Construct Generic class without knowing type

Question

i would like to construct a generic class with generic parameters without knowing the type.

List<T> genericList = new List<T>();

public void AddGenericValue<T>(T t1, T t2)
{
    for (int i = 0; i < genericList.Count; i++)
    {
        if (genericList[i].t1 == t1)
        {
            genericList[i].t2 = t2;
            return;

        }
    }

    genericList.Add(new GenericClass(t1, t2));
}

public class GenericClass<T>
{
    T t1;
    T t2;

    public GenericClass(T t1, T t2)
    {
        this.t1 = t1;
        this.t2 = t2;
    }
}

Now i get error Using the generic type 'GenericClass' requires 1 type of argument.

Answer 1

You need to specify the generic parameters when stating the type:

new GenericClass<T>(t1, t2)

The reason for this is that type parameters cannot be inferred for classes, only for methods. You can make use of this by writing a factory:

static class GenericClassFactory
{
  public static GenericClass<T> Create(T t1, T t2)
  {
    return new GenericClass<T>(t1, t2);
  }
}

Now you can:

var foo = GenericClassFactory.Create(1, 2);

And T will be deduced by the compiler.

Also, the list should be a the right type:

var genericList = new List<GenericClass<T>>();

If you need to store the list as a member variable then you'll need to promote the generic type to the class level rather than the method level:

class Foo<T>
{
    private readonly List<GenericClass<T>> genericList = new List<GenericClass<T>>();

    public void AddGenericValue(T t1, T t2)
    {
        for (int i = 0; i < genericList.Count; i++)
        {
            if (genericList[i].t1 == t1)
            {
                genericList[i].t2 = t2;
                return;

            }
        }

        genericList.Add(new GenericClass<T>(t1, t2));
    }
}

Answer 2

This may fix your issue:

List<GenericClass<T>> genericList = new List<GenericClass<T>>();
genericList.Add(new GenericClass<T>(t1, t2));

Answer 3

Two things you need here,

1. Your instance of class should expect generic parameter ie <T> , so create instance of GenericClass like

    new GenericClass<T>(t1, t2)

2. To avoid compile time error define your List properly

    List<GenericClass<T>> genericList = new List<GenericClass<T>>();