CODEWITHSUNDEEP
c#linqreflectionexpressionconstant-expression
van Nijnatten
van Nijnatten

Reputation: 404

How to get the name and value of ContstantExpression in x => x.property

I'm creating a simple way to get the Name and Value of Expressions in C#. However, I found a case I cannot figure out. See the code below:

public void GetValue_Object_TIn_Property_Test()
{
    string val = "value";
    var tuple = Tuple.Create(new object(), val);
    Expression<Func<object, string>> expression = x => tuple.Item2;

    Assert.AreEqual(val, expression.GetValue());
}

The .GetValue() method is my extension method.

Basically this expression-tree consists of a LambdaExpression, two MemberExpressions and a ConstantExpression, in that order.

When I try to get the name of tuple.Item2 I get memberExpression.Member.Name from the last MemberExpression. This gives me "tuple" instead of "Item2". How can I get "Item2" instead?

Also, when I try to get the value of the expression, I get the entire tuple instead of Item2. I'm using the following method to get the value:

public override bool TryGetValue(
    MemberExpression memberExpression,
    out T value
) {
    value = default(T);
    bool success = false;

    var fieldInfo = memberExpression.Member as FieldInfo;
    if (success = (fieldInfo != null))
    {
        var valueObj = fieldInfo.GetValue(expression.Value);
        if (success = (valueObj is T || valueObj == null))
        {
            value = (T)valueObj;
        }
    }

    return success;
}

Where the MemberExpression again is the last MemberExpression. What am I doing wrong here? What is the exact case I am missing?

Thank you in advance

Upvotes: 0

Views: 407

Answers (1)

NetMage
NetMage

Reputation: 26917

Actually, the tree is a LambdaExpression whose Body is a PropertyExpression that has a Member field with a Name of "Item2" and an Expression that is aFieldExpression for getting the value of tuple. Note that PropertyExpression and FieldExpression are internal types that inherit from MemberExpression.

So you need to get the (Body as MemberExpression).Member.Name instead of the Body.Expression.Member.Name.

Think of the tree as MemberExpression(get Member:Item2 from Expression:MemberExpression(get Member:tuple from Expression:[surrounding class])).

Have you used LINQPad? It's Dump() command can show you this and more.

Upvotes: 1

Related Questions