Perform a query on an object having members of type "dynamic" using Dynamic Linq.

c# dynamic-linq expression-trees lambda

Question

I am trying to use a dynamic linq query to retrieve an IEnumerable<T> from an object collection (Linq to Object), each of the objects in the collection have an internal collection with another set of objects where the data is stored, these values are accessed through an indexer from the outer collection

The dynamic linq query returns the filtered set as expected when you are working with strongly typed objects but my object stores the data in a member of type dynamic, please see the example below:

public class Data
{
    public Data(string name, dynamic value)
    {
        this.Name = name;
        this.Value = value;
    }

    public string Name { get; set; }
    public dynamic Value { get; set; }
}

public class DataItem : IEnumerable
{
    private List<Data> _collection;

    public DataItem()
    { _collection = new List<Data>(); }

    public dynamic this[string name]
    {
        get
        {
            Data d;
            if ((d = _collection.FirstOrDefault(i => i.Name == name)) == null)
                return (null);

            return (d.Value);
        }
    }

    public void Add(Data data)
    { _collection.Add(data); }

    public IEnumerator GetEnumerator()
    {
        return _collection.GetEnumerator();
    }
}

public class Program
{
    public void Example()
    {
        List<DataItem> repository = new List<DataItem>(){
            new DataItem() {
                new Data("Name", "Mike"),
                new Data("Age", 25),
                new Data("BirthDate", new DateTime(1987, 1, 5))
            },
            new DataItem() {
                new Data("Name", "Steve"),
                new Data("Age", 30),
                new Data("BirthDate", new DateTime(1982, 1, 10))
            }
        };

        IEnumerable<DataItem> result = repository.AsQueryable<DataItem>().Where("it[\"Age\"] == 30");
        if (result.Count() == 1)
            Console.WriteLine(result.Single()["Name"]);
    }

When I run the above example I get: Operator '==' incompatible with operand types 'Object' and 'Int32'

Are dynamic members incompatible with Dynamic Linq queries?, or is there another way of constructing expressions that would evaluate properly when dealing with members of type dynamic

Thanks a lot for your help.

1
7
1/12/2012 2:04:09 AM

Fastest Entity Framework Extensions

Bulk Insert
Bulk Delete
Bulk Update
Bulk Merge

Accepted Answer

Are dynamic members incompatible with Dynamic Linq queries?, or is there another way of constructing expressions that would evaluate properly when dealing with members of type dynamic?

Both can work together. Just do a conversion to Int32 before doing the comparison like so:

IEnumerable<DataItem> result = 
           repository.AsQueryable<DataItem>().Where("Int32(it[\"Age\"]) == 30");

Edit 1: Having said that, the use of dynamic binding in connection with Linq is restricted in general as dynamic operations are not allowed in expression trees. Consider the following Linq-To-Objects query:

IEnumerable<DataItem> result = repository.AsQueryable().
                                                  Where(d => d["Age"] == 30);

This code snippet won't compile for the reason mentioned above.

Edit 2: In your case (and in conjunction with Dynamic Linq) there are some ways to hack yourself around the issues mentioned in Edit 1 and in the original question. For example:

// Variant 1: Using strings all the way
public void DynamicQueryExample(string property, dynamic val)
{
   List<DataItem> repository = new List<DataItem>(){
        new DataItem() {
            new Data("Name", "Mike"),
            new Data("Age", 25),
            new Data("BirthDate", new DateTime(1987, 1, 5))
        },
        new DataItem() {
            new Data("Name", "Steve"),
            new Data("Age", 30),
            new Data("BirthDate", new DateTime(1982, 1, 10))
        }
    };

    // Use string comparison all the time        
    string predicate = "it[\"{0}\"].ToString() == \"{1}\"";
    predicate = String.Format(whereClause , property, val.ToString());

    var result = repository.AsQueryable<DataItem>().Where(predicate);
    if (result.Count() == 1)
        Console.WriteLine(result.Single()["Name"]);
}

Program p = new Program();

p.DynamicQueryExample("Age", 30); // Prints "Steve"
p.DynamicQueryExample("BirthDate", new DateTime(1982, 1, 10)); // Prints "Steve"
p.DynamicQueryExample("Name", "Mike"); // Prints "Steve" (nah, just joking...)

or:

// Variant 2: Detecting the type at runtime.
public void DynamicQueryExample(string property, string val)
{
    List<DataItem> repository = new List<DataItem>(){
        new DataItem() {
            new Data("Name", "Mike"),
            new Data("Age", 25),
            new Data("BirthDate", new DateTime(1987, 1, 5))
        },
        new DataItem() {
            new Data("Name", "Steve"),
            new Data("Age", 30),
            new Data("BirthDate", new DateTime(1982, 1, 10))
        }
    };

    string whereClause = "{0}(it[\"{1}\"]) == {2}";


    // Discover the type at runtime (and convert accordingly)
    Type type = repository.First()[property].GetType();
    string stype = type.ToString();
    stype = stype.Substring(stype.LastIndexOf('.') + 1);

    if (type.Equals(typeof(string))) {
        // Need to surround formatting directive with ""
        whereClause = whereClause.Replace("{2}", "\"{2}\"");
    }
    string predicate = String.Format(whereClause, stype, property, val);

    var result = repository.AsQueryable<DataItem>().Where(predicate);
    if (result.Count() == 1)
        Console.WriteLine(result.Single()["Name"]);
}

var p = new Program();
p.DynamicQueryExample("Age", "30");
p.DynamicQueryExample("BirthDate", "DateTime(1982, 1, 10)");
p.DynamicQueryExample("Name", "Mike");
2
1/12/2012 10:52:07 AM

Popular Answer

Is the code below useful for you?

