How to pass anonymous types as parameters?

Question

How can I pass anonymous types as parameters to other functions? Consider this example:

var query = from employee in employees select new { Name = employee.Name, Id = employee.Id };
LogEmployees(query);

The variable query here doesn't have strong type. How should I define my LogEmployees function to accept it?

public void LogEmployees (? list)
{
    foreach (? item in list)
    {

    }
}

In other words, what should I use instead of ? marks.

Answer 1

To pass anonymous types around, consider using dynamic. A longer example is shown below and the technique you can use. For example, consider calling the TreadSafeDynamicObject here 'CustomEmployee' to make more sense of the code. The code has a constructor that accepts a dynamic object (your anonymous, potentially nested class), for example : var someCustomEmploye = new { IsIntern = false, EmployeeFacts = new { IsSenior = true, BirthDate = new DateTime(1960, 1, 1) } };

You can transform someCustomEmployee to a dynamic object using the technique shown below, for example pass in 'someCustomEmployee' into the constructor, in my code it would be:

dynamic someEmp = new ThreadSafeDynamicObject(someCustomEmployee);

Once you have transformed someEmp into a proper dynamic object, your LogEmployee function can for example serialize the object and log it or handle it in some other way (note that you do not have to go via converting it to a dynamic object anyways, if it is enough to just serialize the anonymous class instance).

Example :

dynamic threadSafeToyota = new ThreadSafeDynamicObject(new {
 Make = "Toyota",
 Model = "CR-H",
 Propulsion = new {
    IsHybrid = true,
    UsesPetrol = true,
    ElectricMotor = true
    } 
});

The code accepts a dynamic object and uses a private method 'ToDictionary' to convert the object graph of the anonymous class instance provided as an alternative way to set properties on the dynamic object.

Note that I also have added some more code here to provide thread safety when setting and getting properties.

public class ThreadSafeDynamicObject : DynamicObject, IEnumerable<KeyValuePair<string, object>>
{   

    public ThreadSafeDynamicObject()
    {       
    }
    
    public ThreadSafeDynamicObject(dynamic members)
    {
        var membersDict = ToDictionary(members);        
        InitMembers(membersDict);   
    }

    private IDictionary<string, object> ToDictionary(object data)
    {
        var attr = BindingFlags.Public | BindingFlags.Instance;
        var dict = new Dictionary<string, object>();
        foreach (var property in data.GetType().GetProperties(attr))
        {
            if (property.CanRead)
            {
                dict.Add(property.Name, property.GetValue(data, null));
            }
        }
        return dict;
    }

    private void InitMembers(IDictionary<string, object> membersDict)
    {
        foreach (KeyValuePair<string, object> member in membersDict){
            _members.AddOrUpdate(member.Key, member.Value, (key, oldValue) => member.Value); 
        }
    }

    private readonly ConcurrentDictionary<string, object> _members = new();

    public override bool TryGetMember(GetMemberBinder binder, out object result)
    {
        return _members.TryGetValue(binder.Name, out result);
    }

    public override bool TrySetMember(SetMemberBinder binder, object value)
    {
        _members.AddOrUpdate(binder.Name, value, (key, oldvalue) => value); 
        return true;
    }

    public override IEnumerable<string> GetDynamicMemberNames()
    {
        return _members.Keys.ToList().AsReadOnly(); 
    }

    public override string ToString()
    {
        return JsonSerializer.Serialize(_members);
    }

    public IEnumerator<KeyValuePair<string, object>> GetEnumerator()
    {
        return _members.GetEnumerator(); 
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return _members.GetEnumerator(); 
    }
}

When running the code inside Linqpad 7 I got this output (I am using static System.Console and using System.Dynamic here):

WriteLine(threadSafe.ToString());
WriteLine(threadSafe.Make);
WriteLine(threadSafe.Model);
WriteLine(threadSafe.Propulsion.IsHybrid);
WriteLine(threadSafe.Propulsion.UsesPetrol);
WriteLine(threadSafe.Propulsion.ElectricMotor);

There are several advantages to this. It supports nested levels as you can see in the output and is very flexible. The method 'ToDictionary' is essential here. Also, we do not have to use additional libraries outside the .net framework, so the funtionality is built in. I have not checked all variants of this code, at least it supports the typical scenarios of anonymous type object graphs.

Key thing here is to transform your anonymous type first into a dictionary and then populate the internal concurrent dictionary with the members ('fields' or 'props') of the derived DynamicObject.

There are several ways to solve this :

• You could do boxing. E.g. have a method that accepts object and use reflection to extract the properties and log the properties and their values E.g. :

  public void LogEmployees(object someCustomEmployee) { // .. }

• You could transform the anonymous object into a dynamic object as shown in my sample

• In addition to boxing or converting into a dynamic object, you could avoid reflection by serializing the converted object (either boxed object or dynamic variant).

