Given a type ExpressionType.MemberAccess, how do i get the field value?

[updated for clarity]

First; cast the Expression to a MemberExpression.

A MemberExpression has two things of interest:

• .Member – the PropertyInfo / FieldInfo to the member
• .Expression – the expression to evaluate to get the “obj” for the .Member

i.e. if you can evaluate the .Expression to “obj”, and the .Member is a FieldInfo, then you can get the actual value via .GetValue(obj) on the FieldInfo (and PropertyInfo is very similar).

The problem is that evaluating the .Expression is very tricky ;-p

Obviously you get lucky if it turns out to be a ConstantExpression – but in most cases it isn’t; it could be a ParameterExpression (in which case you’ll need to know the actual parameter value that you want to evaluate), or any other combination of Expressions.

In many cases, a simple (perhaps lazy) option is to use .Compile() to get the .NET framework to do the heavy lifting; you can then evaluate the lambda as a typed delegate (passing in any parameters that the lambda requires). This isn’t always an option, however.

To show how complex this is; consider this trivial example (where I’ve hard-coded at every step, rather than testing etc):

using System;
using System.Linq.Expressions;
using System.Reflection;
class Foo
{
    public string Bar { get; set; }
}

static class Program
{
    static void Main()
    {
        Foo foo = new Foo {Bar = "abc"};
        Expression<Func<string>> func = () => foo.Bar;

        MemberExpression outerMember = (MemberExpression)func.Body;
        PropertyInfo outerProp = (PropertyInfo) outerMember.Member;
        MemberExpression innerMember = (MemberExpression)outerMember.Expression;
        FieldInfo innerField = (FieldInfo)innerMember.Member;
        ConstantExpression ce = (ConstantExpression) innerMember.Expression;
        object innerObj = ce.Value;
        object outerObj = innerField.GetValue(innerObj);
        string value = (string) outerProp.GetValue(outerObj, null);    
    }

}