CODEWITHSUNDEEP
c#genericsreflectioncasting
Dejan
Dejan

Reputation: 1028

Create instance of generic class with dynamic generic type parameter

I need to create instance of a generic class like this:

 Type T = Type.GetType(className).GetMethod(functionName).ReturnType;
 var comparer = new MyComparer<T>(); // ERROR: "The type or namespace name 'T' could not be found"

I found this answer where this is possible only with reflection. But using reflection I get object which I need to cast to my generic type. I tried like this

 Type myGeneric = typeof(MyComparer<>);
 Type constructedClass = myGeneric.MakeGenericType();
 object created = Activator.CreateInstance(constructedClass);
 var comparer = (T)Convert.ChangeType(created, T);// ERROR: "The type or namespace name 'T' could not be found"

but get the same error. How to solve it?

Here is a complete example:

    public static bool Test(string className, string functionName, object[] parameters, object correctResult)
    {
        var method = Type.GetType(className).GetMethod(functionName);
        Type T = method.ReturnType;
        var myResult = method.Invoke(null, parameters);
        dynamic myResultAsT = Convert.ChangeType(myResult, T);
        dynamic correctResultAsT = Convert.ChangeType(correctResult, T);
        var comparer = new MyComparer<T>();    // Problem is here!!!       
        return comparer.Equals(myResultAsT, correctResultAsT);            
    }

The idea is to make a unit test which will call a function with parameters and compare its result with the correct result. But I need custom comparer, so I implement MyComparer which I cannot use because of a compiler error.

public class MyComparer<T> : IEqualityComparer<T>
{
    public bool Equals(T x, T y){/* some implementation*/}
}

Upvotes: 5

Views: 12332

Answers (2)

Dejan
Dejan

Reputation: 1028

I found very simple solution to problem. There is no need to cast object to specific type T, just use dynamic keyword instead of casting

   Type myGeneric = typeof(MyComparer<>);
   Type constructedClass = myGeneric.MakeGenericType(T);
   object created = Activator.CreateInstance(constructedClass);
   dynamic comparer = created; // No need to cast created object to T

and then I can use comparer normally to call its methods like:

   return comparer.Equals(myResultAsT, correctResultAsT);

According to LueTm comments, it is probably possible to use reflection again and call comparer methods, but this solution looks much easier.

Upvotes: 8

LueTm
LueTm

Reputation: 2380

Looks like you're almost there:

// t is a variable, so make it lowercase. This is where some of the confusion comes from
Type t = Type.GetType(className).GetMethod(functionName).ReturnType;
Type myGeneric = typeof(IEqualityComparer<>);

// You need to provide the generic type to make it generic with
// You want IEqualityComparer<T>, so:
Type constructedClass = myGeneric.MakeGenericType(t); 

// Now create the object
object created = Activator.CreateInstance(constructedClass);

// This is tricky without more context...
// You could try this, but to tell you more I would need to know where you use
// the comparer instance. Are you using it in a LINQ query, or in a Sort()?
// If so just cast it to a IEqualityComparer<YourType>, and
// make YourType whaterver you need it to be in the list or the query...
var comparer = (IEqualityComparer<object>)created;

Upvotes: 2

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