score:1

Well, I don't know if you can cut it down to one pass, but you can get information on the member, and if it coincides with another variable you have declared as being part of the query (in this case "o"), you use it to generate your query.

Otherwise, you would assume it is a constant, and then plug that value in.

Unfortunately, because you can have from statements (in addition to the let statement) in multiple places in the query, it doesn't seem like you can do it in just one pass, since you need to know all of the query variables up front.

score:1

Okay, after some quick statistical analysis (ie, comparing individual properties manually), DeclaringType, ReflectedType and Namespace are when Lambda parameter is not in scope" fires.

So unless someone comes up with a better answer, that might be all I have to go on.

score:1

In a Where expression, you are looking at an Expression<Func<T,bool>> - which means that the outermost lambda should have a single ParameterExpression with type T.

If a comparison relates to the row, it will have (as an ancestor) this ParameterExpression; if it is local variable, it will have (as an ancestor) a ConstantExpression - however, the type of this constant expression will be compiler generated to cope with all of the captured variables used in the expression.

Like so:

using System;
using System.Linq.Expressions;
class Foo
{
    public string Name { get; set; }
    static void Main()
    {
        var exp = (LambdaExpression) GetExpression();
        WalkTree(0, exp.Body, exp.Parameters[0]);

    }
    static void WriteLine(int offset, string message)
    {
        Console.WriteLine(new string('>',offset) + message);
    }
    static void WalkTree(int offset, Expression current,
        ParameterExpression param)
    {
        WriteLine(offset, "Node: " + current.NodeType.ToString());
        switch (current.NodeType)
        {
            case ExpressionType.Constant:
                WriteLine(offset, "Constant (non-db)"
                    + current.Type.FullName);
                break;
            case ExpressionType.Parameter:
                if (!ReferenceEquals(param, current))
                {
                    throw new InvalidOperationException(
                        "Unexpected parameter: " + param.Name);
                }
                WriteLine(offset, "db row: " + param.Name);
                break;
            case ExpressionType.Equal:
                BinaryExpression be = (BinaryExpression)current;
                WriteLine(offset, "Left:");
                WalkTree(offset + 1, be.Left, param);
                WriteLine(offset, "Right:");
                WalkTree(offset + 1, be.Right, param);
                break;
            case ExpressionType.MemberAccess:
                MemberExpression me = (MemberExpression)current;
                WriteLine(offset, "Member: " + me.Member.Name);
                WalkTree(offset + 1, me.Expression, param);
                break;
            default:
                throw new NotSupportedException(
                    current.NodeType.ToString());
        }
    }

    static Expression<Func<Foo, bool>> GetExpression()
    {
        Foo foo = new Foo { Name = "abc" };

        return row => row.Name == foo.Name;
    }    
}

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