Access the value of a member expression

c# expression-trees lambda linq

Question

If i have a product.

var p = new Product { Price = 30 };

and i have the following linq query.

var q = repo.Products().Where(x=>x.Price == p.Price).ToList()

In an IQueryable provider, I get a MemberExpression back for the p.Price which contains a Constant Expression, however I can't seem to get the value "30" back from it.

Update I have tried this but it doesn't seem to work.

var memberExpression = (MemberExpression)GetRootConstantExpression(m);
var fi = (PropertyInfo)memberExpression.Member;
var val = fi.GetValue(((ConstantExpression)memberExpression.Expression).Value, null);

Cheers.

Accepted Answer

You can compile and invoke a lambda expression whose body is the member access:

private object GetValue(MemberExpression member)
{
    var objectMember = Expression.Convert(member, typeof(object));

    var getterLambda = Expression.Lambda<Func<object>>(objectMember);

    var getter = getterLambda.Compile();

    return getter();
}

Local evaluation is a common technique when parsing expression trees. LINQ to SQL does this exact thing in quite a few places.


Expert Answer

The constant expression is going to point to a capture-class generated by the compiler. I've not included the decision points etc, but here's how to get 30 from that:

var p = new Product { Price = 30 };
Expression<Func<Product, bool>> predicate = x => x.Price == p.Price;
BinaryExpression eq = (BinaryExpression)predicate.Body;
MemberExpression productToPrice = (MemberExpression)eq.Right;
MemberExpression captureToProduct = (MemberExpression)productToPrice.Expression;
ConstantExpression captureConst = (ConstantExpression)captureToProduct.Expression;
object product = ((FieldInfo)captureToProduct.Member).GetValue(captureConst.Value);
object price = ((PropertyInfo)productToPrice.Member).GetValue(product, null);

price is now 30. Note that I'm assuming that Price is a property, but in reality you would write a GetValue method that handles property / field.




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