass.Field); analyzing this expression yields eventually a MemberExpression whose Expression is NULL and whose member is the FieldInfo corresponding to SomeClass.Field.

What I need to do (and can’t figure out how) is to build that member expression from scratch, knowing only the field’s declaring type (== typeof(SomeClass)) and its name (== “Field”)

Expression<Func<Customer, object>> exp = c => c.Name;

// get PropertyInfo (for .Name)
var property = exp.Body as PropertyInfo;

// evaluate expression for value of Name, passing in the model object
var value = exp(ViewData.Model);

Something along those lines…

Here is my stupid question then:

What if I wanted to get the value?

ie. suspose I wanted to create a new asp.net mvc textbox control that didn’t use ‘magic strings’ anymore.

ie. instead of:
Html.TextBox(“Name”, ViewData.Model.Name) where ViewData.Model is the ‘Customer’

Instead I’d rather have a
Html.TextBox(c => c.Name) (or something like this)- that

1. uses reflection to set the textbox id to ‘name’ and
2. also sets the value

I bring this up, after seeing the code in MvcContrib, much of it is using lamba expressions.

Thanks again for the post, very informative – I’ll go check out the source of the links you provided to help understand this more.

Expression is a collection of classes that describe how methods are written, you can use it to interrogate a method to find any declarations, assignments, and method accesses (and more). In the same way that the reflection API is a programatic way of accessing class definitions, rather than objects; Expression is a way of accessing the contents of a method declaration, rather than the result of the evaluation of that method.

Clear as mud?

“we don’t actually care about the value that’s returned! In-fact, we don’t even evaluate this expression at all.”

Please explain more Makes the head swim a bit.