I'm trying to generate a linq expression in asp.net core for MongoDB using the MongoDriver and I can't access the runtime value of a dictonary with the expression generator. Thanks in advance!
private Expression<Func<Dictionary<string, object>, bool>> GenerateWhereExpression(Dictionary<string, object> filterParams)
{
var pe = Expression.Parameter(typeof(Dictionary<string, object>), "x");
var dictPropery = Expression.PropertyOrField(pe, "name"); // Dictonary value with respect to the name key
var methodCall = Expression.Call(dictPropery, typeof(string).GetMethod("Contains"), Expression.Constant(filterParams["name"], typeof(string)));
var lambda = Expression.Lambda<Func<Dictionary<string, object>, bool>>(methodCall, pe);
return lambda;
}
What I'm trying to accomplish is the where clause of the query to get all the data for a report.
Expresssiom, that should be created as result of the method should look like:
x => ((string)x["name"]).Contains(term)
Ok, you need to change your method this way:
private Expression<Func<Dictionary<string, object>, bool>>
GenerateWhereExpression(Dictionary<string, object> filterParams)
{
var pe = Expression.Parameter(typeof(Dictionary<string, object>), "x");
// it is call of x.getItem("name") what is the same as x["name"]
var dictPropery = Expression.Call(pe,
typeof(Dictionary<string, object>).GetMethod("get_Item"),
Expression.Constant("name"));
//cast to ((string)x.getItem("name"))
var castProperty = Expression.Convert(dictPropery, typeof(string));
var methodCall = Expression.Call(castProperty,
typeof(string).GetMethod("Contains"),
Expression.Constant(filterParams["name"], typeof(string)));
var lambda = Expression.Lambda<Func<Dictionary<string, object>, bool>>(methodCall, pe);
return lambda;
}
I have tested it with my mongodriver and it seems to work.
Actually, for getting Items from dictionary you need to call Item property (Building Expression Tree Using a Parameter's Indexer). But either i have used it wrong way, or MongoDb driver couldn't translate it properly, so i am doing it calling get_Item method, what is the same thing.