Accessing indexer from expression tree

Question

I am working on a filtering function. The filter will be an expression tree build by an user. There will be about 30 fields the user can use for filtering. I think the best way is to create the object model with indexer and to access required values by index of enum type.

See this example:

enum Field
{
    Name,
    Date,
}

class ObjectModel
{
    object this[Field Key]
    {
        get 
        {
            //...
            return xx;
        }
    }
}

I would like to ask how can I access an indexer from an expression tree.

Answer 1

I'll post a complete example on how to use an indexer:

ParameterExpression dictExpr = Expression.Parameter(typeof(Dictionary<string, int>));
ParameterExpression keyExpr = Expression.Parameter(typeof(string));
ParameterExpression valueExpr = Expression.Parameter(typeof(int));

// Simple and direct. Should normally be enough
// PropertyInfo indexer = dictExpr.Type.GetProperty("Item");

// Alternative, note that we could even look for the type of parameters, if there are indexer overloads.
PropertyInfo indexer = (from p in dictExpr.Type.GetDefaultMembers().OfType<PropertyInfo>()
                        // This check is probably useless. You can't overload on return value in C#.
                        where p.PropertyType == typeof(int)
                        let q = p.GetIndexParameters()
                        // Here we can search for the exact overload. Length is the number of "parameters" of the indexer, and then we can check for their type.
                        where q.Length == 1 && q[0].ParameterType == typeof(string)
                        select p).Single();

IndexExpression indexExpr = Expression.Property(dictExpr, indexer, keyExpr);

BinaryExpression assign = Expression.Assign(indexExpr, valueExpr);

var lambdaSetter = Expression.Lambda<Action<Dictionary<string, int>, string, int>>(assign, dictExpr, keyExpr, valueExpr);
var lambdaGetter = Expression.Lambda<Func<Dictionary<string, int>, string, int>>(indexExpr, dictExpr, keyExpr);
var setter = lambdaSetter.Compile();
var getter = lambdaGetter.Compile();

var dict = new Dictionary<string, int>();
setter(dict, "MyKey", 2);
var value = getter(dict, "MyKey");

To read from the indexer the IndexExpression contains directly the value of the indexed property. To write to it we must use Expression.Assign. Everything else is quite vanilla Expression. As written by Daniel the Indexer is normally called "Item". Note that Expression.Property has an overload that accepts directly the name of the indexer (so "Item"), but I chose to find it manually (so it can be reused). I have even put an example on how to use LINQ to find the exact overload of indexer you want.

Just as a curiosity, if you look on MSDN for example for Dictionary, under Properties you'll find Item

Answer 2

The indexer is a simple property, normally called Item. This means, you can access the indexer like any other property by using its name.

The name of the indexer property can be changed by the implementor of the class by means of the IndexerName attribute.

To reliably get the actual name of the indexer property, you have to reflect on the class and obtain the DefaultMember attribute.
More information can be found here.