IEnumerable<DataItem> result = repository.AsQueryable<DataItem>().Where("it[\"Age\"].ToString() == \"30\"");

But for this to work, all your types which can be assigned to the Value member of your Data class needs to have a useful implementation of the ToString method.

Patrick Koorevaar

Related Questions

Partially executing an IQueryable query while leaving the remainder to Linq for Objects

I have a Linq provider that sucessfully goes and gets data from my chosen datasource, but what I would like to do now that I have my filtered resultset, is allow Linq to Objects to process the rest of the Expression tree (for things like Joins, projection etc)...My thought was that I could just replace the expression constant that contains my IQuer...
c# expression-trees iqueryable linq
asked by Tim Jarvis

How to make a dynamic ExpressionFunc, dynamic>> - Is it a flaw?

During my work with expression trees for a few days now, I came across something that I find difficult to understand; hopefully someone will be able so shed some light here....If you code ...Expression<Func<dynamic, dynamic>> expr1 = x => 2 * x;... the compiler will complain and you won't get anywhere. However, it ...seems... that if you create one...
.net-4.0 c# dynamic expression-trees lambda
asked by d..

Linq: Dynamic Query Construction: the query is constructed on the client side.

I've been following with great interest the converstaion here:...Construct Query with Linq rather than SQL strings...with regards to constructing expression trees where even the table name is dynamic....Toward that end, I've created a Extension method, addWhere, that looks like:...static public IQueryable<TResult> addWhere<TResult>(this IQueryable<...
dynamic-linq expression-trees lambda linq linq-to-entities
asked by donundeen

Linq or Expression Tree are the best ways to construct dynamic queries with dynamic results.

I'm looking for best way to write a query with LINQ or an expression tree to return a dynamic result according to dynamic input. For example, consider this pseudocode:...CREATE PROCEDURE Test @Input NVARCHAR(50) AS BEGIN DECLARE @Query NVARCHAR(100); SET @Query=N'SELECT ' + @Input + ' FROM MyTable' EXEC @Query END ...What is best way to:...
c# expression-trees linq linq-to-sql
asked by Arian

Dynamic Linq searching multiple columns of multiple types including nullable types

I need select my columns dynamically and I do not know the types of the columns ahead of time. I've got a string and I want to search all the columns if they contain that string, converting non-string into string for the comparison....string format = "Convert.ToString({0}).Contains(@0)"; Model.Where(String.Format(format, columnName), searchString);...
c# dynamic-linq expression-trees linq
asked by Kyle

Unable to cast object of type 'System.Linq.Expressions.UnaryExpression' to type 'System.Linq.Expressions.MemberExpression'

I created a ...method in C#... to get methodname...public string GetCorrectPropertyName<T>(Expression<Func<T, string>> expression) { return ((MemberExpression)expression.Body).Member.Name; // Failure Point } ...and calling it as...string lcl_name = false; public string Name { get { return lcl_name ; } set { lcl_name = value; ...
c# exception-handling expression-trees linq
asked by Nikhil Agrawal

Dynamic Linq query on relationship with foreign key of type Guid

I'm using ...System.Linq.Dynamic... to query an IQueryable datasource dynamically using a where-clause in a string format, like this:...var result = source.Entities.Where("City = @0", new object[] { "London" }); ...The example above works fine. But now I want to query on a foreign key-property of type Guid like this:...var result = source.Entities....
c# dynamic-linq expression-trees sql
asked by Andreas Zita

C# Dynamic expression on dynamic object

Let's assume the following scenario:...I have two classes:...public class Customer { public string Id { get; set; } public string FirstName { get; set; } public string LastName { get; set; } } public class Employee { public string Id { get; set; } public string FirstName { get; set; } public string LastName { get; set; } ...
c# expression expression-trees
asked by user2818430

C# LINQ - selecting a dynamic object based on properties defined at runtime

I've been trying to create an expression which can project a strongly typed EF Core entity into a dynamic object containing a list which are defined at runtime using a REST API call....This is what I have so far:...Expression<Func<Message, dynamic>> DynamicFields(IEnumerable<string> fields) { var xParameter = Expression.Parameter(typeof(Message...
c# dynamic entity-framework-core expression-trees linq
asked by Stu Ratcliffe

Write Dynamic LINQ queries for sorting and projecting with EF Core

I am having troubles trying to get valid expressions for "inner graph" members....I wrote several code lines based on previous stack overflow answers. but I have issues finding a good "solution approach"....For example trying to express "a string" as a lambda field member......I had refactored the ToMemberOf extension method that indeed does it BUT...
c# entity-framework entity-framework-core expression-trees linq
asked by alhpe

Fastest Entity Framework Extensions

Bulk Insert
Bulk Delete
Bulk Update
Bulk Merge
View more on Stack Overflow

Prime Library

Performance

Expression Evaluator

More Projects...
Licensed under: CC-BY-SA with attribution
Not affiliated with Stack Overflow
Licensed under: CC-BY-SA with attribution
Not affiliated with Stack Overflow