How to pass anonymous types as parameters?

问题:

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);

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.

回答1:

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.

回答2:

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.

回答3:

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>

回答4:

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)
    {

    }
}

回答5:

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);

回答6:

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;
}

回答7:

"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)
   {

   }
}

回答8:

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!

回答9:

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)
    {

    }
}

回答10:

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