Generic Interface and Factory Pattern in C#

There could be several ways to achieve it. I'll be highlighting three different ways to achieve it

First: using strongly type

public static class GenericFactory
{
    public static IGeneric<T> CreateGeneric<T>()
    {
        if (typeof(T) == typeof(string))
        {
            return (IGeneric<T>) new GenericString();
        }

        if (typeof(T) == typeof(int))
        {
            return (IGeneric<T>) new GenericInt();
        }

        throw new InvalidOperationException();
    }
}

You would use it like this:

var a = GenericFactory.CreateGeneric<string>();
var b = GenericFactory.CreateGeneric<int>(); 

Note that this uses a strongly-typed call rather than passing in the type name as a string (which may or may not be what you actually want).

Second: using object

If instead you want to pass a string for the type name, you will have to return an object because there is no way to return the actual type:

public static object CreateGeneric(string type)
{
    switch (type)
    {
        case "string": return new GenericString();
        case "int":    return new GenericInt();
        default:       throw new InvalidOperationException("Invalid type specified.");
    }
}

Obviously if you have an object you would normally have to cast it to the right type in order to use it (which requires that you know the actual type).

Alternatively, you could use reflection to determine what methods it contains, and call them that way. But then you'd still need to know the type in order to pass a parameter of the right type.

I think that what you are attempting to do here is not the right approach, which you will discover once you start trying to use it.

Third: using dynamic

Nevertheless, there is one way you can get something close to what you want: Use dynamic as follows (assuming that you are using the object CreateGeneric(string type) factory method from above):

dynamic a = GenericFactory.CreateGeneric("string");
dynamic b = GenericFactory.CreateGeneric("int");

a.ProcessEntity("A string");
b.ProcessEntity(12345);

Be aware that dynamic uses reflection and code generation behind the scenes, which can make the initial calls relatively slow.

Also be aware that if you pass the wrong type to a method accessed via dynamic, you'll get a nasty runtime exception:

dynamic a = GenericFactory.CreateGeneric("string");
a.ProcessEntity(12345); // Wrong parameter type!

If you run that code, you get this kind of runtime exception:

Unhandled Exception: Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: The best overloaded method match for 'ConsoleApplication1.GenericString.ProcessEntity(string)' has some invalid arguments
   at CallSite.Target(Closure , CallSite , Object , Int32 )
   at System.Dynamic.UpdateDelegates.UpdateAndExecuteVoid2[T0,T1](CallSite site, T0 arg0, T1 arg1)
   at ConsoleApplication1.Program.Main() in D:\Test\CS6\ConsoleApplication1\Program.cs:line 71