Answer 2

Unfortunately, what you're trying to do is impossible. Under the hood, the query variable is typed to be an IEnumerable of an anonymous type. Anonymous type names cannot be represented in user code hence there is no way to make them an input parameter to a function.

Your best bet is to create a type and use that as the return from the query and then pass it into the function. For example,

struct Data {
  public string ColumnName; 
}

var query = (from name in some.Table
            select new Data { ColumnName = name });
MethodOp(query);
...
MethodOp(IEnumerable<Data> enumerable);

In this case though, you are only selecting a single field, so it may be easier to just select the field directly. This will cause the query to be typed as an IEnumerable of the field type. In this case, column name.

var query = (from name in some.Table select name);  // IEnumerable<string>

Answer 3

Instead of passing an anonymous type, pass a List of a dynamic type:

1. var dynamicResult = anonymousQueryResult.ToList<dynamic>();
2. Method signature: DoSomething(List<dynamic> _dynamicResult)
3. Call method: DoSomething(dynamicResult);
4. done.

Thanks to Petar Ivanov!

Answer 4

"dynamic" can also be used for this purpose.

var anonymousType = new { Id = 1, Name = "A" };

var anonymousTypes = new[] { new { Id = 1, Name = "A" }, new { Id = 2, Name = "B" };

private void DisplayAnonymousType(dynamic anonymousType)
{
}

private void DisplayAnonymousTypes(IEnumerable<dynamic> anonymousTypes)
{
   foreach (var info in anonymousTypes)
   {

   }
}

Answer 5

You can use generics with the following trick (casting to anonymous type):

public void LogEmployees<T>(IEnumerable<T> list)
{
    foreach (T item in list)
    {
        var typedItem = Cast(item, new { Name = "", Id = 0 });
        // now you can use typedItem.Name, etc.
    }
}

static T Cast<T>(object obj, T type)
{
    return (T)obj;
}

Answer 6

You can do it like this:

public void LogEmployees<T>(List<T> list) // Or IEnumerable<T> list
{
    foreach (T item in list)
    {

    }
}

... but you won't get to do much with each item. You could call ToString, but you won't be able to use (say) Name and Id directly.

Answer 7

I think you should make a class for this anonymous type. That'd be the most sensible thing to do in my opinion. But if you really don't want to, you could use dynamics:

public void LogEmployees (IEnumerable<dynamic> list)
{
    foreach (dynamic item in list)
    {
        string name = item.Name;
        int id = item.Id;
    }
}

Note that this is not strongly typed, so if, for example, Name changes to EmployeeName, you won't know there's a problem until runtime.

Answer 8

I would use IEnumerable<object> as type for the argument. However not a great gain for the unavoidable explicit cast. Cheers

Answer 9

You can't pass an anonymous type to a non generic function, unless the parameter type is object.

public void LogEmployees (object obj)
{
    var list = obj as IEnumerable(); 
    if (list == null)
       return;

    foreach (var item in list)
    {

    }
}

Anonymous types are intended for short term usage within a method.

From MSDN - Anonymous Types:

You cannot declare a field, a property, an event, or the return type of a method as having an anonymous type. Similarly, you cannot declare a formal parameter of a method, property, constructor, or indexer as having an anonymous type. To pass an anonymous type, or a collection that contains anonymous types, as an argument to a method, you can declare the parameter as type object. However, doing this defeats the purpose of strong typing.

(emphasis mine)

---

Update

You can use generics to achieve what you want:

public void LogEmployees<T>(IEnumerable<T> list)
{
    foreach (T item in list)
    {

    }
}

Answer 10

If you know, that your results implements a certain interface you could use the interface as datatype:

public void LogEmployees<T>(IEnumerable<T> list)
{
    foreach (T item in list)
    {

    }
}

Answer 11

Normally, you do this with generics, for example:

MapEntToObj<T>(IQueryable<T> query) {...}

The compiler should then infer the T when you call MapEntToObj(query). Not quite sure what you want to do inside the method, so I can't tell whether this is useful... the problem is that inside MapEntToObj you still can't name the T - you can either:

• call other generic methods with T
• use reflection on T to do things

but other than that, it is quite hard to manipulate anonymous types - not least because they are immutable ;-p

Another trick (when extracting data) is to also pass a selector - i.e. something like:

Foo<TSource, TValue>(IEnumerable<TSource> source,
        Func<TSource,string> name) {
    foreach(TSource item in source) Console.WriteLine(name(item));
}
...
Foo(query, x=>x.Title);