System.ArgumentNullException when creating a call expression on Xname.Get

Question

I am trying to create a dynamic query on a XElement source by using expression trees. A part of this query needs to compare the value of an XElement Attribute and it is when constructing the expressions for obtaining the attribute value I am getting the ArgumentNullException. It is connected to the Expression.Call for XName.Get but I do not know how to interpret the exception in for this case.

PS: the constant expressions in the code are there just for this example.

The code:

var value = 
            Expression.Property(
                Expression.Call(Expression.Parameter(typeof(XElement), "attr1"), typeof(XElement).GetMethod("Attribute"), 
                    Expression.Call(typeof(XName).GetMethod("Get", BindingFlags.Instance | BindingFlags.Public | BindingFlags.DeclaredOnly), Expression.Constant("id"))), 
            "Value");

Answer 1

typeof(XName).GetMethod(
    "Get", BindingFlags.Instance | BindingFlags.Public | BindingFlags.DeclaredOnly)

This is your problem, as PetSerAl already pointed out. But removing BindingFlags.Instance won't solve anything, you need to specify that you want a static method:

typeof(XName).GetMethod(
    "Get", BindingFlags.Static | BindingFlags.Public | BindingFlags.DeclaredOnly)

This still won't work, it throws AmbiguousMatchException, but we're getting closer. Your Call has a single string parameter, so we need to specify we want that overload of XName.Get:

typeof(XName).GetMethod("Get", new[] { typeof(string) })

(We don't need to specify BindingFlags, because the default works fine.)

With this modification, your snippet seems to work fine.